(Quick Reference)

The Grails Framework

Authors: Graeme Rocher, Peter Ledbrook, Marc Palmer, Jeff Brown, Luke Daley, Burt Beckwith, Lari Hotari

Version: 3.2.3

Table of Contents

1 Introduction

Java web development as it stands today is dramatically more complicated than it needs to be. Most modern web frameworks in the Java space are over complicated and don’t embrace the Don’t Repeat Yourself (DRY) principles.

Dynamic frameworks like Rails, Django and TurboGears helped pave the way to a more modern way of thinking about web applications. Grails builds on these concepts and dramatically reduces the complexity of building web applications on the Java platform. What makes it different, however, is that it does so by building on already established Java technologies like Spring and Hibernate.

Grails is a full stack framework and attempts to solve as many pieces of the web development puzzle through the core technology and its associated plugins. Included out the box are things like:

  • An easy to use Object Relational Mapping (ORM) layer built on Hibernate

  • An expressive view technology called Groovy Server Pages (GSP)

  • A controller layer built on Spring MVC

  • An interactive command line environment and build system based on Gradle

  • An embedded Tomcat container which is configured for on the fly reloading

  • Dependency injection with the inbuilt Spring container

  • Support for internationalization (i18n) built on Spring’s core MessageSource concept

  • A transactional service layer built on Spring’s transaction abstraction

All of these are made easy to use through the power of the Groovy language and the extensive use of Domain Specific Languages (DSLs)

This documentation will take you through getting started with Grails and building web applications with the Grails framework.

1.1 What's new in Grails 3.2?

This section covers all the new features introduced in Grails 3.2.

1.1.1 GORM 6 Suite

Grails 3.2 comes with GORM 6.0, the biggest release of GORM ever! GORM 6 includes the following new features:

  • GORM for Neo4j 3.0 / Bolt Driver support

  • GORM for MongoDB 3.2

  • RxGORM - GORM for RxJava

  • RxGORM for REST built on RxNetty

  • RxGORM for MongoDB Rx Driver

  • Universal Multiple Data Sources Support

  • Multi Tenancy Support

  • Spring Container Free Bootstrapping

  • Improved Unit Testing

  • Unified Configuration API

  • New Standalone Documentation

There are so many new features and novelties in GORM that we had to write its own independent What’s New Guide!

1.1.2 RxJava Support

In addition to RxGORM, support for RxJava has been added to the Grails framework via an RxJava plugin.

Reactive controllers with RxJava

The RxJava plugin allows you to return Observable responses from controllers and integrates seamlessly with RxGORM to make it possible handle requests reactively, in a non-blocking manner. For example:

def show() {
    String author = params.author
    Book.get(params.id)
            .map { Book book ->
        rx.render view:"book", model:[book:book, author:author]
    }
}

Server Sent Events with RxJava

It is now possible to easily issue responses that return Server Sent Events with Grails and RxJava:

def index() {
    rx.stream { Subscriber subscriber ->
       for(i in (0..5)) {
           if(i % 2 == 0) {
               subscriber.onNext(
                   rx.render("Tick")
               )
           }
           else {
               subscriber.onNext(
                   rx.render("Tock")
               )

           }
           sleep 1000
       }
       subscriber.onCompleted()
   }
}
See the sample application for a demonstration of Server Sent Events in action

1.1.3 Angular Support

AngularJS Scaffolding

The angular profile has been refined and now also includes a new Angular Scaffolding plugin.

The Angular scaffolding plugin adds an ng-generate-all command which will generate the necessary AngularJS 1.x client code to perform CRUD operations in conjunction with a Grails 3 backend.

Not only does this serve as a useful tool to get up and running quickly, but (like previous versions of scaffolding) it is a great way for developers to learn how to integrate AngularJS and Grails 3.

Angular 2 Profile (3.2.1+)

Starting with Grails 3.2.1, the Angular 2 profile is available for use. To create a fresh application:

grails create-app test-ng -profile angular2

A multi-project build will be created with a separate project for the client and server applications. To make things easier, the tasks test, integrationTest, and bootRun have been created in the client application to make executing those tasks easier across the whole application.

Since Gradle executes tasks synchronously, and the bootRun task will never finish, it is important to execute it in parallel. At the root of the project:

./gradlew bootRun --parallel

This will start both the client and server side applications simultaneously.

For more information on how the new profile works, see the section in the user guide.

1.1.4 JSON Views 1.1

Version 1.1 of the JSON Views plugin is included with Grails 3.2’s "rest-api" profile and includes a number of new features. Below are some of the highlights:

Template Inheritance

It is now possible for a child JSON template to specify a parent template, thus allowing better template composition. For example given the following parent:

grails-app/views/_parent.gson
model {
    Object object
}
json {
    hal.links(object)
    version "1.0"
}

A child template can be specified as follows:

inherits template:"parent"
model {
    Person person
}
json {
    name person.name
}

Global and Default Templates

Global templates can now be created for any GORM custom types. This allows adding support for external libraries such as JodaTime or custom types provided by datastores such as MongoDB (example GeoJSON).

A global template is simply another JSON template that is named after the class name. See for example the GeoJSON templates.

In addition it is now possible to provide a fallback template named /object/_object.gson that is used if no other template is found.

Better HAL Support

The HAL support has been expanded and now includes greater control over _embedded and _links, for example:

model {
    Book book
}
json {
    hal.links(self: book )
    hal.embedded(authors: book.authors)
    hal.inline(book) {
        pages 300
    }
}

The HAL support has also been improved with support for HAL pagination.

1.1.5 CORS Support

Starting with Grails 3.2.1, we have added support to configure the CORS support provided in Spring Boot.

This feature is disabled by default. Once enabled, the default setting is "wide open". To enable CORS configuration:

application.yml
grails:
    cors:
        enabled: true

To get more information on how to tighten down these settings to match your needs, visit the section on configuring CORS.

1.1.6 Grails Wrapper

The Grails wrapper is back starting with Grails 2.3.2!

You can use it the same way you use any Grails command inside a project.

./grailsw create-controller foo

1.1.7 Updated Dependencies

Grails 3.2 ships with the following dependency upgrades:

  • Hibernate 5.1.1 (now the default version of Hibernate for new applications)

  • Spring Framework 4.3.1

  • Spring Boot 1.4.0

  • Gradle 3.0

1.1.8 Other Novelties

New Asciidoc Reference Documentation

The Grails user guide has been converted to use Asciidoctor for publishing, making it easier for users to contribute improvements to the documentation (Just click the "Improve this doc" link on the right!).

New default date data binding format

Dates formatted like "1970-01-01T00:00:00.000Z" will now be successfully parsed by default. The format is used by common JavaScript libraries.

The run-script command from Grails 2 is back

The run-script command makes a return! It is now possible to run Groovy scripts that are wrapped in a Grails context using Grails 3:

$ grails run-script my-groovy-script.groovy

Refer to the run-script documentation for more information.

Commands, a feature previously only available in plugins, are now available to be created in applications

$ grails create-command MyCommand

Note that commands defined in applications are not executed the same way as commands defined in plugins. See the updated documentation on create-command for details.

REST Profile Refinements

The REST profile has been further refined including more sensible UrlMappings and mime type configuration designed specifically for REST applications.

Ability to skip the Bootstrap process with a system property

When the Grails runtime is started, it will now execute *Bootstrap.groovy classes conditionally. If the system property grails.bootstrap.skip is set to true, the classes will not be executed for that run.

Changes to data binding with the body of a request

To be more inline with the HTTP/1.1 specification, request bodies in GET and DELETE requests will be ignored for data binding. The request body will also be ignored if the specified content length is 0.

Profile improvements

It is now possible to specify credentials for repositories used for profile resolution in your settings.groovy file. In addition, there are other new features useful for creating profiles. See the section on Profiles for the documentation.

Java 8 Date Support

Support for Java 8 date types has been added via a plugin. The tags formatDate and datePicker have been altered to support the new types. Support has been added to databinding to be able to successfully parse Java 8 dates. To take advantage of this functionality, add the new grails-java8 plugin to your application:

compile "org.grails.plugins:grails-java8"

If you are using hibernate and wish to persist the new date types, you should also add a dependency to hibernate-java8 as well:

compile "org.hibernate:hibernate-java8:<your hibernate version here>"

1.2 What's new in Grails 3.1?

Grails 3.1 includes the following new features.

Spring Boot 1.3 and Spring 4.2

Grails 3.1 has been upgraded to Spring Boot 1.3 and Spring 4.2.

1.2.1 Improvements to Grails 3 Profiles

Profile Publishing and Repositories

The following improvements are available in Grails profiles:

  • Profiles are now published as regular JAR files to any Maven compatible repository (Artifactory, Nexus etc.).

  • Additional profiles can be created easily with the new create-profile command.

  • Profiles can now contribute to the generation of the build

  • Profiles can now have one or many features

For more information see the new section on Profiles in the user guide.

1.2.2 REST API and AngularJS Profiles

REST Profile

A new profile is available designed for the creation of pure REST applications without a UI.

To create a REST application use the rest-api profile as an argument to create-app:

$ grails create-app myapp --profile=rest-api
In earlier milestones this profile was named web-api. The profile has been renamed rest-api which more appropriately describes its purpose.

Then start interactive mode to see the available commands for the profile:

$ cd myapp
$ grails

If you hit TAB you will notice code generation commands specific to the profile including:

  • create-domain-resource - Creates a domain class annotated with the Resource annotation)

  • create-restful-controller - Creates a controller that extends RestfulController.

JSON and Markup Views

The REST profile includes the ability to define JSON and Markup views and the build.gradle features the ability to compile these views for production use.

The REST profile also creates JSON views to render the index action and common commands such as generate-views have been overridden to generate JSON views.

AngularJS Profile

An initial version of the AngularJS profile is now available, making it easier to create and integrate AngularJS with Grails 3 applications.

To create a Grails 3 AngularJS application use the angular profile as an argument to create-app:

$ grails create-app myapp --profile=angular

Then start interactive mode to see the available commands for the profile:

$ cd myapp
$ grails

You will notice new commands such as create-ng-component, create-ng-controller etc. that help you get going creating an AngularJS application.

The build.gradle is also preconfigured with the necessary Gradle plugins to integrate AngularJS with Asset Pipeline. The created Angular application can be found in grails-app/assets/javascripts.

For more detail on what the Angular profile provides, see the AngularJS Profile section in the documentation

1.2.3 GORM 5 Suite

Grails 3.1 ships with GORM 5 which is a brand new release of GORM supporting the following technologies:

  • Hibernate 3, 4 and 5 - for SQL databases GORM for Hibernate now supports the latest Hibernate 5.x release

  • MongoDB 3.x - GORM for MongoDB has been upgraded to the MongoDB 3.x Java driver and supports codec based persistence

  • Neo4j 2.3.x - GORM for Neo4j has been significantly improved and support the latest release of Neo4j

  • Cassandra - GORM for Cassandra supports the latest 2.0.x drivers

For more information refer to the new GORM 5 website.

1.2.4 Plugin Publishing Plugins

New Gradle plugins are available to simplify publishing of plugins and profiles.

To utilize the plugin apply the org.grails.grails-plugin-publish plugin (after any existing Grails plugins for Gradle):

apply plugin: "org.grails.grails-plugin"
apply plugin: "org.grails.grails-plugin-publish"

For a profile the grails-profile-publish plugin can be used instead:

apply plugin: "org.grails.grails-profile"
apply plugin: "org.grails.grails-profile-publish"

Then configure the plugin. For example:

grailsPublish {
    user = 'user'
    key = 'key'
    githubSlug = 'foo/bar'
    license {
        name = 'Apache-2.0'
    }
    title = "My Plugin Title"
    desc = "My Plugin Description"
    developers = [johndoe:"John Doe"]
}

The user and key are your Bintray credentials. With this done you can continue to use bintrayUpload to publish your plugin. In addition, if you wish to update the Grails plugin portal, you simply need to configure your grails.org credentials:

grailsPublish {
    ...
    portalUser = "..."
    portalPassword = "..."
}

Then call notifyPluginPortal to update the Grails.org Plugins website:

gradle notifyPluginPortal

2 Getting Started

2.1 Installation Requirements

Before installing Grails 3.0 you will need as a minimum a Java Development Kit (JDK) installed version 1.7 or above. Download the appropriate JDK for your operating system, run the installer, and then set up an environment variable called JAVA_HOME pointing to the location of this installation.

To automate the installation of Grails we recommend SDKMAN which greatly simplifies installing and managing multiple Grails versions.

On some platforms (for example OS X) the Java installation is automatically detected. However in many cases you will want to manually configure the location of Java. For example, if you’re using bash or another variant of the Bourne Shell:

export JAVA_HOME=/Library/Java/Home
export PATH="$PATH:$JAVA_HOME/bin"
On Windows you would have to configure these environment variables in My Computer/Advanced/Environment Variables

2.2 Downloading and Installing

The first step to getting up and running with Grails is to install the distribution.

The best way to install Grails on *nix systems is with SDKMAN which greatly simplifies installing and managing multiple Grails versions.

For manual installation follow these steps:

  • Download a binary distribution of Grails and extract the resulting zip file to a location of your choice

  • Set the GRAILS_HOME environment variable to the location where you extracted the zip

    • On Unix/Linux based systems this is typically a matter of adding something like the following export GRAILS_HOME=/path/to/grails to your profile

    • On Windows this is typically a matter of setting an environment variable under My Computer/Advanced/Environment Variables

  • Then add the bin directory to your PATH variable:

    • On Unix/Linux based systems this can be done by adding export PATH="$PATH:$GRAILS_HOME/bin" to your profile

    • On Windows this is done by modifying the Path environment variable under My Computer/Advanced/Environment Variables

If Grails is working correctly you should now be able to type grails -version in the terminal window and see output similar to this:

Grails version: 3.2.3

2.3 Creating an Application

To create a Grails application you first need to familiarize yourself with the usage of the grails command which is used in the following manner:

grails <<command name>>

Run create-app to create an application:

grails create-app helloworld

This will create a new directory inside the current one that contains the project. Navigate to this directory in your console:

cd helloworld

2.4 A Hello World Example

Let’s now take the new project and turn it into the classic "Hello world!" example. First, change into the "helloworld" directory you just created and start the Grails interactive console:

$ cd helloworld
$ grails

You should see a prompt that looks like this:

interactive helloworld

What we want is a simple page that just prints the message "Hello World!" to the browser. In Grails, whenever you want a new page you just create a new controller action for it. Since we don’t yet have a controller, let’s create one now with the create-controller command:

grails> create-controller hello

Don’t forget that in the interactive console, we have auto-completion on command names. So you can type "cre" and then press <tab> to get a list of all create-* commands. Type a few more letters of the command name and then <tab> again to finish.

The above command will create a new controller in the grails-app/controllers/helloworld directory called HelloController.groovy. Why the extra helloworld directory? Because in Java land, it’s strongly recommended that all classes are placed into packages, so Grails defaults to the application name if you don’t provide one. The reference page for create-controller provides more detail on this.

We now have a controller so let’s add an action to generate the "Hello World!" page. The code looks like this:

package helloworld

class HelloController {

    def index() {
        render "Hello World!"
    }
}

The action is simply a method. In this particular case, it calls a special method provided by Grails to render the page.

Job done. To see your application in action, you just need to start up a server with another command called run-app:

grails> run-app

This will start an embedded server on port 8080 that hosts your application. You should now be able to access your application at the URL http://localhost:8080/ - try it!

Note that in previous versions of Grails the context path was by default the name of the application. If you wish to restore this behavior you can configure a context path in grails-app/conf/application.yml:

server:
    contextPath: /helloworld

With the above configuration in place the server will instead startup at the URL http://localhost:8080/helloworld/.

If you see the error "Server failed to start for port 8080: Address already in use", then it means another server is running on that port. You can easily work around this by running your server on a different port using run-app -port=9090. '9090' is just an example: you can pretty much choose anything within the range 1024 to 49151.

The result will look something like this:

intropage

This is the Grails intro page which is rendered by the grails-app/view/index.gsp file. It detects the presence of your controllers and provides links to them. You can click on the "HelloController" link to see our custom page containing the text "Hello World!". Voila! You have your first working Grails application.

One final thing: a controller can contain many actions, each of which corresponds to a different page (ignoring AJAX at this point). Each page is accessible via a unique URL that is composed from the controller name and the action name: /<appname>/<controller>/<action>. This means you can access the Hello World page via /helloworld/hello/index, where 'hello' is the controller name (remove the 'Controller' suffix from the class name and lower-case the first letter) and 'index' is the action name. But you can also access the page via the same URL without the action name: this is because 'index' is the default action. See the end of the controllers and actions section of the user guide to find out more on default actions.

2.5 Using Interactive Mode

Grails 3.0 features an interactive mode which makes command execution faster since the JVM doesn’t have to be restarted for each command. To use interactive mode simple type 'grails' from the root of any projects and use TAB completion to get a list of available commands. See the screenshot below for an example:

interactive output

For more information on the capabilities of interactive mode refer to the section on Interactive Mode in the user guide.

2.6 Getting Set Up in an IDE

IntelliJ IDEA

IntelliJ IDEA is an excellent IDE for Grails 3.0 development. It comes in 2 editions, the free community edition and the paid-for ultimate edition.

The community edition can be used for most things, although GSP syntax higlighting is only part of the ultimate edition

You can always open GSP files in the HTML editor if you just want highlighting in the community edition.

To get started with Intellij IDEA and Grails 3.0 simply go to File / Import Project and point IDEA at your build.gradle file to import and configure the project.

TextMate, Sublime, VIM etc.

There are several excellent text editors that work nicely with Groovy and Grails. See below for references:

2.7 Convention over Configuration

Grails uses "convention over configuration" to configure itself. This typically means that the name and location of files is used instead of explicit configuration, hence you need to familiarize yourself with the directory structure provided by Grails.

Here is a breakdown and links to the relevant sections:

2.8 Running and Debugging an Application

Grails applications can be run with the built in Tomcat server using the run-app command which will load a server on port 8080 by default:

grails run-app

You can specify a different port by using the server.port argument:

grails -Dserver.port=8090 run-app

Note that it is better to start up the application in interactive mode since a container restart is much quicker:

$ grails
grails> run-app
| Server running. Browse to http://localhost:8080/helloworld
| Application loaded in interactive mode. Type 'stop-app' to shutdown.
| Downloading: plugins-list.xml
grails> stop-app
| Stopping Grails server
grails> run-app
| Server running. Browse to http://localhost:8080/helloworld
| Application loaded in interactive mode. Type 'stop-app' to shutdown.
| Downloading: plugins-list.xml

You can debug a grails app by simply right-clicking on the Application.groovy class in your IDE and choosing the appropriate action (since Grails 3).

Alternatively, you can run your app with the following command and then attach a remote debugger to it.

grails run-app --debug-jvm

More information on the run-app command can be found in the reference guide.

2.9 Testing an Application

The create-* commands in Grails automatically create unit or integration tests for you within the src/test/groovy directory. It is of course up to you to populate these tests with valid test logic, information on which can be found in the section on Unit and integration tests.

To execute tests you run the test-app command as follows:

grails test-app

2.10 Deploying an Application

Grails applications can be deployed in a number of different ways.

If you are deploying to a traditional container (Tomcat, Jetty etc.) you can create a Web Application Archive (WAR file), and Grails includes the war command for performing this task:

grails war

This will produce a WAR file under the build/libs directory which can then be deployed as per your container’s instructions.

Note that by default Grails will include an embeddable version of Tomcat inside the WAR file, this can cause problems if you deploy to a different version of Tomcat. If you don’t intend to use the embedded container then you should change the scope of the Tomcat dependencies to provided prior to deploying to your production container in build.gradle:

provided "org.springframework.boot:spring-boot-starter-tomcat"

If you are building a WAR file to deploy on Tomcat 7 then in addition you will need to change the target Tomcat version in the build. Grails is built against Tomcat 8 APIs by default. To target a Tomcat 7 container, insert a line to build.gradle above the dependencies { } section:

ext['tomcat.version'] = '7.0.59'

Unlike most scripts which default to the development environment unless overridden, the war command runs in the production environment by default. You can override this like any script by specifying the environment name, for example:

grails dev war

If you prefer not to operate a separate Servlet container then you can simply run the Grails WAR file as a regular Java application. Example:

grails war
java -Dgrails.env=prod -jar build/libs/mywar-0.1.war

When deploying Grails you should always run your containers JVM with the -server option and with sufficient memory allocation. A good set of VM flags would be:

-server -Xmx768M -XX:MaxPermSize=256m

2.11 Supported Java EE Containers

Grails runs on any container that supports Servlet 3.0 and above and is known to work on the following specific container products:

  • Tomcat 7

  • GlassFish 3 or above

  • Resin 4 or above

  • JBoss 6 or above

  • Jetty 8 or above

  • Oracle Weblogic 12c or above

  • IBM WebSphere 8.0 or above

It’s required to set "-Xverify:none" in "Application servers > server > Process Definition > Java Virtual Machine > Generic JVM arguments" for older versions of WebSphere. This is no longer needed for WebSphere version 8 or newer.

Some containers have bugs however, which in most cases can be worked around. A list of known deployment issues can be found on the Grails wiki.

2.12 Creating Artefacts

Grails ships with a few convenience targets such as create-controller, create-domain-class and so on that will create controllers and different artefact types for you. NOTE: These are just for your convenience and you can just as easily use an IDE or your favourite text editor. For example to create the basis of an application you typically need a domain model:

grails create-app helloworld
cd helloworld
grails create-domain-class book

This will result in the creation of a domain class at grails-app/domain/helloworld/Book.groovy such as:

package helloworld

class Book {
}

There are many such create-* commands that can be explored in the command line reference guide.

To decrease the amount of time it takes to run Grails scripts, use the interactive mode.

2.13 Generating an Application

To get started quickly with Grails it is often useful to use a feature called scaffolding to generate the skeleton of an application. To do this use one of the generate-* commands such as generate-all, which will generate a controller (and its unit test) and the associated views:

grails generate-all helloworld.Book

3 Upgrading

Grails 3.0 is a complete ground up rewrite of Grails and introduces new concepts and components for many parts of the framework.

When upgrading an application or plugin from Grails 3.0 there are many areas to consider including:

  • Removal of dynamic scaffolding from Grails 3.0.0 till 3.0.4 when it was re-introduced

  • Removal of before and after interceptors

  • Project structure differences

  • File location differences

  • Configuration differences

  • Package name differences

  • Legacy Gant Scripts

  • Gradle Build System

  • Changes to Plugins

  • Source vs Binary Plugins

The best approach to take when upgrading a plugin or application (and if your application is using several plugins the plugins will need upgrading first) is to create a new Grails 3.0 application of the same name and copy the source files into the correct locations in the new application.

Removal of before and after interceptors

Before and after interceptors were removed. So all beforeInterceptor and afterInterceptor need to be replaced by Stand alone interceptors.

File Location Differences

The location of certain files have changed or been replaced with other files in Grails 3.0. The following table lists old default locations and their respective new locations:

Old Location New Location Description

grails-app/conf/BuildConfig.groovy

build.gradle

Build time configuration is now defined in a Gradle build file

grails-app/conf/Config.groovy

grails-app/conf/application.groovy

Renamed for consistency with Spring Boot

grails-app/conf/UrlMappings.groovy

grails-app/controllers/UrlMappings.groovy

Moved since grails-app/conf is not a source directory anymore

grails-app/conf/BootStrap.groovy

grails-app/init/BootStrap.groovy

Moved since grails-app/conf is not a source directory anymore

scripts

src/main/scripts

Moved for consistency with Gradle

src/groovy

src/main/groovy

Moved for consistency with Gradle

src/java

src/main/groovy (yes

groovy!)

Moved for consistency with Gradle

test/unit

src/test/groovy

Moved for consistency with Gradle

test/integration

src/integration-test/groovy

Moved for consistency with Gradle

web-app

src/main/webapp or src/main/resources/

Moved for consistency with Gradle

\*GrailsPlugin.groovy

src/main/groovy

src/main/resources/public is recommended as src/main/webapp only gets included in WAR packaging but not in JAR packaging.

It is recommended to merge Java source files from src/java into src/main/groovy. You can create a src/main/java directory if you want to and it will be used but it is generally better to combine the folders. (The Groovy and Java sources compile together.)

For plugins the plugin descriptor (a Groovy file ending with "GrailsPlugin") which was previously located in the root of the plugin directory should be moved to the src/main/groovy directory under an appropriate package.

New Files Not Present in Grails 2.x

The reason it is best to create a new application and copy your original sources to it is because there are a number of new files that are not present in Grails 2.x by default. These include:

File Description

build.gradle

The Gradle build descriptor located in the root of the project

gradle.properties

Properties file defining the Grails and Gradle versions

grails-app/conf/logback.groovy

Logging previously defined in Config.groovy is now defined using Logback

grails-app/conf/application.yml

Configuration can now also be defined using YAML

grails-app/init/PACKAGE_PATH/Application.groovy

The Application class used By Spring Boot to start the application

Files Not Present in Grails 3.x

Some files that were previously created by Grails 2.x are no longer created. These have either been removed or an appropriate replacement added. The following table lists files no longer in use:

File Description

application.properties

The application version is now defined in build.gradle. The application name defaults to the directory name which can be overridden by creating a settings.gradle file and setting the rootProject.name property

grails-app/conf/DataSource.groovy

Merged together into application.yml

lib

Dependency resolution should be used to resolve JAR files

web-app/WEB-INF/applicationContext.xml

Removed. Beans can be defined in grails-app/conf/spring/resources.groovy

src/templates/war/web.xml

Grails 3.0 no longer requires web.xml. Customizations can be done via Spring

web-app/WEB-INF/sitemesh.xml

Removed. Sitemesh filter no longer present.

web-app/WEB-INF/tld

Removed. Can be restored in src/main/webapp or src/main/resources/WEB-INF

3.1 Upgrading from Grails 3.1

If you are upgrading from Grails 3.1 there are a few items to take into consideration.

Deprecated Classes and Methods Removed

Classes and methods deprecated in Grails 3.0.x have been removed in Grails 3.2. This includes all classes in the org.codehaus.groovy.grails package. If your application or plugin uses deprecated classes they should be updated to use non-deprecated replacements.

Slf4j Now Default

The log property injected at compile time into all classes is now an Slf4j Logger instance and not an instance of the Commons Logging Log class.

This should be a simple upgrade for most use cases, however this change does have some implications, for example it is no longer possible to pass non-string types to the log method. Example:

log.info "this works"
Double notAString = 9.2
log.info notAString

The latter call to the info method will throw an exception as it is not a String.

Instead you should use Slf4j’s formatting anchors to log. The advantage is the toString() method is not called unless the message will be logged.

log.info "{}", 9.2D
log.debug "Key: {}, Value: {}", key, value
log.error "{}", exception.message, exception

See the Slf4j FAQ for more information.

Spring 4.3

Grails 3.2 comes with Spring 4.3 which no longer supports Hibernate 3 and hence Grails 3.2 no longer supports Hibernate 3 either and you will need to upgrade to Hibernate 4 or above.

Spring Boot 1.4

Spring Boot 1.4, through its dependency management mechanism, enforces the upgrade for many depndencies. You should review your dependencies following the upgrade to ensure the new versions are compatible with your application.

Spring Boot 1.4 also deprecates many testing annotations (such as WebIntegrationTest).

See the Spring Boot 1.4 release notes for more information on the changes required at the Boot level.

Hibernate 4 Usage

Related to Spring Boot 1.4, one important change is that Hibernate 5 is now the default version, so if you have declared a dependency on the hibernate4 plugin in Grails such as:

compile "org.grails.plugins:hibernate4"

This will not be enough to ensure that Hibernate 4 is used. You must instead also directly declare the Hibernate 4 dependencies:

dependencies {
    compile "org.grails.plugins:hibernate4"
    compile "org.hibernate:hibernate-core:4.3.10.Final"
    compile "org.hibernate:hibernate-ehcache:4.3.10.Final"
}

GORM 6 Configuration Model

In preparation for Hibernate 5.2 support the previous "SessionFactoryBean" notion has been removed. Now if you wish to customize SessionFactory creation you should instead register a custom org.grails.orm.hibernate.connections.HibernateConnectionSourceFactory in Spring.

HibernateTestMixin Dependency Changes

The grails-datastore-test-support dependency has been removed and the HibernateTestMixin class integrated directly into the plugin, so if you receive a resolve error remove the following dependency from your build.gradle:

dependencies {
    testCompile "org.grails:grails-datastore-test-support"
}

application.groovy Changes

An improvement was added to make groovy configuration behave like yml configuration when it comes to the CLI. Previously, configuration values in application.groovy were not available to commands like grails create-controller. A side effect of this change causes an exception when those commands are executed if the configuration relies on classes in the runtime.

Error occurred running Grails CLI: startup failed:
script14738267015581837265078.groovy: 13: unable to resolve class com.foo.Bar

The solution is to create a separate file called runtime.groovy in grails-app/conf. That file will not be parsed by the CLI and will only be included at runtime.

Stop using the default namespace

Using the default package in places like UrlMappings.groovy, BootStrap.groovy or in a taglib can cause that code to fail or not execute at all when packaged in a JAR or WAR file. Make sure all Groovy/Java files start with the package statement and move any affected files to the respective folder. For example, change:

class UrlMappings {

    static mappings = {

    ...

to:

package myapp

class UrlMappings {

    static mappings = {

    ...

and copy the file from grails-app/controllers/UrlMappings.groovy into .grails-app/controllers/myapp/UrlMappings.groovy

3.2 Upgrading from Grails 3.0

Generally to upgrade an application from Grails 3.0 you can simply modify the version of Grails in gradle.properties.

There are however some differences to Grails 3.0.x that are documented below.

GORM 5 Upgrade

Grails 3.1 ships with GORM 5, which is a near complete rewrite of GORM ontop of Groovy traits and is not binary compatible with the previous version of GORM.

If you receive an error such as:

Caused by: java.lang.ClassNotFoundException: org.grails.datastore.gorm.GormEntity$Trait$FieldHelper
    ... 8 more

You are using a plugin or class that was compiled with a previous version of GORM and these will need to be recompiled to be Grails 3.1 and GORM 5 compatible.

Hibernate Plugin

For the GORM 5 release the hibernate plugin has been renamed to hibernate4 (and there are hibernate3 and hibernate5 versions too). You should change your build.gradle to reflect that:

compile 'org.grails.plugins:hibernate4'

Static Resources Path

The default path for static resources resolved from src/main/resources/public has been changed to be nested under the static/* pattern instead of directly under the root of the application. For example a link in GSP pages such as:

${g.resource(dir:'files', file:'mydoc.pdf')}

Will produce a URI such as /static/files/mydoc.pdf instead of /files/mydoc.pdf. If you wish to revert to the previous behavior you can configure this in application.yml:

grails:
    resources:
        pattern: '/**'

Filters Plugin Removed

The Filters plugin was replaced by Interceptors in Grails 3.0.x, although the plugin was still present. In Grails 3.1.x the Filters plugin has been removed. If you still wish to use the filters plugin you can add a dependency on the previous version provided by Grails 3.0.x. For example:

compile 'org.grails:grails-plugin-filters:3.0.12'

You would also need to move the filters to any other source directory (e.g. grails-app/controllers) as grails-app/conf is not considered a source directory anymore.

Spring Transactional Proxies

Because the grails.transactional.Transactional transform already provides the ability to create transactional services without the need for proxies, traditional support for transactional proxies has been disabled by default for new applications.

This means that if you have any services that use the transactional property and not the Transactional annotation they should be altered. For example the following service:

class FooService {
    static transactional = true
}

Becomes:

import grails.transaction.Transactional

@Transactional
class FooService {

}

In addition because in previous versions of a Grails transactional defaulted to true any services that do not declare transactional should be altered too.

If you wish to adopt the new behavior then transctional proxies can be disabled with the following configuration:

grails:
    spring:
        transactionManagement:
            proxies: false

JSON Converter changes

The default JSON converter no longer includes the class property by default. This can be re-enable with the following configuration:

grails:
    converters:
        domain:
            include:
                class: true

In addition the default JSON converter will no longer render the id property if it is null.

JSON Builder Groovy Alignment

The class grails.web.JSONBuilder has been deprecated and replaced with groovy.json.StreamingJsonBuilder, the default JSON builder within Groovy. This avoids confusion with the differences between JSON builders and better aligns with Groovy’s core libraries.

This also means that any render blocks that rendered JSON will need to be updated to use the groovy.json.StreamingJsonBuilder syntax. For example the following code:

render(contentType:"application/json") {
    title = "The Stand"
}

Should instead be written as:

render(contentType:"application/json") {
    title "The Stand"
}

If you are upgrading and prefer to continue to use the previous implementation then you can re-enable the deprecated JSONBuilder with the following configuration:

grails:
    json:
        legacy:
            builder: true

JSON Views Replace JSON Converters

With the addition of JSON views the previous API for using JSON converters is largely discouraged in favour of views. The converters plugin will in the future be separated into an external plugin and JSON views phased in to replace it. The JSON converter API is not deprecated, however JSON views provide a more fully featured, elegant API that is superior to writing JSON converters and/or marshallers.

Spring Boot 1.3 and Spring 4.2

Grails 3.1 ships with upgraded third party libraries that may require changes. See the Spring Boot upgrade notes for information.

Unlike Spring Boot 1.2, Spring Boot 1.3 no longer uses the Gradle Application Plugin so if you relied on any behavior the application plugin then the plugin will need to be re-applied to your build.gradle.

Spring Boot 1.3 also uses Spring Security 4.x by default, so if you project depends on Spring Security 3.x you have to force a downgrade. For example:

compile 'org.springframework.security:spring-security-core:3.2.9.RELEASE'
compile 'org.springframework.security:spring-security-web:3.2.9.RELEASE'

Gradle run task no longer available by default

Because the Gradle run task for application startup was provided by the Gradle Application Plugin (see above), it is no longer available by default. If you use Gradle to start up your application, use the bootRun task instead, or re-apply the Application plugin in your build.gradle.

Note: If you don’t have need of the Gradle Application plugin’s features, but have custom Gradle tasks or IDE configurations that depend on run, you can supply your own run task that depends on bootRun in your build.gradle:

task run(dependsOn: ['bootRun'])

Resource annotation defaults to JSON instead of XML

The Resource annotation applied to domain classes defaults to XML in Grails 3.0.x, but in Grails 3.1.x and above it defaults to JSON.

If you use this annotation with the expecation of produces XML responses as the default you can modify the definition as follows:

import grails.rest.*

@Resource(formats=['xml', 'json'])
class MyDomainClass {}

This will restore the Grails 3.0.x behavior.

Geb and HTMLUnit 2.18

If you use Geb with HTMLUnit (something that is not recommended, as a more native driver such as PhantomJS is recommended) you will need to upgrade your dependencies in build.grade:

testRuntime 'org.seleniumhq.selenium:selenium-htmlunit-driver:2.47.1'
    testRuntime 'net.sourceforge.htmlunit:htmlunit:2.18'

Note that there are also some changes in behavior in HTMLUnit 2.18 that may cause issues in existing tests including:

  • Expressions that evaluate the title (Example $('title')) now return blank and should be replaced with just title

  • If you return plain text in a response without surrounding HTML tags, these are no longer regarded as valid responses and should be wrapped in the required tags.

application.groovy Changes (3.1.11+)

An improvement was added to make groovy configuration behave like yml configuration when it comes to the CLI. Previously, configuration values in application.groovy were not available to commands like grails create-controller. A side effect of this change causes an exception when those commands are executed if the configuration relies on classes in the runtime.

Error occurred running Grails CLI: startup failed:
script14738267015581837265078.groovy: 13: unable to resolve class com.foo.Bar

The solution is to create a separate file called runtime.groovy in grails-app/conf. That file will not be parsed by the CLI and will only be included at runtime.

3.3 Upgrading from Grails 2.x

This guide takes you through the fundamentals of upgrading a Grails 2.x application or plugins to Grails 3.x.

3.3.1 Upgrading Plugins

To upgrade a Grails 2.x plugin to Grails 3.x you need to make a number of different changes. This documentation will outline the steps that were taken to upgrade the Quartz plugin to Grails 3, each individual plugin may differ.

Step 1 - Create a new Grails 3 plugin

The first step is to create a new Grails 3 plugin using the command line:

$ grails create-plugin quartz

This will create a Grails 3 plugin in the quartz directory.

Step 2 - Copy sources from the original Grails 2 plugin

The next step is to copy the sources from the original Grails 2 plugin to the Grails 3 plugin:

# first the sources
cp -rf ../quartz-2.x/src/groovy/ src/main/groovy
cp -rf ../quartz-2.x/src/java/ src/main/groovy
cp -rf ../quartz-2.x/grails-app/ grails-app
cp -rf ../quartz-2.x/QuartzGrailsPlugin.groovy src/main/groovy/grails/plugins/quartz

# then the tests
cp -rf ../quartz-2.x/test/unit/* src/test/groovy
mkdir -p src/integration-test/groovy
cp -rf ../quartz-2.x/test/integration/* src/integration-test/groovy

# then templates / other resources
cp -rf ../quartz-2.x/src/templates/ src/main/templates

Step 3 - Alter the plugin descriptor

You will need to add a package declaration to the plugin descriptor. In this case QuartzGrailsPlugin is modified as follows:

// add package declaration
package grails.plugins.quartz
...
class QuartzGrailsPlugin extends Plugin {
 ...
}

In addition you should remove the version property from the descriptor as this is now defined in build.gradle.

Step 4 - Update the Gradle build with required dependencies

The repositories and dependencies defined in grails-app/conf/BuildConfig.groovy of the original Grails 2.x plugin will need to be defined in build.gradle of the new Grails 3.x plugin:

compile("org.quartz-scheduler:quartz:2.2.1") {
    exclude group: 'slf4j-api', module: 'c3p0'
  }

It is recommended to use the latest stable, Grails 3+ compatible version of plugins. (Grails 2.x plugin versions will not work.)

Step 5 - Modify Package Imports

In Grails 3.x all internal APIs can be found in the org.grails package and public facing APIs in the grails package. The org.codehaus.groovy.grails package no longer exists.

All package declaration in sources should be modified for the new location of the respective classes. Example org.codehaus.groovy.grails.commons.GrailsApplication is now grails.core.GrailsApplication.

Step 5 - Migrate Plugin Specific Config to application.yml

Some plugins define a default configuration file. For example the Quartz plugin defines a file called grails-app/conf/DefaultQuartzConfig.groovy. In Grails 3.x this default configuration can be migrated to grails-app/conf/application.yml and it will automatically be loaded by Grails without requiring manual configuration merging.

Step 6 - Update plugin exclusions

Old plugins may have a pluginExcludes property defined that lists the patterns for any files that should not be included in the plugin package. This is normally used to exclude artifacts such as domain classes that are used in the plugin’s integration tests. You generally don’t want these polluting the target application.

This property is no longer sufficient in Grails 3, and nor can you use source paths. Instead, you must specify patterns that match the paths of the compiled classes. For example, imagine you have some test domain classes in the grails-app/domain/plugin/tests directory. You should first change the pluginExcludes value to

def pluginExcludes = ["plugin/test/**"]

and then add this block to the build file:

jar {
    exclude "plugin/test/**"
}

The easiest way to ensure these patterns work effectively is to put all your non-packaged class into a distinct Java package so that there is a clean separation between the main plugin classes and the rest.

Step 7 - Register ArtefactHandler Definitions

In Grails 3.x ArtefactHandler definitions written in Java need to be declared in a file called src/main/resources/META-INF/grails.factories since these need to be known at compile time.

If the ArtefactHandler is written in Groovy this step can be skipped as Grails will automatically create the grails.factories file during compilation.

The Quartz plugin requires the following definition to register the ArtrefactHandler:

grails.core.ArtefactHandler=grails.plugins.quartz.JobArtefactHandler

Step 8 - Migrate Code Generation Scripts

Many plugins previously defined command line scripts in Gant. In Grails 3.x command line scripts have been replaced by two new features: Code generation scripts and Gradle tasks.

If your script is doing simple code generation then for many cases a code generation script can replace an old Gant script.

The create-job script provided by the Quartz plugin in Grails 2.x was defined in scripts/CreateJob.groovy as:

includeTargets << grailsScript("_GrailsCreateArtifacts")

target(createJob: "Creates a new Quartz scheduled job") {
    depends(checkVersion, parseArguments)

    def type = "Job"
    promptForName(type: type)

    for (name in argsMap.params) {
        name = purgeRedundantArtifactSuffix(name, type)
        createArtifact(name: name, suffix: type, type: type, path: "grails-app/jobs")
        createUnitTest(name: name, suffix: type)
    }
}

setDefaultTarget 'createJob'

A replacement Grails 3.x compatible script can be created using the create-script command:

$ grails create-script create-job

Which creates a new script called src/main/scripts/create-job.groovy. Using the new code generation API it is simple to implement:

description("Creates a new Quartz scheduled job") {
    usage "grails create-job <<JOB NAME>>"
    argument name:'Job Name', description:"The name of the job"
}

model = model( args[0] )
render  template:"Job.groovy",
        destination: file( "grails-app/jobs/$model.packagePath/${model.simpleName}Job.groovy"),
        model: model

Please refer to the documentation on Creating Custom Scripts for more information.

Migrating More Complex Scripts Using Gradle Tasks

Using the old Grails 2.x build system it was relatively common to spin up Grails inside the command line. In Grails 3.x it is not possible to load a Grails application within a code generation script created by the create-script command.

Instead a new mechanism specific to plugins exists via the create-command command. The create-command command will create a new ApplicationCommand, for example the following command will execute a query:

import grails.dev.commands.*
import javax.sql.*
import groovy.sql.*
import org.springframework.beans.factory.annotation.*

class RunQueryCommand implements ApplicationCommand {

  @Autowired
  DataSource dataSource

  boolean handle(ExecutionContext ctx) {
      def sql = new Sql(dataSource)
      println sql.executeQuery("select * from foo")
      return true
  }
}

With this command in place once the plugin is installed into your local Maven cache you can add the plugin to both the build classpath and the runtime classpath of the application’s build.gradle file:

buildscript {
  ...
  dependencies {
    classpath "org.grails.plugins:myplugin:0.1-SNAPSHOT"
  }
}
...
dependencies {
  runtime "org.grails.plugins:myplugin:0.1-SNAPSHOT"
}

Grails will automatically create a Gradle task called runQuery and a command named run-query so both the following examples will execute the command:

$ grails run-query
$ gradle runQuery

Step 8 - Delete Files that were migrated or no longer used

You should now delete and cleanup the project of any files no longer required by Grails 3.x (BuildConfig.groovy, Config.groovy, DataSource.groovy etc.)

3.3.2 Upgrading Applications

Upgrading applications to Grails 3.x will require that you upgrade all plugins the application uses first, hence you should follow the steps in the previous section to first upgrade your plugins.

Step 1 - Create a New Application

Once the plugins are Grails 3.x compatible you can upgrade the application. To upgrade an application it is again best to create a new Grails 3 application using the "web" profile:

$ grails create-app myapp
$ cd myapp

Step 2 - Migrate Sources

The next step is to copy the sources from the original Grails 2 application to the Grails 3 application:

# first the sources
cp -rf ../old_app/src/groovy/ src/main/groovy
cp -rf ../old_app/src/java/ src/main/groovy
cp -rf ../old_app/grails-app/ grails-app

# then the tests
cp -rf ../old_app/test/unit/ src/test/groovy
mkdir -p src/integration-test/groovy
cp -rf ../old_app/test/integration/ src/integration-test/groovy

Step 3 - Update the Gradle build with required dependencies

The repositories and dependencies defined in grails-app/conf/BuildConfig.groovy of the original Grails 2.x application will need to be defined in build.gradle of the new Grails 3.x application.

Step 4 - Modify Package Imports

In Grails 3.x all internal APIs can be found in the org.grails package and public facing APIs in the grails package. The org.codehaus.groovy.grails package no longer exists.

All package declaration in sources should be modified for the new location of the respective classes. Example org.codehaus.groovy.grails.commons.GrailsApplication is now grails.core.GrailsApplication.

Step 5 - Migrate Configuration

The configuration of the application will need to be migrated, this can normally be done by simply renaming grails-app/conf/Config.groovy to grails-app/conf/application.groovy and merging the content of grails-app/conf/DataSource.groovy into grails-app/conf/application.groovy.

Note however that Log4j has been replaced by grails-app/conf/logback.groovy for logging, so any logging configuration in grails-app/conf/Config.groovy should be migrated to logback format.

Step 6 - Migrate web.xml Modifications to Spring

If you have a modified web.xml template then you will need to migrate this to Spring as Grails 3.x does not use a web.xml (although it is still possible to have on in src/main/webapp/WEB-INF/web.xml).

New servlets and filters can be registered as Spring beans or with ServletRegistrationBean and FilterRegistrationBean respectively.

Step 7 - Migrate Static Assets not handled by Asset Pipeline

If you have static assets in your web-app directory of your Grails 2.x application such as HTML files, TLDs etc. these need to be moved. For public assets such as static HTML pages and so on these should go in src/main/resources/public.

TLD descriptors and non public assets should go in src/main/resources/WEB-INF.

As noted earlier, src/main/webapp folder can also be used for this purpose but it is not recommended.

Step 8 - Migrate Tests

Once the package names are corrected unit tests will continue to run, however any tests that extend the deprecated and removed JUnit 3 hierarchy will need to be migrated to Spock or JUnit 4.

Integration tests will need to be annotated with the Integration annotation and should not extend GroovyTestCase or any JUnit 3 super class.

3.3.3 General Changes to be aware of when migrating apps

There are other miscellaneous changes between Grails 2.x and Grails 3.x that it may help to be aware of when migrating your applications and plugins. Minor changes may be required.

Domain classes

The Constraints section of a Domain Class (or other validateable object) looks like this:

static constraints = {
  name nullable: true, blank: false
  myField nullable: true
  another unique: true
}

In Grails 2.x, fields with no constraints could be declared in the Constraints block, as a method call with no arguments. Example (NB. the following syntax is no longer supported):

static constraints = {
  name nullable: true, blank: false
  mySimpleProperty()                  // <- A field that has no constraints. This syntax is not supported in Grails 3.
  anotherProperty unique: true
}

A different syntax has to be used in Grails 3. Either remove the field declaration from the constraints block (if there are no constraints to specify for it), or to keep the field placeholder, pass an empty map argument: \[:\] instead of ().

Replacement code for Grails 3.x:

static constraints = {
  name nullable: true, blank: false
  mySimpleProperty [:]                // <- Empty map argument instead of ()
  anotherProperty unique: true
}

If such declarations have not yet been changed then a log message like this emits on startup:

ORM Mapping Invalid: Specified config option <<mySimpleProperty>> does not exist for class [example.MyDomainClass]

Multi-project builds (Grails 2.x inline plugins)

If your project had inline plugins in Grails 2.x, contains ASTs, or if your project is composed of several modules or related applications then you may decide to restructure your project as a Gradle multi-project build.

Sample multi-project structure:

+ example
    + example-app   <-- Main app
    + example-core  <-- Shared code plugin
    + example-ast   <-- AST transformations plugin

How to configure this is documented in the Plugins section under the heading 'Inline Plugins in Grails 3.0'.

Migrating from Grails 2.x to Grails 3.1+

During the progress of migrating code from Grails 2.4 to Grails 3.1+, your project (and the plugins that your project depends on) will be moving to GORM 5 (or higher) and other newer library versions. You might also wish to familiarise yourself with the differences mentioned in the section Upgrading from Grails 3.0.

AST Transformations

If your application contains AST transformations, please be aware that for these to be applied to your application code, they must now be contained within a plugin. (In Grails 2.x it was possible to pre-compile AST transformations then apply them to your application code by hooking into compile events in _Events.groovy. This is no longer possible. Move your AST Transformation classes and associated annotations into a plugin for this purpose.)

There are two AST patterns on which you can base migration of your AST transformer code:

  • Groovy way: Use Groovy AST transformation annotations.

  • Grails way: Use Grails AST transformer annotations.

Groovy AST transformations
  • Import org.codehaus.groovy.transform.GroovyASTTransformation

  • Annotate your transformation class with GroovyASTTransformation(phase=CompilePhase.CANONICALIZATION)

  • A useful example app can be found here: grails3ast

Grails AST transformations
  • Import grails.compiler.ast.AstTransformer

  • Annotate your transformation class with AstTransformer

  • Implement applicable interfaces, particularly if you are transforming Artefacts, e.g. implements GrailsArtefactClassInjector, AnnotatedClassInjector

  • Your Transformer class must reside in a package under org.grails.compiler, otherwise it will not be detected. Example: org.grails.compiler.myapp

  • Examples can be found in the Grails source code

  • Example reference: ControllerActionTransformer.java

Deployment to containers

Grails uses Spring Boot to embed a Tomcat or Jetty instance by default. To build a war file for deployment to a container you will need to make a simple change to build.gradle (so that a container is not embedded).

If you deploy to a Tomcat 7 container then there is an additional step. Grails 3 is built against Tomcat 8 APIs by default. You will need to change the target Tomcat version in the build to 7.

There are standalone deployment options available.

Refer to the Deployment guide for further details.

Multiple datasources

If your application uses multiple datasources, then be aware that the way these are declared in application.yml or application.groovy (previously DataSources.groovy) has changed.

If there is more than one DataSource in an application there is now a dataSources { …​ } configuration block to contain them all. Previously, multiple dataSource declarations were used, with an underscore and suffix on the additional datasources, e.g. dataSource_lookup { …​ }.

Please refer to the user guide section on Multiple Datasources for examples.

Improvements to dependency injection

In your Grails 2.x app you may have used Spring @Autowired in a few situations, such as dependency injection into certain base classes, and for typed field dependency injection. For example:

@Autowired
org.quartz.Scheduler quartzScheduler

Grails now has support for dependency injection into typed fields in addition to untyped def fields, following the usual Grails conventions of field name matching the bean property name. Example:

GrailsApplication grailsApplication

You may find that @Autowired no longer works as it did previously in your code on artefacts or base classes, in certain scenarios, resulting in NULL for these fields. Changing these to a simple typed Grails dependency following the Grails naming convention and removing @Autowired should resolve this.

4 Configuration

It may seem odd that in a framework that embraces "convention-over-configuration" that we tackle this topic now. With Grails' default settings you can actually develop an application without doing any configuration whatsoever, as the quick start demonstrates, but it’s important to learn where and how to override the conventions when you need to. Later sections of the user guide will mention what configuration settings you can use, but not how to set them. The assumption is that you have at least read the first section of this chapter!

4.1 Basic Configuration

Configuration in Grails is generally split across 2 areas: build configuration and runtime configuration.

Build configuration is generally done via Gradle and the build.gradle file. Runtime configuration is by default specified in YAML in the grails-app/conf/application.yml file.

If you prefer to use Grails 2.0-style Groovy configuration then you can create an additional grails-app/conf/application.groovy file to specify configuration using Groovy’s ConfigSlurper syntax.

For Groovy configuration the following variables are available to the configuration script:

Variable Description

userHome

Location of the home directory for the account that is running the Grails application.

grailsHome

Location of the directory where you installed Grails. If the GRAILS_HOME environment variable is set, it is used.

appName

The application name as it appears in build.gradle.

appVersion

The application version as it appears in build.gradle.

For example:

my.tmp.dir = "${userHome}/.grails/tmp"

If you want to read runtime configuration settings, i.e. those defined in application.yml, use the grailsApplication object, which is available as a variable in controllers and tag libraries:

class MyController {
    def hello() {
        def recipient = grailsApplication.config.getProperty('foo.bar.hello')

        render "Hello ${recipient}"
    }
}

The config property of the grailsApplication object is an instance of the Config interface and provides a number of useful methods to read the configuration of the application.

In particular, the getProperty method (seen above) is useful for efficiently retrieving configuration properties, while specifying the property type (the default type is String) and/or providing a default fallback value.

class MyController {

    def hello(Recipient recipient) {
        //Retrieve Integer property 'foo.bar.max.hellos', otherwise use value of 5
        def max = grailsApplication.config.getProperty('foo.bar.max.hellos', Integer, 5)

        //Retrieve property 'foo.bar.greeting' without specifying type (default is String), otherwise use value "Hello"
        def greeting = grailsApplication.config.getProperty('foo.bar.greeting', "Hello")

        def message = (recipient.receivedHelloCount >= max) ?
          "Sorry, you've been greeted the max number of times" :  "${greeting}, ${recipient}"
        }

        render message
    }
}

Notice that the Config instance is a merged configuration based on Spring’s PropertySource concept and reads configuration from the environment, system properties and the local application configuration merging them into a single object.

GrailsApplication can be easily injected into services and other Grails artifacts:

import grails.core.*

class MyService {
    GrailsApplication grailsApplication

    String greeting() {
        def recipient = grailsApplication.config.getProperty('foo.bar.hello')
        return "Hello ${recipient}"
    }
}

Finally, you can also use Spring’s Value annotation to inject configuration values:

import org.springframework.beans.factory.annotation.*

class MyController {
    @Value('${foo.bar.hello}')
    String recipient

    def hello() {
        render "Hello ${recipient}"
    }
}
In Groovy code you must use single quotes around the string for the value of the Value annotation otherwise it is interpreted as a GString not a Spring expression.

As you can see, when accessing configuration settings you use the same dot notation as when you define them.

4.1.1 Options for the YML format Config

application.yml was introduced in Grails 3.0 for an alternative format for the configuration tasks.

Using system properties / command line arguments

Suppose you are using the JDBC_CONNECTION_STRING command line argument and you want to access the same in the yml file then it can be done in the following manner:

production:
    dataSource:
        url: '${JDBC_CONNECTION_STRING}'

Similarly system arguments can be accessed.

You will need to have this in build.gradle to modify the bootRun target if grails run-app is used to start the application

bootRun {
    systemProperties = System.properties
}

For testing the following will need to change the test task as follows

test {
    systemProperties = System.properties
}

4.1.2 Built in options

Grails has a set of core settings that are worth knowing about. Their defaults are suitable for most projects, but it’s important to understand what they do because you may need one or more of them later.

Runtime settings

On the runtime front, i.e. grails-app/conf/application.yml, there are quite a few more core settings:

  • grails.enable.native2ascii - Set this to false if you do not require native2ascii conversion of Grails i18n properties files (default: true).

  • grails.views.default.codec - Sets the default encoding regime for GSPs - can be one of 'none', 'html', or 'base64' (default: 'none'). To reduce risk of XSS attacks, set this to 'html'.

  • grails.views.gsp.encoding - The file encoding used for GSP source files (default: 'utf-8').

  • grails.mime.file.extensions - Whether to use the file extension to dictate the mime type in Content Negotiation (default: true).

  • grails.mime.types - A map of supported mime types used for Content Negotiation.

  • grails.serverURL - A string specifying the server URL portion of absolute links, including server name e.g. grails.serverURL="http://my.yourportal.com". See createLink. Also used by redirects.

  • grails.views.gsp.sitemesh.preprocess - Determines whether SiteMesh preprocessing happens. Disabling this slows down page rendering, but if you need SiteMesh to parse the generated HTML from a GSP view then disabling it is the right option. Don’t worry if you don’t understand this advanced property: leave it set to true.

  • grails.reload.excludes and grails.reload.includes - Configuring these directives determines the reload behavior for project specific source files. Each directive takes a list of strings that are the class names for project source files that should be excluded from reloading behavior or included accordingly when running the application in development with the run-app command. If the grails.reload.includes directive is configured, then only the classes in that list will be reloaded.

4.1.3 Logging

By default logging in Grails 3.0 is handled by the Logback logging framework and can be configured with the grails-app/conf/logback.groovy file.

If you prefer XML you can replace the logback.groovy file with a logback.xml file instead.

For more information on configuring logging refer to the Logback documentation on the subject.

4.1.4 GORM

Grails provides the following GORM configuration options:

  • grails.gorm.failOnError - If set to true, causes the save() method on domain classes to throw a grails.validation.ValidationException if validation fails during a save. This option may also be assigned a list of Strings representing package names. If the value is a list of Strings then the failOnError behavior will only be applied to domain classes in those packages (including sub-packages). See the save method docs for more information.

For example, to enable failOnError for all domain classes:

grails:
    gorm:
        failOnError: true

and to enable failOnError for domain classes by package:

grails:
    gorm:
        failOnError:
            - com.companyname.somepackage
            - com.companyname.someotherpackage
  • grails.gorm.autoFlush - If set to true, causes the merge, save and delete methods to flush the session, replacing the need to explicitly flush using save(flush: true).

4.1.5 Configuring an HTTP proxy

To setup Grails to use an HTTP proxy there are two steps. Firstly you need to configure the grails CLI to be aware of the proxy if you wish to use it to create applications and so on. This can be done using the GRAILS_OPTS environment variable, for example on Unix systems:

export GRAILS_OPTS="-Dhttps.proxyHost=127.0.0.1 -Dhttps.proxyPort=3128 -Dhttp.proxyUser=test -Dhttp.proxyPassword=test"
The default profile repository is resolved over HTTPS so https.proxyPort and https.proxyUser are used, however the username and password are specified with http.proxyUser and http.proxyPassword

For Windows systems the environment variable can be configured under My Computer/Advanced/Environment Variables.

With this configuration in place the grails command can connect and authenticate via a proxy.

Secondly, since Grails uses Gradle as the build system, you need to configure Gradle to authenticate via the proxy. For instructions on how to do this see the Gradle user guide section on the topic.

4.2 The Application Class

Every new Grails application features an Application class within the grails-app/init directory.

The Application class subclasses the GrailsAutoConfiguration class and features a static void main method, meaning it can be run as a regular application.

4.2.1 Executing the Application Class

There are several ways to execute the Application class, if you are using an IDE then you can simply right click on the class and run it directly from your IDE which will start your Grails application.

This is also useful for debugging since you can debug directly from the IDE without having to connect a remote debugger when using the run-app --debug-jvm command from the command line.

You can also package your application into a runnable WAR file, for example:

$ grails package
$ java -jar build/libs/myapp-0.1.war

This is useful if you plan to deploy your application using a container-less approach.

4.2.2 Customizing the Application Class

There are several ways in which you can customize the Application class.

Customizing Scanning

By default Grails will scan all known source directories for controllers, domain class etc., however if there are packages in other JAR files you wish to scan you can do so by overriding the packageNames() method of the Application class:

class Application extends GrailsAutoConfiguration {
    @Override
    Collection<String> packageNames() {
        super.packageNames() + ['my.additional.package']
    }

    ...
}

Registering Additional Beans

The Application class can also be used as a source for Spring bean definitions, simply define a method annotated with the Bean and the returned object will become a Spring bean. The name of the method is used as the bean name:

class Application extends GrailsAutoConfiguration {
    @Bean
    MyType myBean() {
        return new MyType()
    }

    ...
}

4.2.3 The Application LifeCycle

The Application class also implements the GrailsApplicationLifeCycle interface which all plugins implement.

This means that the Application class can be used to perform the same functions as a plugin. You can override the regular plugins hooks such as doWithSpring, doWithApplicationContext and so on by overriding the appropriate method:

class Application extends GrailsAutoConfiguration {
    @Override
    Closure doWithSpring() {
        {->
            mySpringBean(MyType)
        }
    }

    ...
}

4.3 Environments

Per Environment Configuration

Grails supports the concept of per environment configuration. The application.yml and application.groovy files in the grails-app/conf directory can use per-environment configuration using either YAML or the syntax provided by ConfigSlurper. As an example consider the following default application.yml definition provided by Grails:

environments:
    development:
        dataSource:
            dbCreate: create-drop
            url: jdbc:h2:mem:devDb;MVCC=TRUE;LOCK_TIMEOUT=10000;DB_CLOSE_ON_EXIT=FALSE
    test:
        dataSource:
            dbCreate: update
            url: jdbc:h2:mem:testDb;MVCC=TRUE;LOCK_TIMEOUT=10000;DB_CLOSE_ON_EXIT=FALSE
    production:
        dataSource:
            dbCreate: update
            url: jdbc:h2:prodDb;MVCC=TRUE;LOCK_TIMEOUT=10000;DB_CLOSE_ON_EXIT=FALSE
        properties:
           jmxEnabled: true
           initialSize: 5
        ...

The above can be expressed in Groovy syntax in application.groovy as follows:

dataSource {
    pooled = false
    driverClassName = "org.h2.Driver"
    username = "sa"
    password = ""
}
environments {
    development {
        dataSource {
            dbCreate = "create-drop"
            url = "jdbc:h2:mem:devDb"
        }
    }
    test {
        dataSource {
            dbCreate = "update"
            url = "jdbc:h2:mem:testDb"
        }
    }
    production {
        dataSource {
            dbCreate = "update"
            url = "jdbc:h2:prodDb"
        }
    }
}

Notice how the common configuration is provided at the top level and then an environments block specifies per environment settings for the dbCreate and url properties of the DataSource.

Packaging and Running for Different Environments

Grails' command line has built in capabilities to execute any command within the context of a specific environment. The format is:

grails <<environment>> <<command name>>

In addition, there are 3 preset environments known to Grails: dev, prod, and test for development, production and test. For example to create a WAR for the test environment you wound run:

grails test war

To target other environments you can pass a grails.env variable to any command:

grails -Dgrails.env=UAT run-app

Programmatic Environment Detection

Within your code, such as in a Gant script or a bootstrap class you can detect the environment using the Environment class:

import grails.util.Environment

...

switch (Environment.current) {
    case Environment.DEVELOPMENT:
        configureForDevelopment()
        break
    case Environment.PRODUCTION:
        configureForProduction()
        break
}

Per Environment Bootstrapping

It’s often desirable to run code when your application starts up on a per-environment basis. To do so you can use the grails-app/init/BootStrap.groovy file’s support for per-environment execution:

def init = { ServletContext ctx ->
    environments {
        production {
            ctx.setAttribute("env", "prod")
        }
        development {
            ctx.setAttribute("env", "dev")
        }
    }
    ctx.setAttribute("foo", "bar")
}

Generic Per Environment Execution

The previous BootStrap example uses the grails.util.Environment class internally to execute. You can also use this class yourself to execute your own environment specific logic:

Environment.executeForCurrentEnvironment {
    production {
        // do something in production
    }
    development {
        // do something only in development
    }
}

4.4 The DataSource

Since Grails is built on Java technology setting up a data source requires some knowledge of JDBC (the technology that stands for Java Database Connectivity).

If you use a database other than H2 you need a JDBC driver. For example for MySQL you would need Connector/J.

Drivers typically come in the form of a JAR archive. It’s best to use the dependency resolution to resolve the jar if it’s available in a Maven repository, for example you could add a dependency for the MySQL driver like this:

dependencies {
    runtime 'mysql:mysql-connector-java:5.1.29'
}

Once you have the JAR resolved you need to get familiar with how Grails manages its database configuration. The configuration can be maintained in either grails-app/conf/application.groovy or grails-app/conf/application.yml. These files contain the dataSource definition which includes the following settings:

  • driverClassName - The class name of the JDBC driver

  • username - The username used to establish a JDBC connection

  • password - The password used to establish a JDBC connection

  • url - The JDBC URL of the database

  • dbCreate - Whether to auto-generate the database from the domain model - one of 'create-drop', 'create', 'update' or 'validate'

  • pooled - Whether to use a pool of connections (defaults to true)

  • logSql - Enable SQL logging to stdout

  • formatSql - Format logged SQL

  • dialect - A String or Class that represents the Hibernate dialect used to communicate with the database. See the org.hibernate.dialect package for available dialects.

  • readOnly - If true makes the DataSource read-only, which results in the connection pool calling setReadOnly(true) on each Connection

  • transactional - If false leaves the DataSource’s transactionManager bean outside the chained BE1PC transaction manager implementation. This only applies to additional datasources.

  • persistenceInterceptor - The default datasource is automatically wired up to the persistence interceptor, other datasources are not wired up automatically unless this is set to true

  • properties - Extra properties to set on the DataSource bean. See the Tomcat Pool documentation. There is also a Javadoc format documentation of the properties.

  • jmxExport - If false, will disable registration of JMX MBeans for all DataSources. By default JMX MBeans are added for DataSources with jmxEnabled = true in properties.

A typical configuration for MySQL in application.groovy may be something like:

dataSource {
    pooled = true
    dbCreate = "update"
    url = "jdbc:mysql://localhost:3306/my_database"
    driverClassName = "com.mysql.jdbc.Driver"
    dialect = org.hibernate.dialect.MySQL5InnoDBDialect
    username = "username"
    password = "password"
    properties {
       jmxEnabled = true
       initialSize = 5
       maxActive = 50
       minIdle = 5
       maxIdle = 25
       maxWait = 10000
       maxAge = 10 * 60000
       timeBetweenEvictionRunsMillis = 5000
       minEvictableIdleTimeMillis = 60000
       validationQuery = "SELECT 1"
       validationQueryTimeout = 3
       validationInterval = 15000
       testOnBorrow = true
       testWhileIdle = true
       testOnReturn = false
       jdbcInterceptors = "ConnectionState;StatementCache(max=200)"
       defaultTransactionIsolation = java.sql.Connection.TRANSACTION_READ_COMMITTED
    }
}
When configuring the DataSource do not include the type or the def keyword before any of the configuration settings as Groovy will treat these as local variable definitions and they will not be processed. For example the following is invalid:
dataSource {
    boolean pooled = true // type declaration results in ignored local variable
    ...
}

Example of advanced configuration using extra properties:

dataSource {
    pooled = true
    dbCreate = "update"
    url = "jdbc:mysql://localhost:3306/my_database"
    driverClassName = "com.mysql.jdbc.Driver"
    dialect = org.hibernate.dialect.MySQL5InnoDBDialect
    username = "username"
    password = "password"
    properties {
       // Documentation for Tomcat JDBC Pool
       // http://tomcat.apache.org/tomcat-7.0-doc/jdbc-pool.html#Common_Attributes
       // https://tomcat.apache.org/tomcat-7.0-doc/api/org/apache/tomcat/jdbc/pool/PoolConfiguration.html
       jmxEnabled = true
       initialSize = 5
       maxActive = 50
       minIdle = 5
       maxIdle = 25
       maxWait = 10000
       maxAge = 10 * 60000
       timeBetweenEvictionRunsMillis = 5000
       minEvictableIdleTimeMillis = 60000
       validationQuery = "SELECT 1"
       validationQueryTimeout = 3
       validationInterval = 15000
       testOnBorrow = true
       testWhileIdle = true
       testOnReturn = false
       ignoreExceptionOnPreLoad = true
       // http://tomcat.apache.org/tomcat-7.0-doc/jdbc-pool.html#JDBC_interceptors
       jdbcInterceptors = "ConnectionState;StatementCache(max=200)"
       defaultTransactionIsolation = java.sql.Connection.TRANSACTION_READ_COMMITTED // safe default
       // controls for leaked connections
       abandonWhenPercentageFull = 100 // settings are active only when pool is full
       removeAbandonedTimeout = 120
       removeAbandoned = true
       // use JMX console to change this setting at runtime
       logAbandoned = false // causes stacktrace recording overhead, use only for debugging
       // JDBC driver properties
       // Mysql as example
       dbProperties {
           // Mysql specific driver properties
           // http://dev.mysql.com/doc/connector-j/en/connector-j-reference-configuration-properties.html
           // let Tomcat JDBC Pool handle reconnecting
           autoReconnect=false
           // truncation behaviour
           jdbcCompliantTruncation=false
           // mysql 0-date conversion
           zeroDateTimeBehavior='convertToNull'
           // Tomcat JDBC Pool's StatementCache is used instead, so disable mysql driver's cache
           cachePrepStmts=false
           cacheCallableStmts=false
           // Tomcat JDBC Pool's StatementFinalizer keeps track
           dontTrackOpenResources=true
           // performance optimization: reduce number of SQLExceptions thrown in mysql driver code
           holdResultsOpenOverStatementClose=true
           // enable MySQL query cache - using server prep stmts will disable query caching
           useServerPrepStmts=false
           // metadata caching
           cacheServerConfiguration=true
           cacheResultSetMetadata=true
           metadataCacheSize=100
           // timeouts for TCP/IP
           connectTimeout=15000
           socketTimeout=120000
           // timer tuning (disable)
           maintainTimeStats=false
           enableQueryTimeouts=false
           // misc tuning
           noDatetimeStringSync=true
       }
    }
}

More on dbCreate

Hibernate can automatically create the database tables required for your domain model. You have some control over when and how it does this through the dbCreate property, which can take these values:

  • create - Drops the existing schema and creates the schema on startup, dropping existing tables, indexes, etc. first.

  • create-drop - Same as create, but also drops the tables when the application shuts down cleanly.

  • update - Creates missing tables and indexes, and updates the current schema without dropping any tables or data. Note that this can’t properly handle many schema changes like column renames (you’re left with the old column containing the existing data).

  • validate - Makes no changes to your database. Compares the configuration with the existing database schema and reports warnings.

  • any other value - does nothing

Setting the dbCreate setting to "none" is recommended once your schema is relatively stable and definitely when your application and database are deployed in production. Database changes are then managed through proper migrations, either with SQL scripts or a migration tool like Flyway or Liquibase. The Database Migration plugin uses Liquibase.

4.4.1 DataSources and Environments

The previous example configuration assumes you want the same config for all environments: production, test, development etc.

Grails' DataSource definition is "environment aware", however, so you can do:

dataSource {
    pooled = true
    driverClassName = "com.mysql.jdbc.Driver"
    dialect = org.hibernate.dialect.MySQL5InnoDBDialect
    // other common settings here
}

environments {
    production {
        dataSource {
            url = "jdbc:mysql://liveip.com/liveDb"
            // other environment-specific settings here
        }
    }
}

4.4.2 Automatic Database Migration

The dbCreate property of the DataSource definition is important as it dictates what Grails should do at runtime with regards to automatically generating the database tables from GORM classes. The options are described in the DataSource section:

  • create

  • create-drop

  • update

  • validate

  • no value

In development mode dbCreate is by default set to "create-drop", but at some point in development (and certainly once you go to production) you’ll need to stop dropping and re-creating the database every time you start up your server.

It’s tempting to switch to update so you retain existing data and only update the schema when your code changes, but Hibernate’s update support is very conservative. It won’t make any changes that could result in data loss, and doesn’t detect renamed columns or tables, so you’ll be left with the old one and will also have the new one.

Grails supports migrations with Flyway or Liquibase via plugins.

4.4.3 Transaction-aware DataSource Proxy

The actual dataSource bean is wrapped in a transaction-aware proxy so you will be given the connection that’s being used by the current transaction or Hibernate Session if one is active.

If this were not the case, then retrieving a connection from the dataSource would be a new connection, and you wouldn’t be able to see changes that haven’t been committed yet (assuming you have a sensible transaction isolation setting, e.g. READ_COMMITTED or better).

The "real" unproxied dataSource is still available to you if you need access to it; its bean name is dataSourceUnproxied.

You can access this bean like any other Spring bean, i.e. using dependency injection:

class MyService {

   def dataSourceUnproxied
   ...
}

or by pulling it from the ApplicationContext:

def dataSourceUnproxied = ctx.dataSourceUnproxied

4.4.4 Database Console

The H2 database console is a convenient feature of H2 that provides a web-based interface to any database that you have a JDBC driver for, and it’s very useful to view the database you’re developing against. It’s especially useful when running against an in-memory database.

You can access the console by navigating to http://localhost:8080/dbconsole in a browser. The URI can be configured using the grails.dbconsole.urlRoot attribute in application.groovy and defaults to '/dbconsole'.

The console is enabled by default in development mode and can be disabled or enabled in other environments by using the grails.dbconsole.enabled attribute in application.groovy. For example, you could enable the console in production like this:

environments {
    production {
        grails.serverURL = "http://www.changeme.com"
        grails.dbconsole.enabled = true
        grails.dbconsole.urlRoot = '/admin/dbconsole'
    }
    development {
        grails.serverURL = "http://localhost:8080/${appName}"
    }
    test {
        grails.serverURL = "http://localhost:8080/${appName}"
    }
}
If you enable the console in production be sure to guard access to it using a trusted security framework.

Configuration

By default the console is configured for an H2 database which will work with the default settings if you haven’t configured an external database - you just need to change the JDBC URL to jdbc:h2:mem:devDb. If you’ve configured an external database (e.g. MySQL, Oracle, etc.) then you can use the Saved Settings dropdown to choose a settings template and fill in the url and username/password information from your application.groovy.

4.4.5 Multiple Datasources

By default all domain classes share a single DataSource and a single database, but you have the option to partition your domain classes into two or more data sources.

Configuring Additional DataSources

The default DataSource configuration in grails-app/conf/application.yml looks something like this:

dataSource:
    pooled: true
    jmxExport: true
    driverClassName: org.h2.Driver
    username: sa
    password:

environments:
    development:
        dataSource:
            dbCreate: create-drop
            url: jdbc:h2:mem:devDb;MVCC=TRUE;LOCK_TIMEOUT=10000;DB_CLOSE_ON_EXIT=FALSE
    test:
        dataSource:
            dbCreate: update
            url: jdbc:h2:mem:testDb;MVCC=TRUE;LOCK_TIMEOUT=10000;DB_CLOSE_ON_EXIT=FALSE
    production:
        dataSource:
            dbCreate: update
            url: jdbc:h2:prodDb;MVCC=TRUE;LOCK_TIMEOUT=10000;DB_CLOSE_ON_EXIT=FALSE
            properties:
               jmxEnabled: true
               initialSize: 5

This configures a single DataSource with the Spring bean named dataSource. To configure extra data sources, add a dataSources block (at the top level, in an environment block, or both, just like the standard DataSource definition) with a custom name. For example, this configuration adds a second DataSource, using MySQL in the development environment and Oracle in production:

dataSources:
    dataSource:
        pooled: true
        jmxExport: true
        driverClassName: org.h2.Driver
        username: sa
        password:
    lookup:
        dialect: org.hibernate.dialect.MySQLInnoDBDialect
        driverClassName: com.mysql.jdbc.Driver
        username: lookup
        password: secret
        url: jdbc:mysql://localhost/lookup
        dbCreate: update

environments:
    development:
        dataSources:
            dataSource:
                dbCreate: create-drop
                url: jdbc:h2:mem:devDb;MVCC=TRUE;LOCK_TIMEOUT=10000;DB_CLOSE_ON_EXIT=FALSE
    test:
        dataSources:
            dataSource:
                dbCreate: update
                url: jdbc:h2:mem:testDb;MVCC=TRUE;LOCK_TIMEOUT=10000;DB_CLOSE_ON_EXIT=FALSE
    production:
        dataSources:
            dataSource:
                dbCreate: update
                url: jdbc:h2:prodDb;MVCC=TRUE;LOCK_TIMEOUT=10000;DB_CLOSE_ON_EXIT=FALSE
                properties:
                   jmxEnabled: true
                   initialSize: 5
                   ...
            lookup:
                dialect: org.hibernate.dialect.Oracle10gDialect
                driverClassName: oracle.jdbc.driver.OracleDriver
                username: lookup
                password: secret
                url: jdbc:oracle:thin:@localhost:1521:lookup
                dbCreate: update

You can use the same or different databases as long as they’re supported by Hibernate.

If you need to inject the lookup datasource in a Grails artefact, you can do it like this:

DataSource dataSource_lookup
While defining multiple data sources, one of them must be named "dataSource". Grails uses that reserved name to determine which data source is the default.

Configuring Domain Classes

If a domain class has no DataSource configuration, it defaults to the standard 'dataSource'. Set the datasource property in the mapping block to configure a non-default DataSource. For example, if you want to use the ZipCode domain to use the 'lookup' DataSource, configure it like this:

class ZipCode {

   String code

   static mapping = {
      datasource 'lookup'
   }
}

A domain class can also use two or more data sources. Use the datasources property with a list of names to configure more than one, for example:

class ZipCode {

   String code

   static mapping = {
      datasources(['lookup', 'auditing'])
   }
}

If a domain class uses the default DataSource and one or more others, use the special name 'DEFAULT' to indicate the default DataSource:

class ZipCode {

   String code

   static mapping = {
      datasources(['lookup', 'DEFAULT'])
   }
}

If a domain class uses all configured data sources, use the special value 'ALL':

class ZipCode {

   String code

   static mapping = {
      datasource 'ALL'
   }
}

Namespaces and GORM Methods

If a domain class uses more than one DataSource then you can use the namespace implied by each DataSource name to make GORM calls for a particular DataSource. For example, consider this class which uses two data sources:

class ZipCode {

   String code

   static mapping = {
      datasources(['lookup', 'auditing'])
   }
}

The first DataSource specified is the default when not using an explicit namespace, so in this case we default to 'lookup'. But you can call GORM methods on the 'auditing' DataSource with the DataSource name, for example:

def zipCode = ZipCode.auditing.get(42)
...
zipCode.auditing.save()

As you can see, you add the DataSource to the method call in both the static case and the instance case.

Hibernate Mapped Domain Classes

You can also partition annotated Java classes into separate datasources. Classes using the default datasource are registered in grails-app/conf/hibernate.cfg.xml. To specify that an annotated class uses a non-default datasource, create a hibernate.cfg.xml file for that datasource with the file name prefixed with the datasource name.

For example if the Book class is in the default datasource, you would register that in grails-app/conf/hibernate.cfg.xml:

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
          '-//Hibernate/Hibernate Configuration DTD 3.0//EN'
          'http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd'>
<hibernate-configuration>
   <session-factory>
      <mapping class='org.example.Book'/>
   </session-factory>
</hibernate-configuration>

and if the Library class is in the "ds2" datasource, you would register that in grails-app/conf/ds2_hibernate.cfg.xml:

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
          '-//Hibernate/Hibernate Configuration DTD 3.0//EN'
          'http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd'>
<hibernate-configuration>
   <session-factory>
      <mapping class='org.example.Library'/>
   </session-factory>
</hibernate-configuration>

The process is the same for classes mapped with hbm.xml files - just list them in the appropriate hibernate.cfg.xml file.

Services

Like Domain classes, by default Services use the default DataSource and PlatformTransactionManager. To configure a Service to use a different DataSource, use the static datasource property, for example:

class DataService {

   static datasource = 'lookup'

   void someMethod(...) {
      ...
   }
}

A transactional service can only use a single DataSource, so be sure to only make changes for domain classes whose DataSource is the same as the Service.

Note that the datasource specified in a service has no bearing on which datasources are used for domain classes; that’s determined by their declared datasources in the domain classes themselves. It’s used to declare which transaction manager to use.

If you have a Foo domain class in dataSource1 and a Bar domain class in dataSource2, if WahooService uses dataSource1, a service method that saves a new Foo and a new Bar will only be transactional for Foo since they share the same datasource. The transaction won’t affect the Bar instance. If you want both to be transactional you’d need to use two services and XA datasources for two-phase commit, e.g. with the Atomikos plugin.

Transactions across multiple data sources

Grails uses the Best Efforts 1PC pattern for handling transactions across multiple datasources.

The Best Efforts 1PC pattern is fairly general but can fail in some circumstances that the developer must be aware of. This is a non-XA pattern that involves a synchronized single-phase commit of a number of resources. Because the 2PC is not used, it can never be as safe as an XA transaction, but is often good enough if the participants are aware of the compromises.

The basic idea is to delay the commit of all resources as late as possible in a transaction so that the only thing that can go wrong is an infrastructure failure (not a business-processing error). Systems that rely on Best Efforts 1PC reason that infrastructure failures are rare enough that they can afford to take the risk in return for higher throughput. If business-processing services are also designed to be idempotent, then little can go wrong in practice.

The BE1PC implementation was added in Grails 2.3.6. . Before this change additional datasources didn’t take part in transactions initiated in Grails. The transactions in additional datasources were basically in auto commit mode. In some cases this might be the wanted behavior. One reason might be performance: on the start of each new transaction, the BE1PC transaction manager creates a new transaction to each datasource. It’s possible to leave an additional datasource out of the BE1PC transaction manager by setting transactional = false in the respective configuration block of the additional dataSource. Datasources with readOnly = true will also be left out of the chained transaction manager (since 2.3.7).

By default, the BE1PC implementation will add all beans implementing the Spring PlatformTransactionManager interface to the chained BE1PC transaction manager. For example, a possible JMSTransactionManager bean in the Grails application context would be added to the Grails BE1PC transaction manager’s chain of transaction managers.

You can exclude transaction manager beans from the BE1PC implementation with the this configuration option:

grails.transaction.chainedTransactionManagerPostProcessor.blacklistPattern = '.*'

The exclude matching is done on the name of the transaction manager bean. The transaction managers of datasources with transactional = false or readOnly = true will be skipped and using this configuration option is not required in that case.

XA and Two-phase Commit

When the Best Efforts 1PC pattern isn’t suitable for handling transactions across multiple transactional resources (not only datasources), there are several options available for adding XA/2PC support to Grails applications.

The Spring transactions documentation contains information about integrating the JTA/XA transaction manager of different application servers. In this case, you can configure a bean with the name transactionManager manually in resources.groovy or resources.xml file.

4.5 Versioning

Detecting Versions at Runtime

You can detect the application version using Grails' support for application metadata using the GrailsApplication class. For example within controllers there is an implicit grailsApplication variable that can be used:

def version = grailsApplication.metadata.getApplicationVersion()

You can retrieve the version of Grails that is running with:

def grailsVersion = grailsApplication.metadata.getGrailsVersion()

or the GrailsUtil class:

import grails.util.GrailsUtil
...
def grailsVersion = GrailsUtil.grailsVersion

4.6 Dependency Resolution

Dependency resolution is handled by the Gradle build tool, all dependencies are defined in the build.gradle file. Refer to the Gradle user guide for more information.

5 The Command Line

Grails 3.0’s command line system differs greatly from previous versions of Grails and features APIs for invoking Gradle for build related tasks, as well as performing code generation.

When you type:

grails <<command name>>

Grails searches the profile repository based on the profile of the current application. If the profile is for a web application then commands are read from the web profile and the base profile which it inherits from.

Since command behavior is profile specific the web profile may provide different behavior for the run-app command then say a profile for running batch applications.

When you type the following command:

grails run-app

It results in a search for the following files:

  • PROJECT_HOME/scripts/RunApp.groovy

  • PROFILE_REPOSITORY_PATH/profiles/web/commands/run-app.groovy (if the web profile is active)

  • PROFILE_REPOSITORY_PATH/profiles/web/commands/run-app.yml (for YAML defined commands)

To get a list of all commands and some help about the available commands type:

grails help

which outputs usage instructions and the list of commands Grails is aware of:

grails <<environment>>* <<target>> <<arguments>>*'

| Examples:
$ grails dev run-app
$ grails create-app books

| Available Commands (type grails help 'command-name' for more info):
| Command Name                          Command Description
----------------------------------------------------------------------------------------------------
clean                                   Cleans a Grails application's compiled sources
compile                                 Compiles a Grails application
...
Refer to the Command Line reference in the Quick Reference menu of the reference guide for more information about individual commands

non-interactive mode

When you run a script manually and it prompts you for information, you can answer the questions and continue running the script. But when you run a script as part of an automated process, for example a continuous integration build server, there’s no way to "answer" the questions. So you can pass the \--non-interactive switch to the script command to tell Grails to accept the default answer for any questions, for example whether to install a missing plugin.

For example:

grails war --non-interactive

5.1 Interactive Mode

Interactive mode is a feature of the Grails command line which keeps the JVM running and allows for quicker execution of commands. To activate interactive mode type 'grails' at the command line and then use TAB completion to get a list of commands:

interactive output

If you need to open a file whilst within interactive mode you can use the open command which will TAB complete file paths:

interactive open cmd

Even better, the open command understands the logical aliases 'test-report' and 'dep-report', which will open the most recent test and dependency reports respectively. In other words, to open the test report in a browser simply execute open test-report. You can even open multiple files at once: open test-report test/unit/MyTests.groovy will open the HTML test report in your browser and the MyTests.groovy source file in your text editor.

TAB completion also works for class names after the create-* commands:

interactive complete class

If you need to run an external process whilst interactive mode is running you can do so by starting the command with a !:

interactive run external

Note that with ! (bang) commands, you get file path auto completion - ideal for external commands that operate on the file system such as 'ls', 'cat', 'git', etc.

To exit interactive mode enter the exit command. Note that if the Grails application has been run with run-app normally it will terminate when the interactive mode console exits because the JVM will be terminated. An exception to this would be if the application were running in forked mode which means the application is running in a different JVM. In that case the application will be left running after the interactive mode console terminates. If you want to exit interactive mode and stop an application that is running in forked mode, use the quit command. The quit command will stop the running application and then close interactive mode.

5.2 Creating Custom Scripts

You can create your own Command scripts by running the create-script command from the root of your project. For example the following command will create a script called src/main/scripts/hello-world.groovy:

grails create-script hello-world
In general Grails scripts should be used for scripting the Gradle based build system and code generation. Scripts cannot load application classes and in fact should not since Gradle is required to construct the application classpath.

See below for an example script that prints "Hello World":

description "Example description", "grails hello-world"

println "Hello World"

The description method is used to define the output seen by grails help and to aid users of the script. The following is a more complete example of providing a description taken from the generate-all command:

description( "Generates a controller that performs CRUD operations and the associated views" ) {
  usage "grails generate-all <<DOMAIN CLASS>>"
  flag name:'force', description:"Whether to overwrite existing files"
  argument name:'Domain Class', description:'The name of the domain class'
}

As you can see this description profiles usage instructions, a flag and an argument. This allows the command to be used as follows:

grails generate-all MyClass --force

Template Generation

Plugins and applications that need to define template generation tasks can do so using scripts. A example of this is the Scaffolding plugin which defines the generate-all and generate-controllers commands.

Every Grails script implements the TemplateRenderer interface which makes it trivial to render templates to the users project workspace.

The following is an example of the create-script command written in Groovy:

description( "Creates a Grails script" ) {
  usage "grails create-script <<SCRIPT NAME>>"
  argument name:'Script Name', description:"The name of the script to create"
  flag name:'force', description:"Whether to overwrite existing files"
}

def scriptName = args[0]
def model = model(scriptName)
def overwrite = flag('force') ? true : false

render  template: template('artifacts/Script.groovy'),
        destination: file("src/main/scripts/${model.lowerCaseName}.groovy"),
        model: model,
        overwrite: overwrite

If a script is defined in a plugin or profile, the template(String) method will search for the template in the application before using the template provided by your plugin or profile. This allows users of your plugin or profile to customize what gets generated.

It is common to provide an easy way to allow users to copy the templates from your plugin or profile. Here is one example on how the angular scaffolding copies templates.

templates("angular/**/*").each { Resource r ->
    String path = r.URL.toString().replaceAll(/^.*?META-INF/, "src/main")
    if (path.endsWith('/')) {
        mkdir(path)
    } else {
        File to = new File(path)
        SpringIOUtils.copy(r, to)
        println("Copied ${r.filename} to location ${to.canonicalPath}")
    }
}

The "model"

Executing the model method with a Class/String/File/Resource will return an instance of Model. The model contains several properties that can help you generate code.

Example:

def domain = model(com.foo.Bar)

domain.className == "FooBar"
domain.fullName == "com.foo.FooBar"
domain.packageName == "com.foo"
domain.packagePath == "com/foo"
domain.propertyName == "fooBar"
domain.lowerCaseName == "foo-bar"

In addition, an asMap method is available to turn all of the properties into a map to pass to the render method.

Working with files

All scripts have access to methods on the FileSystemInteraction class. It contains helpful methods to copy, delete, and create files.

5.3 Creating Custom Commands

You can create your own commands by running the create-command command from the root of your project. For example the following command will create a command called grails-app/commands/HelloWorldCommand:

grails create-command HelloWorld
Unlike scripts, commands cause the Grails environment to start and you have full access to the application context and the runtime.

Since Grails 3.2.0, commands have similar abilities as scripts in regards to retrieving arguments, template generation, file access, and model building.

If you created a command in a previous version of grails, you can update your command to have those abilities by changing which trait you are implementing.

Commands created in Grails 3.1.x or lower implement the ApplicationCommand trait by default which requires your command to implement the following method:

boolean handle(ExecutionContext executionContext)

Commands created in Grails 3.2.0 or higher implement the GrailsApplicationCommand trait by default which requires your command to implement the following method:

boolean handle()
Commands defined this way still have access to the execution context via a variable called "executionContext".

5.4 Re-using Grails scripts

Grails ships with a lot of command line functionality out of the box that you may find useful in your own scripts (See the command line reference in the reference guide for info on all the commands).

Any script you create can invoke another Grails script simply by invoking a method:

testApp()

The above will invoke the test-app command. You can also pass arguments using the method arguments:

testApp('--debug-jvm')

Invoking Gradle

Instead of invoking another Grails CLI command you can invoke Gradle directory using the gradle property.

gradle.compileGroovy()

Invoking Ant

You can also invoke Ant tasks from scripts which can help if you need to writing code generation and automation tasks:

ant.mkdir(dir:"path")

5.5 Building with Gradle

Grails 3.1 uses the Gradle Build System for build related tasks such as compilation, runnings tests and producing binary distrubutions of your project. It is recommended to use Gradle 2.2 or above with Grails 3.1.

The build is defined by the build.gradle file which specifies the version of your project, the dependencies of the project and the repositories where to find those dependencies (amongst other things).

When you invoke the grails command the version of Gradle that ships with Grails 3.1 (currently 2.9) is invoked by the grails process via the Gradle Tooling API:

# Equivalent to 'gradle classes'
$ grails compile

You can invoke Gradle directly using the gradle command and use your own local version of Gradle, however you will need Gradle 2.2 or above to work with Grails 3.0:

$ gradle assemble

5.5.1 Defining Dependencies with Gradle

Dependencies for your project are defined in the dependencies block. In general you can follow the Gradle documentation on dependency management to understand how to configure additional dependencies.

The default dependencies for the "web" profile can be seen below:

dependencies {
  compile 'org.springframework.boot:spring-boot-starter-logging'
  compile('org.springframework.boot:spring-boot-starter-actuator')
  compile 'org.springframework.boot:spring-boot-autoconfigure'
  compile 'org.springframework.boot:spring-boot-starter-tomcat'
  compile 'org.grails:grails-dependencies'
  compile 'org.grails:grails-web-boot'

  compile 'org.grails.plugins:hibernate'
  compile 'org.grails.plugins:cache'
  compile 'org.hibernate:hibernate-ehcache'

  runtime 'org.grails.plugins:asset-pipeline'
  runtime 'org.grails.plugins:scaffolding'

  testCompile 'org.grails:grails-plugin-testing'
  testCompile 'org.grails.plugins:geb'

  // Note: It is recommended to update to a more robust driver (Chrome, Firefox etc.)
  testRuntime 'org.seleniumhq.selenium:selenium-htmlunit-driver:2.44.0'

  console 'org.grails:grails-console'
}

Note that version numbers are not present in the majority of the dependencies. This is thanks to the dependency management plugin which configures a Maven BOM that defines the default dependency versions for certain commonly used dependencies and plugins:

dependencyManagement {
    imports {
        mavenBom 'org.grails:grails-bom:' + grailsVersion
    }
    applyMavenExclusions false
}

5.5.2 Working with Gradle Tasks

As mentioned previously the grails command uses an embedded version of Gradle and certain Grails commands that existed in previous versions of Grails map onto their Gradle equivalents. The following table shows which Grails command invoke which Gradle task:

Grails Command Gradle Task

clean

clean

compile

classes

package

assemble

run-app

bootRun

test-app

test

test-app --integration

integrationTest

war

assemble

You can invoke any of these Grails commands using their Gradle equivalents if you prefer:

$ gradle test

Note however that you will need to use a version of Gradle compatible with Grails 3.1 (Gradle 2.2 or above). If you wish to invoke a Gradle task using the version of Gradle used by Grails you can do so with the grails command:

$ grails gradle compileGroovy

However, it is recommended you do this via interactive mode, as it greatly speeds up execution and provides TAB completion for the available Gradle tasks:

$ grails
| Enter a command name to run. Use TAB for completion:
 grails> gradle compileGroovy
 ...

To find out what Gradle tasks are available without using interactive mode TAB completion you can use the Gradle tasks task:

gradle tasks

5.5.3 Grails plugins for Gradle

When you create a new project with the create-app command, a default build.gradle is created. The default build.gradle configures the build with a set of Gradle plugins that allow Gradle to build the Grails project:

apply plugin:"war"
apply plugin:"org.grails.grails-web"
apply plugin:"org.grails.grails-gsp"
apply plugin:"asset-pipeline"

The default plugins are as follows:

  • war - The WAR plugin changes the packaging so that Gradle creates as WAR file from you application. You can comment out this plugin if you wish to create only a runnable JAR file for standalone deployment.

  • asset-pipeline - The asset pipeline] plugin enables the compilation of static assets (JavaScript, CSS etc.)

Many of these are built in plugins provided by Gradle or third party plugins. The Gradle plugins that Grails provides are as follows:

  • org.grails.grails-core - The primary Grails plugin for Gradle, included by all other plugins and designed to operate with all profiles.

  • org.grails.grails-gsp - The Grails GSP plugin adds precompilation of GSP files for production deployments.

  • org.grails.grails-doc - A plugin for Gradle for using Grails 2.0’s documentation engine.

  • org.grails.grails-plugin - A plugin for Gradle for building Grails plugins.

  • org.grails.grails-plugin-publish - A plugin for publishing Grails plugins to the central repository.

  • org.grails.grails-profile - A plugin for use when creating Grails Profiles.

  • org.grails.grails-profile-publish - A plugin for publishing Grails profiles to the central repository.

  • org.grails.grails-web - The Grails Web gradle plugin configures Gradle to understand the Grails conventions and directory structure.

6 Application Profiles

When you create a Grails application with the create-app command by default the "web" profile is used:

grails create-app myapp

You can specify a different profile with the profile argument:

grails create-app myapp --profile=rest-api

Profiles encapsulate the project commands, templates and plugins that are designed to work for a given profile. The source for the profiles can be found on Github, whilst the profiles themselves are published as JAR files to the Grails central repository.

To find out what profiles are available use the list-profiles command:

$ grails list-profiles

For more information on a particular profile use the profile-info command:

$ grails profile-info rest-api

Profile Repositories

By default Grails will resolve profiles from the Grails central repository. However, you can override what repositories will be searched by specifying repositories in the USER_HOME/.grails/settings.groovy file.

If you want profiles to be resolved with a custom repository in addition to the Grails central repository, you must specify Grails central in the file as well:

grails {
  profiles {
    repositories {
      myRepo {
        url = "http://foo.com/repo"
        snapshotsEnabled = true
      }
      grailsCentral {
        url = "https://repo.grails.org/grails/core"
        snapshotsEnabled = true
      }
    }
  }
}
Grails uses Aether to resolve profiles, as a Gradle instance is not yet available when the create-app command is executed. This means that you can also define repositories and more advanced configuration (proxies, authentication etc.) in your USER_HOME/.m2/settings.xml file if you wish.

It is also possible to store simple credentials for profile repositories directly in the USER_HOME/.grails/settings.groovy file.

grails {
  profiles {
    repositories {
      myRepo {
        url = "http://foo.com/repo"
        snapshotsEnabled = true
        username = "user"
        password = "pass"
      }
      ...
    }
  }
}

Profile Defaults

To create an application that uses a custom profile, you must specify the full artifact.

$ grails create-app myapp --profile=com.mycompany.grails.profiles:myprofile:1.0.0

To make this process easier, you can define defaults for a given profile in the USER_HOME/.grails/settings.groovy file.

grails {
  profiles {
    myprofile {
      groupId = "com.mycompany.grails.profiles"
      version = "1.0.0"
    }
    repositories {
      ...
    }
  }
}

With the default values specified, the command to create an application using that profile becomes:

$ grails create-app myapp --profile=myprofile

6.1 Creating Profiles

The idea behind creating a new profile is that you can setup a default set of commands and plugins that are tailored to a particular technology or organisation.

To create a new profile you can use the create-profile command which will create a new empty profile that extends the base profile:

$ grails create-profile mycompany

The above command will create a new profile in the "mycompany" directory where the command is executed. If you start interactive mode within the directory you will get a set of commands for creating profiles:

$ cd mycompany
$ grails
| Enter a command name to run. Use TAB for completion:
grails>

create-command      create-creator-command      create-feature      create-generator-command    create-gradle-command   create-template

The commands are as follows:

  • create-command - creates a new command that will be available from the Grails CLI when the profile is used

  • create-creator-command - creates a command available to the CLI that renders a template (Example: create-controller)

  • create-generator-command - creates a command available to the CLI that renders a template based on a domain class (Example: generate-controller)

  • create-feature - creates a feature that can be used with this profile

  • create-gradle-command - creates a CLI command that can invoke gradle

  • create-template - creates a template that can be rendered by a command

To customize the dependencies for your profile you can specify additional dependencies in profile.yml.

Below is an example profile.yml file:

features:
    defaults:
        - hibernate
        - asset-pipeline
build:
    plugins:
        - org.grails.grails-web
    excludes:
        - org.grails.grails-core
dependencies:
    compile:
        - "org.mycompany:myplugin:1.0.1"

With the above configuration in place you can publish the profile to your local repository with gradle install:

$ gradle install

Your profile is now usable with the create-app command:

$ grails create-app myapp --profile mycompany

With the above command the application will be created with the "mycompany" profile which includes an additional dependency on the "myplugin" plugin and also includes the "hibernate" and "asset-pipeline" features (more on features later).

Note that if you customize the dependency coordinates of the profile (group, version etc.) then you may need to use the fully qualified coordinates to create an application:

$ grails create-app myapp --profile com.mycompany:mycompany:1.0.1

6.2 Profile Inheritance

One profile can extend one or many different parent profiles. To define profile inheritance you can modify the build.gradle of a profile and define the profile dependences. For example typically you want to extend the base profile:

dependencies {
    runtime project(':base')
}

By inheriting from a parent profile you get the following benefits:

  • When the create-app command is executed the parent profile’s skeleton is copied first

  • Dependencies and build.gradle is merged from the parent(s)

  • The application.yml file is merged from the parent(s)

  • CLI commands from the parent profile are inherited

  • Features from the parent profile are inherited

To define the order of inheritance ensure that your dependencies are declared in the correct order. For example:

dependencies {
    runtime project(':plugin')
    runtime project(':web')
}

In the above snippet the skeleton from the "plugin" profile is copied first, followed by the "web" profile. In addition, the "web" profile overrides commands from the "plugin" profile, whilst if the dependency order was reversed the "plugin" profile would override the "web" profile.

6.3 Publishing Profiles

Publishing Profiles to the Grails Central Repository

Any profile created with the create-profile command already comes configured with a grails-profile-publish plugin defined in build.gradle:

apply plugin: "org.grails.grails-profile-publish"

To publish a profile using this plugin to the Grails central repository first upload the source to Github (closed source profiles will not be accepted). Then register for an account on Bintray and configure your keys as follows in the profile’s build.gradle file:

grailsPublish {
  user = 'YOUR USERNAME'
  key = 'YOUR KEY'
  githubSlug = 'your-repo/your-profile'
  license = 'Apache-2.0'
}
The githubSlug argument should point to the path to your Github repository. For example if your repository is located at https://github.com/foo/bar then your githubSlug is foo/bar

With this in place you can run gradle publishProfile to publish your profile:

$ gradle publishProfile

The profile will be uploaded to Bintray. You can then go the Grails profiles repository and request to have your profile included by clicking "Include My Package" button on Bintray’s interface (you must be logged in to see this).

Publishing Profiles to an Internal Repository

The aforementioned grails-profile-publish plugin configures Gradle’s Maven Publish plugin. In order to publish to an internal repository all you need to do is define the repository in build.gradle. For example:

publishing {
    repositories {
        maven {
            credentials {
                username "foo"
                password "bar"
            }

            url "http://foo.com/repo"
        }
    }
}

Once configured you can publish your plugin with gradle publish:

$ gradle publish

6.4 Understanding Profiles

A profile is a simple directory that contains a profile.yml file and directories containing the "commands", "skeleton" and "templates" defined by the profile. Example:

/web
    commands/
        create-controller.yml
        run-app.groovy
        ...
    features/
        asset-pipeline/
            skeleton
            feature.yml
    skeleton/
        grails-app/
            controllers/
            ...
        build.gradle
    templates/
        artifacts/
            Controller.groovy
    profile.yml

The above example is a snippet of structure of the 'web' profile. The profile.yml file is used to describe the profile and control how the build is configured.

Understanding the profile.yml descriptor

The profile.yml can contain the following child elements.

1) repositories

A list of Maven repositories to include in the generated build. Example:

repositories:
    - "https://repo.grails.org/grails/core"

2) build.repositories

A list of Maven repositories to include in the buildscript section of the generated build. Example:

build:
    repositories:
        - "https://repo.grails.org/grails/core"

3) build.plugins

A list of Gradle plugins to configure in the generated build. Example:

build:
    plugins:
        - eclipse
        - idea
        - org.grails.grails-core

4) build.excludes

A list of Gradle plugins to exclude from being inherited from the parent profile:

build:
    excludes:
        - org.grails.grails-core

5) dependencies

A map of scopes and dependencies to configure. The excludes scope can be used to exclude from the parent profile. Example:

dependencies:
    excludes:
        - "org.grails:hibernate"
    build:
        - "org.grails:grails-gradle-plugin:$grailsVersion"
    compile:
        - "org.springframework.boot:spring-boot-starter-logging"
        - "org.springframework.boot:spring-boot-autoconfigure"

6) features.defaults

A default list of features to use if no explicit features are specified.

features:
    defaults:
        - hibernate
        - asset-pipeline

7) skeleton.excludes

A list of files to exclude from parent profile’s skeletons (supports wildcards).

skeleton:
    excludes:
        - gradlew
        - gradlew.bat
        - gradle/

8) skeleton.parent.target

The target folder that parent profile’s skeleton should be copied into. This can be used to create multi-project builds.

skeleton:
    parent:
        target: app

9) instructions

Text to be displayed to the user after the application is created

instructions: Here are some instructions

What happens when a profile is used?

When the create-app command runs it takes the skeleton of the parent profiles and copies the skeletons into a new project structure.

The build.gradle file is generated is result of obtaining all of the dependency information defined in the profile.yml files and produces the required dependencies.

The command will also merge any build.gradle files defined within a profile and its parent profiles.

The grails-app/conf/application.yml file is also merged into a single YAML file taking into account the profile and all of the parent profiles.

6.5 Creating Profile Commands

A profile can define new commands that apply only to that profile using YAML or Groovy scripts. Below is an example of the create-controller command defined in YAML:

description:
    - Creates a controller
    - usage: 'create-controller <<controller name>>'
    - completer: org.grails.cli.interactive.completers.DomainClassCompleter
    - argument: "Controller Name"
      description: "The name of the controller"
steps:
 - command: render
   template: templates/artifacts/Controller.groovy
   destination: grails-app/controllers/`artifact.package.path`/`artifact.name`Controller.groovy
 - command: render
   template: templates/testing/Controller.groovy
   destination: src/test/groovy/`artifact.package.path`/`artifact.name`ControllerSpec.groovy
 - command: mkdir
   location: grails-app/views/`artifact.propertyName`

Commands defined in YAML must define one or many steps. Each step is a command in itself. The available step types are:

  • render - To render a template to a given destination (as seen in the previous example)

  • mkdir - To make a directory specified by the location parameter

  • execute - To execute a command specified by the class parameter. Must be a class that implements the Command interface.

  • gradle - To execute one or many Gradle tasks specified by the tasks parameter.

For example to invoke a Gradle task, you can define the following YAML:

description: Creates a WAR file for deployment to a container (like Tomcat)
minArguments: 0
usage: |
 war
steps:
 - command: gradle
   tasks:
     - war

If you need more flexiblity than what the declarative YAML approach provides you can create Groovy script commands. Each Command script is extends from the GroovyScriptCommmand class and hence has all of the methods of that class available to it.

The following is an example of the create-script command written in Groovy:

description( "Creates a Grails script" ) {
  usage "grails create-script <<SCRIPT NAME>>"
  argument name:'Script Name', description:"The name of the script to create"
  flag name:'force', description:"Whether to overwrite existing files"
}

def scriptName = args[0]
def model = model(scriptName)
def overwrite = flag('force') ? true : false

render  template: template('artifacts/Script.groovy'),
        destination: file("src/main/scripts/${model.lowerCaseName}.groovy"),
        model: model,
        overwrite: overwrite

For more information on creating CLI commands see the section on creating custom scripts in the Command Line section of the user guide.

6.6 Creating Profile Features

A Profile feature is a shareable set of templates and dependencies that may span multiple profiles. Typically you create a base profile that has multiple features and child profiles that inherit from the parent and hence can use the features available from the parent.

To create a feature use the create-feature command from the root directory of your profile:

$ grails create-feature myfeature

This will create a myfeature/feature.yml file that looks like the following:

description: Description of the feature
# customize versions here
# dependencies:
#   compile:
#     - "org.grails.plugins:myplugin2:1.0"
#

As a more concrete example. The following is the feature.yml file from the "asset-pipeline" feature:

description: Adds Asset Pipeline to a Grails project
build:
    plugins:
        - asset-pipeline
dependencies:
    build:
        - 'com.bertramlabs.plugins:asset-pipeline-gradle:2.5.0'
    runtime:
        - "org.grails.plugins:asset-pipeline"

The structure of a feature is as follows:

FEATURE_DIR
    feature.yml
    skeleton/
        grails-app/
            conf/
                application.yml
        build.gradle

The contents of the skeleton get copied into the application tree, whilst the application.yml and build.gradle get merged with their respective counterparts in the profile by used.

With the feature.yml you can define additional dependencies. This allows users to create applications with optional features. For example:

$ grails create-app myapp --profile myprofile --features myfeature,hibernate

The above example will create a new application using your new feature and the "hibernate" feature.

7 Object Relational Mapping (GORM)

Domain classes are core to any business application. They hold state about business processes and hopefully also implement behavior. They are linked together through relationships; one-to-one, one-to-many, or many-to-many.

GORM is Grails' object relational mapping (ORM) implementation. Under the hood it uses Hibernate (a very popular and flexible open source ORM solution) and thanks to the dynamic nature of Groovy with its static and dynamic typing, along with the convention of Grails, there is far less configuration involved in creating Grails domain classes.

You can also write Grails domain classes in Java. See the section on Hibernate Integration for how to write domain classes in Java but still use dynamic persistent methods. Below is a preview of GORM in action:

def book = Book.findByTitle("Groovy in Action")

book
  .addToAuthors(name:"Dierk Koenig")
  .addToAuthors(name:"Guillaume LaForge")
  .save()

7.1 Quick Start Guide

A domain class can be created with the create-domain-class command:

grails create-domain-class helloworld.Person
If no package is specified with the create-domain-class script, Grails automatically uses the application name as the package name.

This will create a class at the location grails-app/domain/helloworld/Person.groovy such as the one below:

package helloworld

class Person {
}
If you have the dbCreate property set to "update", "create" or "create-drop" on your DataSource, Grails will automatically generate/modify the database tables for you.

You can customize the class by adding properties:

class Person {
    String name
    Integer age
    Date lastVisit
}

Once you have a domain class try and manipulate it with the shell or console by typing:

grails console

This loads an interactive GUI where you can run Groovy commands with access to the Spring ApplicationContext, GORM, etc.

7.1.1 Basic CRUD

Try performing some basic CRUD (Create/Read/Update/Delete) operations.

Create

To create a domain class use Map constructor to set its properties and call save:

def p = new Person(name: "Fred", age: 40, lastVisit: new Date())
p.save()

The save method will persist your class to the database using the underlying Hibernate ORM layer.

Read

Grails transparently adds an implicit id property to your domain class which you can use for retrieval:

def p = Person.get(1)
assert 1 == p.id

This uses the get method that expects a database identifier to read the Person object back from the database. You can also load an object in a read-only state by using the read method:

def p = Person.read(1)

In this case the underlying Hibernate engine will not do any dirty checking and the object will not be persisted. Note that if you explicitly call the save method then the object is placed back into a read-write state.

In addition, you can also load a proxy for an instance by using the load method:

def p = Person.load(1)

This incurs no database access until a method other than getId() is called. Hibernate then initializes the proxied instance, or throws an exception if no record is found for the specified id.

Update

To update an instance, change some properties and then call save again:

def p = Person.get(1)
p.name = "Bob"
p.save()

Delete

To delete an instance use the delete method:

def p = Person.get(1)
p.delete()

7.2 Further Reading on GORM

For more information on using GORM see the dedicated documentation for the GORM project.

8 The Web Layer

8.1 Controllers

A controller handles requests and creates or prepares the response. A controller can generate the response directly or delegate to a view. To create a controller, simply create a class whose name ends with Controller in the grails-app/controllers directory (in a subdirectory if it’s in a package).

The default URL Mapping configuration ensures that the first part of your controller name is mapped to a URI and each action defined within your controller maps to URIs within the controller name URI.

8.1.1 Understanding Controllers and Actions

Creating a controller

Controllers can be created with the create-controller or generate-controller command. For example try running the following command from the root of a Grails project:

grails create-controller book

The command will create a controller at the location grails-app/controllers/myapp/BookController.groovy:

package myapp

class BookController {

    def index() { }
}

where "myapp" will be the name of your application, the default package name if one isn’t specified.

BookController by default maps to the /book URI (relative to your application root).

The create-controller and generate-controller commands are just for convenience and you can just as easily create controllers using your favorite text editor or IDE

Creating Actions

A controller can have multiple public action methods; each one maps to a URI:

class BookController {

    def list() {

        // do controller logic
        // create model

        return model
    }
}

This example maps to the /book/list URI by default thanks to the property being named list.

The Default Action

A controller has the concept of a default URI that maps to the root URI of the controller, for example /book for BookController. The action that is called when the default URI is requested is dictated by the following rules:

  • If there is only one action, it’s the default

  • If you have an action named index, it’s the default

  • Alternatively you can set it explicitly with the defaultAction property:

static defaultAction = "list"

8.1.2 Controllers and Scopes

Available Scopes

Scopes are hash-like objects where you can store variables. The following scopes are available to controllers:

  • servletContext - Also known as application scope, this scope lets you share state across the entire web application. The servletContext is an instance of ServletContext

  • session - The session allows associating state with a given user and typically uses cookies to associate a session with a client. The session object is an instance of HttpSession

  • request - The request object allows the storage of objects for the current request only. The request object is an instance of HttpServletRequest

  • params - Mutable map of incoming request query string or POST parameters

  • flash - See below

Accessing Scopes

Scopes can be accessed using the variable names above in combination with Groovy’s array index operator, even on classes provided by the Servlet API such as the HttpServletRequest:

class BookController {
    def find() {
        def findBy = params["findBy"]
        def appContext = request["foo"]
        def loggedUser = session["logged_user"]
    }
}

You can also access values within scopes using the de-reference operator, making the syntax even more clear:

class BookController {
    def find() {
        def findBy = params.findBy
        def appContext = request.foo
        def loggedUser = session.logged_user
    }
}

This is one of the ways that Grails unifies access to the different scopes.

Using Flash Scope

Grails supports the concept of flash scope as a temporary store to make attributes available for this request and the next request only. Afterwards the attributes are cleared. This is useful for setting a message directly before redirecting, for example:

def delete() {
    def b = Book.get(params.id)
    if (!b) {
        flash.message = "User not found for id ${params.id}"
        redirect(action:list)
    }
    ... // remaining code
}

When the list action is requested, the message value will be in scope and can be used to display an information message. It will be removed from the flash scope after this second request.

Note that the attribute name can be anything you want, and the values are often strings used to display messages, but can be any object type.

Scoped Controllers

Newly created applications have the grails.controllers.defaultScope property set to a value of "singleton" in application.yml. You may change this value to any of the supported scopes listed below. If the property is not assigned a value at all, controllers will default to "prototype" scope.

Supported controller scopes are:

  • prototype (default) - A new controller will be created for each request (recommended for actions as Closure properties)

  • session - One controller is created for the scope of a user session

  • singleton - Only one instance of the controller ever exists (recommended for actions as methods)

To enable one of the scopes, add a static scope property to your class with one of the valid scope values listed above, for example

static scope = "singleton"

You can define the default strategy in application.yml with the grails.controllers.defaultScope key, for example:

grails:
    controllers:
        defaultScope: singleton
Use scoped controllers wisely. For instance, we don’t recommend having any properties in a singleton-scoped controller since they will be shared for all requests.

8.1.3 Models and Views

Returning the Model

A model is a Map that the view uses when rendering. The keys within that Map correspond to variable names accessible by the view. There are a couple of ways to return a model. First, you can explicitly return a Map instance:

def show() {
    [book: Book.get(params.id)]
}
The above does not reflect what you should use with the scaffolding views - see the scaffolding section for more details.

A more advanced approach is to return an instance of the Spring ModelAndView class:

import org.springframework.web.servlet.ModelAndView

def index() {
    // get some books just for the index page, perhaps your favorites
    def favoriteBooks = ...

    // forward to the list view to show them
    return new ModelAndView("/book/list", [ bookList : favoriteBooks ])
}

One thing to bear in mind is that certain variable names can not be used in your model:

  • attributes

  • application

Currently, no error will be reported if you do use them, but this will hopefully change in a future version of Grails.

Selecting the View

In both of the previous two examples there was no code that specified which view to render. So how does Grails know which one to pick? The answer lies in the conventions. Grails will look for a view at the location grails-app/views/book/show.gsp for this show action:

class BookController {
    def show() {
         [book: Book.get(params.id)]
    }
}

To render a different view, use the render method:

def show() {
    def map = [book: Book.get(params.id)]
    render(view: "display", model: map)
}

In this case Grails will attempt to render a view at the location grails-app/views/book/display.gsp. Notice that Grails automatically qualifies the view location with the book directory of the grails-app/views directory. This is convenient, but to access shared views you need instead you can use an absolute path instead of a relative one:

def show() {
    def map = [book: Book.get(params.id)]
    render(view: "/shared/display", model: map)
}

In this case Grails will attempt to render a view at the location grails-app/views/shared/display.gsp.

Grails also supports JSPs as views, so if a GSP isn’t found in the expected location but a JSP is, it will be used instead.

Selecting Views For Namespaced Controllers

If a controller defines a namespace for itself with the namespace property that will affect the root directory in which Grails will look for views which are specified with a relative path. The default root directory for views rendered by a namespaced controller is grails-app/views/<namespace name>/<controller name>/. If the view is not found in the namespaced directory then Grails will fallback to looking for the view in the non-namespaced directory.

See the example below.

class ReportingController {
    static namespace = 'business'

    def humanResources() {
        // This will render grails-app/views/business/reporting/humanResources.gsp
        // if it exists.

        // If grails-app/views/business/reporting/humanResources.gsp does not
        // exist the fallback will be grails-app/views/reporting/humanResources.gsp.

        // The namespaced GSP will take precedence over the non-namespaced GSP.

        [numberOfEmployees: 9]
    }


    def accountsReceivable() {
        // This will render grails-app/views/business/reporting/accounting.gsp
        // if it exists.

        // If grails-app/views/business/reporting/accounting.gsp does not
        // exist the fallback will be grails-app/views/reporting/accounting.gsp.

        // The namespaced GSP will take precedence over the non-namespaced GSP.

        render view: 'numberCrunch', model: [numberOfEmployees: 13]
    }
}

Rendering a Response

Sometimes it’s easier (for example with Ajax applications) to render snippets of text or code to the response directly from the controller. For this, the highly flexible render method can be used:

render "Hello World!"

The above code writes the text "Hello World!" to the response. Other examples include:

// write some markup
render {
   for (b in books) {
      div(id: b.id, b.title)
   }
}
// render a specific view
render(view: 'show')
// render a template for each item in a collection
render(template: 'book_template', collection: Book.list())
// render some text with encoding and content type
render(text: "<xml>some xml</xml>", contentType: "text/xml", encoding: "UTF-8")

If you plan on using Groovy’s MarkupBuilder to generate HTML for use with the render method be careful of naming clashes between HTML elements and Grails tags, for example:

import groovy.xml.MarkupBuilder
...
def login() {
    def writer = new StringWriter()
    def builder = new MarkupBuilder(writer)
    builder.html {
        head {
            title 'Log in'
        }
        body {
            h1 'Hello'
            form {
            }
        }
    }

    def html = writer.toString()
    render html
}

This will actually call the form tag (which will return some text that will be ignored by the MarkupBuilder). To correctly output a <form> element, use the following:

def login() {
    // ...
    body {
        h1 'Hello'
        builder.form {
        }
    }
    // ...
}

8.1.4 Redirects and Chaining

Redirects

Actions can be redirected using the redirect controller method:

class OverviewController {

    def login() {}

    def find() {
        if (!session.user)
            redirect(action: 'login')
            return
        }
        ...
    }
}

Internally the redirect method uses the HttpServletResponse object’s sendRedirect method.

The redirect method expects one of:

  • The name of an action (and controller name if the redirect isn’t to an action in the current controller):

// Also redirects to the index action in the home controller
redirect(controller: 'home', action: 'index')
  • A URI for a resource relative the application context path:

// Redirect to an explicit URI
redirect(uri: "/login.html")
  • Or a full URL:

// Redirect to a URL
redirect(url: "http://grails.org")
// Redirect to the domain instance
Book book = ... // obtain a domain instance
redirect book

In the above example Grails will construct a link using the domain class id (if present).

Parameters can optionally be passed from one action to the next using the params argument of the method:

redirect(action: 'myaction', params: [myparam: "myvalue"])

These parameters are made available through the params dynamic property that accesses request parameters. If a parameter is specified with the same name as a request parameter, the request parameter is overridden and the controller parameter is used.

Since the params object is a Map, you can use it to pass the current request parameters from one action to the next:

redirect(action: "next", params: params)

Finally, you can also include a fragment in the target URI:

redirect(controller: "test", action: "show", fragment: "profile")

which will (depending on the URL mappings) redirect to something like "/myapp/test/show#profile".

Chaining

Actions can also be chained. Chaining allows the model to be retained from one action to the next. For example calling the first action in this action:

class ExampleChainController {

    def first() {
        chain(action: second, model: [one: 1])
    }

    def second () {
        chain(action: third, model: [two: 2])
    }

    def third() {
        [three: 3])
    }
}

results in the model:

[one: 1, two: 2, three: 3]

The model can be accessed in subsequent controller actions in the chain using the chainModel map. This dynamic property only exists in actions following the call to the chain method:

class ChainController {

    def nextInChain() {
        def model = chainModel.myModel
        ...
    }
}

Like the redirect method you can also pass parameters to the chain method:

chain(action: "action1", model: [one: 1], params: [myparam: "param1"])
The chain method uses the HTTP session and hence should only be used if your application is stateful.

8.1.5 Data Binding

Data binding is the act of "binding" incoming request parameters onto the properties of an object or an entire graph of objects. Data binding should deal with all necessary type conversion since request parameters, which are typically delivered by a form submission, are always strings whilst the properties of a Groovy or Java object may well not be.

Map Based Binding

The data binder is capable of converting and assigning values in a Map to properties of an object. The binder will associate entries in the Map to properties of the object using the keys in the Map that have values which correspond to property names on the object. The following code demonstrates the basics:

grails-app/domain/Person.groovy
class Person {
    String firstName
    String lastName
    Integer age
}
def bindingMap = [firstName: 'Peter', lastName: 'Gabriel', age: 63]

def person = new Person(bindingMap)

assert person.firstName == 'Peter'
assert person.lastName == 'Gabriel'
assert person.age == 63

To update properties of a domain object you may assign a Map to the properties property of the domain class:

def bindingMap = [firstName: 'Peter', lastName: 'Gabriel', age: 63]

def person = Person.get(someId)
person.properties = bindingMap

assert person.firstName == 'Peter'
assert person.lastName == 'Gabriel'
assert person.age == 63

The binder can populate a full graph of objects using Maps of Maps.

class Person {
    String firstName
    String lastName
    Integer age
    Address homeAddress
}

class Address {
    String county
    String country
}
def bindingMap = [firstName: 'Peter', lastName: 'Gabriel', age: 63, homeAddress: [county: 'Surrey', country: 'England'] ]

def person = new Person(bindingMap)

assert person.firstName == 'Peter'
assert person.lastName == 'Gabriel'
assert person.age == 63
assert person.homeAddress.county == 'Surrey'
assert person.homeAddress.country == 'England'

Binding To Collections And Maps

The data binder can populate and update Collections and Maps. The following code shows a simple example of populating a List of objects in a domain class:

class Band {
    String name
    static hasMany = [albums: Album]
    List albums
}

class Album {
    String title
    Integer numberOfTracks
}
def bindingMap = [name: 'Genesis',
                  'albums[0]': [title: 'Foxtrot', numberOfTracks: 6],
                  'albums[1]': [title: 'Nursery Cryme', numberOfTracks: 7]]

def band = new Band(bindingMap)

assert band.name == 'Genesis'
assert band.albums.size() == 2
assert band.albums[0].title == 'Foxtrot'
assert band.albums[0].numberOfTracks == 6
assert band.albums[1].title == 'Nursery Cryme'
assert band.albums[1].numberOfTracks == 7

That code would work in the same way if albums were an array instead of a List.

Note that when binding to a Set the structure of the Map being bound to the Set is the same as that of a Map being bound to a List but since a Set is unordered, the indexes don’t necessarily correspond to the order of elements in the Set. In the code example above, if albums were a Set instead of a List, the bindingMap could look exactly the same but 'Foxtrot' might be the first album in the Set or it might be the second. When updating existing elements in a Set the Map being assigned to the Set must have id elements in it which represent the element in the Set being updated, as in the following example:

/*
 * The value of the indexes 0 and 1 in albums[0] and albums[1] are arbitrary
 * values that can be anything as long as they are unique within the Map.
 * They do not correspond to the order of elements in albums because albums
 * is a Set.
 */
def bindingMap = ['albums[0]': [id: 9, title: 'The Lamb Lies Down On Broadway']
                  'albums[1]': [id: 4, title: 'Selling England By The Pound']]

def band = Band.get(someBandId)

/*
 * This will find the Album in albums that has an id of 9 and will set its title
 * to 'The Lamb Lies Down On Broadway' and will find the Album in albums that has
 * an id of 4 and set its title to 'Selling England By The Pound'.  In both
 * cases if the Album cannot be found in albums then the album will be retrieved
 * from the database by id, the Album will be added to albums and will be updated
 * with the values described above.  If a Album with the specified id cannot be
 * found in the database, then a binding error will be created and associated
 * with the band object.  More on binding errors later.
 */
band.properties = bindingMap

When binding to a Map the structure of the binding Map is the same as the structure of a Map used for binding to a List or a Set and the index inside of square brackets corresponds to the key in the Map being bound to. See the following code:

class Album {
    String title
    static hasMany = [players: Player]
    Map players
}

class Player {
    String name
}
def bindingMap = [title: 'The Lamb Lies Down On Broadway',
                  'players[guitar]': [name: 'Steve Hackett'],
                  'players[vocals]': [name: 'Peter Gabriel'],
                  'players[keyboards]': [name: 'Tony Banks']]

def album = new Album(bindingMap)

assert album.title == 'The Lamb Lies Down On Broadway'
assert album.players.size() == 3
assert album.players.guitar.name == 'Steve Hackett'
assert album.players.vocals.name == 'Peter Gabriel'
assert album.players.keyboards.name == 'Tony Banks'

When updating an existing Map, if the key specified in the binding Map does not exist in the Map being bound to then a new value will be created and added to the Map with the specified key as in the following example:

def bindingMap = [title: 'The Lamb Lies Down On Broadway',
                  'players[guitar]': [name: 'Steve Hackett'],
                  'players[vocals]': [name: 'Peter Gabriel']
                  'players[keyboards]': [name: 'Tony Banks']]

def album = new Album(bindingMap)

assert album.title == 'The Lamb Lies Down On Broadway'
assert album.players.size() == 3
assert album.players.guitar == 'Steve Hackett'
assert album.players.vocals == 'Peter Gabriel'
assert album.players.keyboards == 'Tony Banks'

def updatedBindingMap = ['players[drums]': [name: 'Phil Collins'],
                         'players[keyboards]': [name: 'Anthony George Banks']]

album.properties = updatedBindingMap

assert album.title == 'The Lamb Lies Down On Broadway'
assert album.players.size() == 4
assert album.players.guitar.name == 'Steve Hackett'
assert album.players.vocals.name == 'Peter Gabriel'
assert album.players.keyboards.name == 'Anthony George Banks'
assert album.players.drums.name == 'Phil Collins'

Binding Request Data to the Model

The params object that is available in a controller has special behavior that helps convert dotted request parameter names into nested Maps that the data binder can work with. For example, if a request includes request parameters named person.homeAddress.country and person.homeAddress.city with values 'USA' and 'St. Louis' respectively, params would include entries like these:

[person: [homeAddress: [country: 'USA', city: 'St. Louis']]]

There are two ways to bind request parameters onto the properties of a domain class. The first involves using a domain classes' Map constructor:

def save() {
    def b = new Book(params)
    b.save()
}

The data binding happens within the code new Book(params). By passing the params object to the domain class constructor Grails automatically recognizes that you are trying to bind from request parameters. So if we had an incoming request like:

/book/save?title=The%20Stand&author=Stephen%20King

Then the title and author request parameters would automatically be set on the domain class. You can use the properties property to perform data binding onto an existing instance:

def save() {
    def b = Book.get(params.id)
    b.properties = params
    b.save()
}

This has the same effect as using the implicit constructor.

When binding an empty String (a String with no characters in it, not even spaces), the data binder will convert the empty String to null. This simplifies the most common case where the intent is to treat an empty form field as having the value null since there isn’t a way to actually submit a null as a request parameter. When this behavior is not desirable the application may assign the value directly.

The mass property binding mechanism will by default automatically trim all Strings at binding time. To disable this behavior set the grails.databinding.trimStrings property to false in grails-app/conf/application.groovy.

// the default value is true
grails.databinding.trimStrings = false

// ...

The mass property binding mechanism will by default automatically convert all empty Strings to null at binding time. To disable this behavior set the grails.databinding.convertEmptyStringsToNull property to false in grails-app/conf/application.groovy.

// the default value is true
grails.databinding.convertEmptyStringsToNull = false

// ...

The order of events is that the String trimming happens and then null conversion happens so if trimStrings is true and convertEmptyStringsToNull is true, not only will empty Strings be converted to null but also blank Strings. A blank String is any String such that the trim() method returns an empty String.

These forms of data binding in Grails are very convenient, but also indiscriminate. In other words, they will bind all non-transient, typed instance properties of the target object, including ones that you may not want bound. Just because the form in your UI doesn’t submit all the properties, an attacker can still send malign data via a raw HTTP request. Fortunately, Grails also makes it easy to protect against such attacks - see the section titled "Data Binding and Security concerns" for more information.

Data binding and Single-ended Associations

If you have a one-to-one or many-to-one association you can use Grails' data binding capability to update these relationships too. For example if you have an incoming request such as:

/book/save?author.id=20

Grails will automatically detect the .id suffix on the request parameter and look up the Author instance for the given id when doing data binding such as:

def b = new Book(params)

An association property can be set to null by passing the literal String "null". For example:

/book/save?author.id=null

Data Binding and Many-ended Associations

If you have a one-to-many or many-to-many association there are different techniques for data binding depending of the association type.

If you have a Set based association (the default for a hasMany) then the simplest way to populate an association is to send a list of identifiers. For example consider the usage of <g:select> below:

<g:select name="books"
          from="${Book.list()}"
          size="5" multiple="yes" optionKey="id"
          value="${author?.books}" />

This produces a select box that lets you select multiple values. In this case if you submit the form Grails will automatically use the identifiers from the select box to populate the books association.

However, if you have a scenario where you want to update the properties of the associated objects the this technique won’t work. Instead you use the subscript operator:

<g:textField name="books[0].title" value="the Stand" />
<g:textField name="books[1].title" value="the Shining" />

However, with Set based association it is critical that you render the mark-up in the same order that you plan to do the update in. This is because a Set has no concept of order, so although we’re referring to books[0] and books[1] it is not guaranteed that the order of the association will be correct on the server side unless you apply some explicit sorting yourself.

This is not a problem if you use List based associations, since a List has a defined order and an index you can refer to. This is also true of Map based associations.

Note also that if the association you are binding to has a size of two and you refer to an element that is outside the size of association:

<g:textField name="books[0].title" value="the Stand" />
<g:textField name="books[1].title" value="the Shining" />
<g:textField name="books[2].title" value="Red Madder" />

Then Grails will automatically create a new instance for you at the defined position.

You can bind existing instances of the associated type to a List using the same .id syntax as you would use with a single-ended association. For example:

<g:select name="books[0].id" from="${bookList}"
          value="${author?.books[0]?.id}" />

<g:select name="books[1].id" from="${bookList}"
          value="${author?.books[1]?.id}" />

<g:select name="books[2].id" from="${bookList}"
          value="${author?.books[2]?.id}" />

Would allow individual entries in the books List to be selected separately.

Entries at particular indexes can be removed in the same way too. For example:

<g:select name="books[0].id"
          from="${Book.list()}"
          value="${author?.books[0]?.id}"
          noSelection="['null': '']"/>

Will render a select box that will remove the association at books[0] if the empty option is chosen.

Binding to a Map property works the same way except that the list index in the parameter name is replaced by the map key:

<g:select name="images[cover].id"
          from="${Image.list()}"
          value="${book?.images[cover]?.id}"
          noSelection="['null': '']"/>

This would bind the selected image into the Map property images under a key of "cover".

When binding to Maps, Arrays and Collections the data binder will automatically grow the size of the collections as necessary.

The default limit to how large the binder will grow a collection is 256. If the data binder encounters an entry that requires the collection be grown beyond that limit, the entry is ignored. The limit may be configured by assigning a value to the grails.databinding.autoGrowCollectionLimit property in application.groovy.
grails-app/conf/application.groovy
// the default value is 256
grails.databinding.autoGrowCollectionLimit = 128

// ...

Data binding with Multiple domain classes

It is possible to bind data to multiple domain objects from the params object.

For example so you have an incoming request to:

/book/save?book.title=The%20Stand&author.name=Stephen%20King

You’ll notice the difference with the above request is that each parameter has a prefix such as author. or book. which is used to isolate which parameters belong to which type. Grails' params object is like a multi-dimensional hash and you can index into it to isolate only a subset of the parameters to bind.

def b = new Book(params.book)

Notice how we use the prefix before the first dot of the book.title parameter to isolate only parameters below this level to bind. We could do the same with an Author domain class:

def a = new Author(params.author)

Data Binding and Action Arguments

Controller action arguments are subject to request parameter data binding. There are 2 categories of controller action arguments. The first category is command objects. Complex types are treated as command objects. See the Command Objects section of the user guide for details. The other category is basic object types. Supported types are the 8 primitives, their corresponding type wrappers and java.lang.String. The default behavior is to map request parameters to action arguments by name:

class AccountingController {

   // accountNumber will be initialized with the value of params.accountNumber
   // accountType will be initialized with params.accountType
   def displayInvoice(String accountNumber, int accountType) {
       // ...
   }
}

For primitive arguments and arguments which are instances of any of the primitive type wrapper classes a type conversion has to be carried out before the request parameter value can be bound to the action argument. The type conversion happens automatically. In a case like the example shown above, the params.accountType request parameter has to be converted to an int. If type conversion fails for any reason, the argument will have its default value per normal Java behavior (null for type wrapper references, false for booleans and zero for numbers) and a corresponding error will be added to the errors property of the defining controller.

/accounting/displayInvoice?accountNumber=B59786&accountType=bogusValue

Since "bogusValue" cannot be converted to type int, the value of accountType will be zero, the controller’s errors.hasErrors() will be true, the controller’s errors.errorCount will be equal to 1 and the controller’s errors.getFieldError('accountType') will contain the corresponding error.

If the argument name does not match the name of the request parameter then the @grails.web.RequestParameter annotation may be applied to an argument to express the name of the request parameter which should be bound to that argument:

import grails.web.RequestParameter

class AccountingController {

   // mainAccountNumber will be initialized with the value of params.accountNumber
   // accountType will be initialized with params.accountType
   def displayInvoice(@RequestParameter('accountNumber') String mainAccountNumber, int accountType) {
       // ...
   }
}

Data binding and type conversion errors

Sometimes when performing data binding it is not possible to convert a particular String into a particular target type. This results in a type conversion error. Grails will retain type conversion errors inside the errors property of a Grails domain class. For example:

class Book {
    ...
    URL publisherURL
}

Here we have a domain class Book that uses the java.net.URL class to represent URLs. Given an incoming request such as:

/book/save?publisherURL=a-bad-url

it is not possible to bind the string a-bad-url to the publisherURL property as a type mismatch error occurs. You can check for these like this:

def b = new Book(params)

if (b.hasErrors()) {
    println "The value ${b.errors.getFieldError('publisherURL').rejectedValue}" +
            " is not a valid URL!"
}

Although we have not yet covered error codes (for more information see the section on validation), for type conversion errors you would want a message from the grails-app/i18n/messages.properties file to use for the error. You can use a generic error message handler such as:

typeMismatch.java.net.URL=The field {0} is not a valid URL

Or a more specific one:

typeMismatch.Book.publisherURL=The publisher URL you specified is not a valid URL

The BindUsing Annotation

The BindUsing annotation may be used to define a custom binding mechanism for a particular field in a class. Any time data binding is being applied to the field the closure value of the annotation will be invoked with 2 arguments. The first argument is the object that data binding is being applied to and the second argument is DataBindingSource which is the data source for the data binding. The value returned from the closure will be bound to the property. The following example would result in the upper case version of the name value in the source being applied to the name field during data binding.

import grails.databinding.BindUsing

class SomeClass {
    @BindUsing({obj, source ->

        //source is DataSourceBinding which is similar to a Map
        //and defines getAt operation but source.name cannot be used here.
        //In order to get name from source use getAt instead as shown below.

        source['name']?.toUpperCase()
    })
    String name
}
Note that data binding is only possible when the name of the request parameter matches with the field name in the class. Here, name from request parameters matches with name from SomeClass.

The BindUsing annotation may be used to define a custom binding mechanism for all of the fields on a particular class. When the annotation is applied to a class, the value assigned to the annotation should be a class which implements the BindingHelper interface. An instance of that class will be used any time a value is bound to a property in the class that this annotation has been applied to.

@BindUsing(SomeClassWhichImplementsBindingHelper)
class SomeClass {
    String someProperty
    Integer someOtherProperty
}

Custom Data Converters

The binder will do a lot of type conversion automatically. Some applications may want to define their own mechanism for converting values and a simple way to do this is to write a class which implements ValueConverter and register an instance of that class as a bean in the Spring application context.

package com.myapp.converters

import grails.databinding.converters.ValueConverter

/**
 * A custom converter which will convert String of the
 * form 'city:state' into an Address object.
 */
class AddressValueConverter implements ValueConverter {

    boolean canConvert(value) {
        value instanceof String
    }

    def convert(value) {
        def pieces = value.split(':')
        new com.myapp.Address(city: pieces[0], state: pieces[1])
    }

    Class<?> getTargetType() {
        com.myapp.Address
    }
}

An instance of that class needs to be registered as a bean in the Spring application context. The bean name is not important. All beans that implemented ValueConverter will be automatically plugged in to the data binding process.

grails-app/conf/spring/resources.groovy
beans = {
    addressConverter com.myapp.converters.AddressValueConverter
    // ...
}
class Person {
    String firstName
    Address homeAddress
}

class Address {
    String city
    String state
}

def person = new Person()
person.properties = [firstName: 'Jeff', homeAddress: "O'Fallon:Missouri"]
assert person.firstName == 'Jeff'
assert person.homeAddress.city = "O'Fallon"
assert person.homeAddress.state = 'Missouri'

Date Formats For Data Binding

A custom date format may be specified to be used when binding a String to a Date value by applying the BindingFormat annotation to a Date field.

import grails.databinding.BindingFormat

class Person {
    @BindingFormat('MMddyyyy')
    Date birthDate
}

A global setting may be configured in application.groovy to define date formats which will be used application wide when binding to Date.

grails-app/conf/application.groovy
grails.databinding.dateFormats = ['MMddyyyy', 'yyyy-MM-dd HH:mm:ss.S', "yyyy-MM-dd'T'hh:mm:ss'Z'"]

The formats specified in grails.databinding.dateFormats will be attempted in the order in which they are included in the List. If a property is marked with @BindingFormat, the @BindingFormat will take precedence over the values specified in grails.databinding.dateFormats.

The formats configured by default are:

  • yyyy-MM-dd HH:mm:ss.S

  • yyyy-MM-dd’T’hh:mm:ss’Z'

  • yyyy-MM-dd HH:mm:ss.S z

  • yyyy-MM-dd’T’HH:mm:ss.SSSX

Custom Formatted Converters

You may supply your own handler for the BindingFormat annotation by writing a class which implements the FormattedValueConverter interface and registering an instance of that class as a bean in the Spring application context. Below is an example of a trivial custom String formatter that might convert the case of a String based on the value assigned to the BindingFormat annotation.

package com.myapp.converters

import grails.databinding.converters.FormattedValueConverter

class FormattedStringValueConverter implements FormattedValueConverter {
    def convert(value, String format) {
        if('UPPERCASE' == format) {
            value = value.toUpperCase()
        } else if('LOWERCASE' == format) {
            value = value.toLowerCase()
        }
        value
    }

    Class getTargetType() {
        // specifies the type to which this converter may be applied
        String
    }
}

An instance of that class needs to be registered as a bean in the Spring application context. The bean name is not important. All beans that implemented FormattedValueConverter will be automatically plugged in to the data binding process.

grails-app/conf/spring/resources.groovy
beans = {
    formattedStringConverter com.myapp.converters.FormattedStringValueConverter
    // ...
}

With that in place the BindingFormat annotation may be applied to String fields to inform the data binder to take advantage of the custom converter.

import grails.databinding.BindingFormat

class Person {
    @BindingFormat('UPPERCASE')
    String someUpperCaseString

    @BindingFormat('LOWERCASE')
    String someLowerCaseString

    String someOtherString
}

Localized Binding Formats

The BindingFormat annotation supports localized format strings by using the optional code attribute. If a value is assigned to the code attribute that value will be used as the message code to retrieve the binding format string from the messageSource bean in the Spring application context and that lookup will be localized.

import grails.databinding.BindingFormat

class Person {
    @BindingFormat(code='date.formats.birthdays')
    Date birthDate
}
# grails-app/conf/i18n/messages.properties
date.formats.birthdays=MMddyyyy
# grails-app/conf/i18n/messages_es.properties
date.formats.birthdays=ddMMyyyy

Structured Data Binding Editors

A structured data binding editor is a helper class which can bind structured request parameters to a property. The common use case for structured binding is binding to a Date object which might be constructed from several smaller pieces of information contained in several request parameters with names like birthday_month, birthday_date and birthday_year. The structured editor would retrieve all of those individual pieces of information and use them to construct a Date.

The framework provides a structured editor for binding to Date objects. An application may register its own structured editors for whatever types are appropriate. Consider the following classes:

src/main/groovy/databinding/Gadget.groovy
package databinding

class Gadget {
    Shape expandedShape
    Shape compressedShape
}
src/main/groovy/databinding/Shape.groovy
package databinding

class Shape {
    int area
}

A Gadget has 2 Shape fields. A Shape has an area property. It may be that the application wants to accept request parameters like width and height and use those to calculate the area of a Shape at binding time. A structured binding editor is well suited for that.

The way to register a structured editor with the data binding process is to add an instance of the grails.databinding.TypedStructuredBindingEditor interface to the Spring application context. The easiest way to implement the TypedStructuredBindingEditor interface is to extend the org.grails.databinding.converters.AbstractStructuredBindingEditor abstract class and override the getPropertyValue method as shown below:

src/main/groovy/databinding/converters/StructuredShapeEditor.groovy
package databinding.converters

import databinding.Shape

import org.grails.databinding.converters.AbstractStructuredBindingEditor

class StructuredShapeEditor extends AbstractStructuredBindingEditor<Shape> {

    public Shape getPropertyValue(Map values) {
        // retrieve the individual values from the Map
        def width = values.width as int
        def height = values.height as int

        // use the values to calculate the area of the Shape
        def area = width * height

        // create and return a Shape with the appropriate area
        new Shape(area: area)
    }
}

An instance of that class needs to be registered with the Spring application context:

grails-app/conf/spring/resources.groovy
beans = {
    shapeEditor databinding.converters.StructuredShapeEditor
    // ...
}

When the data binder binds to an instance of the Gadget class it will check to see if there are request parameters with names compressedShape and expandedShape which have a value of "struct" and if they do exist, that will trigger the use of the StructuredShapeEditor. The individual components of the structure need to have parameter names of the form propertyName_structuredElementName. In the case of the Gadget class above that would mean that the compressedShape request parameter should have a value of "struct" and the compressedShape_width and compressedShape_height parameters should have values which represent the width and the height of the compressed Shape. Similarly, the expandedShape request parameter should have a value of "struct" and the expandedShape_width and expandedShape_height parameters should have values which represent the width and the height of the expanded Shape.

grails-app/controllers/demo/DemoController.groovy
class DemoController {

    def createGadget(Gadget gadget) {
        /*
        /demo/createGadget?expandedShape=struct&expandedShape_width=80&expandedShape_height=30
                          &compressedShape=struct&compressedShape_width=10&compressedShape_height=3

        */

        // with the request parameters shown above gadget.expandedShape.area would be 2400
        // and gadget.compressedShape.area would be 30
        // ...
    }
}

Typically the request parameters with "struct" as their value would be represented by hidden form fields.

Data Binding Event Listeners

The DataBindingListener interface provides a mechanism for listeners to be notified of data binding events. The interface looks like this:

package grails.databinding.events;

import grails.databinding.errors.BindingError;

/**
 * A listener which will be notified of events generated during data binding.
 *
 * @author Jeff Brown
 * @since 3.0
 * @see DataBindingListenerAdapter
 */
public interface DataBindingListener {

    /**
     * @return true if the listener is interested in events for the specified type.
     */
    boolean supports(Class<?> clazz);

    /**
     * Called when data binding is about to start.
     *
     * @param target The object data binding is being imposed upon
     * @param errors the Spring Errors instance (a org.springframework.validation.BindingResult)
     * @return true if data binding should continue
     */
    Boolean beforeBinding(Object target, Object errors);

    /**
     * Called when data binding is about to imposed on a property
     *
     * @param target The object data binding is being imposed upon
     * @param propertyName The name of the property being bound to
     * @param value The value of the property being bound
     * @param errors the Spring Errors instance (a org.springframework.validation.BindingResult)
     * @return true if data binding should continue, otherwise return false
     */
    Boolean beforeBinding(Object target, String propertyName, Object value, Object errors);

    /**
     * Called after data binding has been imposed on a property
     *
     * @param target The object data binding is being imposed upon
     * @param propertyName The name of the property that was bound to
     * @param errors the Spring Errors instance (a org.springframework.validation.BindingResult)
     */
    void afterBinding(Object target, String propertyName, Object errors);

    /**
     * Called after data binding has finished.
     *
     * @param target The object data binding is being imposed upon
     * @param errors the Spring Errors instance (a org.springframework.validation.BindingResult)
     */
    void afterBinding(Object target, Object errors);

    /**
     * Called when an error occurs binding to a property
     * @param error encapsulates information about the binding error
     * @param errors the Spring Errors instance (a org.springframework.validation.BindingResult)
     * @see BindingError
     */
    void bindingError(BindingError error, Object errors);
}

Any bean in the Spring application context which implements that interface will automatically be registered with the data binder. The DataBindingListenerAdapter class implements the DataBindingListener interface and provides default implementations for all of the methods in the interface so this class is well suited for subclassing so your listener class only needs to provide implementations for the methods your listener is interested in.

Using The Data Binder Directly

There are situations where an application may want to use the data binder directly. For example, to do binding in a Service on some arbitrary object which is not a domain class. The following will not work because the properties property is read only.

src/main/groovy/bindingdemo/Widget.groovy
package bindingdemo

class Widget {
    String name
    Integer size
}
grails-app/services/bindingdemo/WidgetService.groovy
package bindingdemo

class WidgetService {

    def updateWidget(Widget widget, Map data) {
        // this will throw an exception because
        // properties is read-only
        widget.properties = data
    }
}

An instance of the data binder is in the Spring application context with a bean name of grailsWebDataBinder. That bean implements the DataBinder interface. The following code demonstrates using the data binder directly.

grails-app/services/bindingdmeo/WidgetService
package bindingdemo

import grails.databinding.SimpleMapDataBindingSource

class WidgetService {

    // this bean will be autowired into the service
    def grailsWebDataBinder

    def updateWidget(Widget widget, Map data) {
        grailsWebDataBinder.bind widget, data as SimpleMapDataBindingSource
    }

}

See the DataBinder documentation for more information about overloaded versions of the bind method.

Data Binding and Security Concerns

When batch updating properties from request parameters you need to be careful not to allow clients to bind malicious data to domain classes and be persisted in the database. You can limit what properties are bound to a given domain class using the subscript operator:

def p = Person.get(1)

p.properties['firstName','lastName'] = params

In this case only the firstName and lastName properties will be bound.

Another way to do this is is to use Command Objects as the target of data binding instead of domain classes. Alternatively there is also the flexible bindData method.

The bindData method allows the same data binding capability, but to arbitrary objects:

def p = new Person()
bindData(p, params)

The bindData method also lets you exclude certain parameters that you don’t want updated:

def p = new Person()
bindData(p, params, [exclude: 'dateOfBirth'])

Or include only certain properties:

def p = new Person()
bindData(p, params, [include: ['firstName', 'lastName']])
If an empty List is provided as a value for the include parameter then all fields will be subject to binding if they are not explicitly excluded.

The bindable constraint can be used to globally prevent data binding for certain properties.

8.1.6 Responding with JSON

Using the respond method to output JSON

The respond method is the preferred way to return JSON and integrates with Content Negotiation and JSON Views.

The respond method provides content negotiation strategies to intelligently produce an appropriate response for the given client.

For example given the following controller and action:

grails-app/controllers/example/BookController.groovy
package example

class BookController {
    def index() {
        respond Book.list()
    }
}

The respond method will take the followings steps:

  1. If the client Accept header specifies a media type (for example application/json) use that

  2. If the file extension of the URI (for example /books.json) includes a format defined in the grails.mime.types property of grails-app/conf/application.yml use the media type defined in the configuration

The respond method will then look for an appriopriate Renderer for the object and the calculated media type from the RendererRegistry.

Grails includes a number of pre-configured Renderer implementations that will produce default representations of JSON responses for the argument passed to respond. For example going to the /book.json URI will produce JSON such as:

[
    {id:1,"title":"The Stand"},
    {id:2,"title":"Shining"}
]

Controlling the Priority of Media Types

By default if you define a controller there is no priority in terms of which format is sent back to the client and Grails assumes you wish to serve HTML as a response type.

However if your application is primarily an API, then you can specify the priorty using the responseFormats property:

grails-app/controllers/example/BookController.groovy
package example

class BookController {
    static responseFormats = ['json', 'html']
    def index() {
        respond Book.list()
    }
}

In the above example Grails will respond by default with json if the media type to respond with cannot be calculated from the Accept header or file extension.

Using Views to Output JSON Responses

If you define a view (either a GSP or a JSON View) then Grails will render the view when using the respond method by calculating a model from the argument passed to respond.

For example, in the previous listing, if you were to define grails-app/views/index.gson and grails-app/views/index.gsp views, these would be used if the client requested application/json or text/html media types respectively. Thus allowing you to define a single backend capible of serving responses to a web browser or representing your application’s API.

When rendering the view, Grails will calculate a model to pass to the view based on the type of the value passed to the respond method.

The following table summarizes this convention:

Example Argument Type Calculated Model Variable

respond Book.list()

java.util.List

bookList

respond( [] )

java.util.List

emptyList

respond Book.get(1)

example.Book

book

respond( [1,2] )

java.util.List

integerList

respond( [1,2] as Set )

java.util.Set

integerSet

respond( [1,2] as Integer[] )

Integer[]

integerArray

Using this convention you can reference the argument passed to respond from within your view:

grails-app/views/book/index.gson
@Field List<Book> bookList = []

json bookList, { Book book ->
    title book.title
}

You will notice that if Book.list() returns an empty list then the model variable name is translated to emptyList. This is by design and you should provide a default value in the view if no model variable is specified, such as the List in the example above:

grails-app/views/book/index.gson
// defaults to an empty list
@Field List<Book> bookList = []
...

There are cases where you may wish to be more explicit and control the name of the model variable. For example if you have a domain inheritance hierarchy where a call to list() my return different child classes relying on automatic calculation may not be reliable.

In this case you should pass the model directly using respond and a map argument:

respond bookList: Book.list()
When responding with any kind of mixed argument types in a collection, always use an explicit model name.

If you simply wish to augment the calculated model then you can do so by passing a model argument:

respond Book.list(), [model: [bookCount: Book.count()]]

The above example will produce a model like [bookList:books, bookCount:totalBooks], where the calculated model is combined with the model passed in the model argument.

Using the render method to output JSON

The render method can also be used to output JSON, but should only be used for simple cases that don’t warrant the creation of a JSON view:

def list() {

    def results = Book.list()

    render(contentType: "application/json") {
        books(results) { Book b ->
            title b.title
        }
    }
}

In this case the result would be something along the lines of:

[
    {"title":"The Stand"},
    {"title":"Shining"}
]
This technique for rendering JSON may be ok for very simple responses, but in general you should favour the use of JSON Views and use the view layer rather than embedding logic in your application.

The same dangers with naming conflicts described above for XML also apply to JSON building.

8.1.7 More on JSONBuilder

The previous section on on XML and JSON responses covered simplistic examples of rendering XML and JSON responses. Whilst the XML builder used by Grails is the standard XmlSlurper found in Groovy.

For JSON, since Grails 3.1, Grails uses Groovy’s StreamingJsonBuilder by default and you can refer to the Groovy documentation and StreamingJsonBuilder API documentation on how to use it.

In older version of Grails a custom JSON builder was used called grails.web.JSONBuilder. This class is considered deprecated. However, if you are upgrading an application you can re-enable it with the following configuration in application.groovy:

grails.json.legacy.builder = false

8.1.8 Responding with XML

8.1.9 Uploading Files

Programmatic File Uploads

Grails supports file uploads using Spring’s MultipartHttpServletRequest interface. The first step for file uploading is to create a multipart form like this:

Upload Form: <br />
    <g:uploadForm action="upload">
        <input type="file" name="myFile" />
        <input type="submit" />
    </g:uploadForm>

The uploadForm tag conveniently adds the enctype="multipart/form-data" attribute to the standard <g:form> tag.

There are then a number of ways to handle the file upload. One is to work with the Spring MultipartFile instance directly:

def upload() {
    def f = request.getFile('myFile')
    if (f.empty) {
        flash.message = 'file cannot be empty'
        render(view: 'uploadForm')
        return
    }

    f.transferTo(new File('/some/local/dir/myfile.txt'))
    response.sendError(200, 'Done')
}

This is convenient for doing transfers to other destinations and manipulating the file directly as you can obtain an InputStream and so on with the MultipartFile interface.

File Uploads through Data Binding

File uploads can also be performed using data binding. Consider this Image domain class:

class Image {
    byte[] myFile

    static constraints = {
        // Limit upload file size to 2MB
        myFile maxSize: 1024 * 1024 * 2
    }
}

If you create an image using the params object in the constructor as in the example below, Grails will automatically bind the file’s contents as a byte[] to the myFile property:

def img = new Image(params)

It’s important that you set the size or maxSize constraints, otherwise your database may be created with a small column size that can’t handle reasonably sized files. For example, both H2 and MySQL default to a blob size of 255 bytes for byte[] properties.

It is also possible to set the contents of the file as a string by changing the type of the myFile property on the image to a String type:

class Image {
   String myFile
}

8.1.10 Command Objects

Grails controllers support the concept of command objects. A command object is a class that is used in conjunction with data binding, usually to allow validation of data that may not fit into an existing domain class.

A class is only considered to be a command object when it is used as a parameter of an action.

Declaring Command Objects

Command object classes are defined just like any other class.

class LoginCommand implements grails.validation.Validateable {
    String username
    String password

    static constraints = {
        username(blank: false, minSize: 6)
        password(blank: false, minSize: 6)
    }
}

In this example, the command object class implements the Validateable trait. The Validateable trait allows the definition of Constraints just like in domain classes. If the command object is defined in the same source file as the controller that is using it, Grails will automatically make it Validateable. It is not required that command object classes be validateable.

By default, all Validateable object properties are nullable: false which matches the behavior of GORM domain objects. If you want a Validateable that has nullable: true properties by default, you can specify this by defining a defaultNullable method in the class:

class AuthorSearchCommand implements grails.validation.Validateable {
    String  name
    Integer age

    static boolean defaultNullable() {
        true
    }
}

In this example, both name and age will allow null values during validation.

Using Command Objects

To use command objects, controller actions may optionally specify any number of command object parameters. The parameter types must be supplied so that Grails knows what objects to create and initialize.

Before the controller action is executed Grails will automatically create an instance of the command object class and populate its properties by binding the request parameters. If the command object class is marked with Validateable then the command object will be validated. For example:

class LoginController {

    def login(LoginCommand cmd) {
        if (cmd.hasErrors()) {
            redirect(action: 'loginForm')
            return
        }

        // work with the command object data
    }
}

If the command object’s type is that of a domain class and there is an id request parameter then instead of invoking the domain class constructor to create a new instance a call will be made to the static get method on the domain class and the value of the id parameter will be passed as an argument.

Whatever is returned from that call to get is what will be passed into the controller action. This means that if there is an id request parameter and no corresponding record is found in the database then the value of the command object will be null. If an error occurs retrieving the instance from the database then null will be passed as an argument to the controller action and an error will be added the controller’s errors property.

If the command object’s type is a domain class and there is no id request parameter or there is an id request parameter and its value is empty then null will be passed into the controller action unless the HTTP request method is "POST", in which case a new instance of the domain class will be created by invoking the domain class constructor. For all of the cases where the domain class instance is non-null, data binding is only performed if the HTTP request method is "POST", "PUT" or "PATCH".

Command Objects And Request Parameter Names

Normally request parameter names will be mapped directly to property names in the command object. Nested parameter names may be used to bind down the object graph in an intuitive way.

In the example below a request parameter named name will be bound to the name property of the Person instance and a request parameter named address.city will be bound to the city property of the address property in the Person.

class StoreController {
    def buy(Person buyer) {
        // ...
    }
}

class Person {
    String name
    Address address
}

class Address {
    String city
}

A problem may arise if a controller action accepts multiple command objects which happen to contain the same property name. Consider the following example.

class StoreController {
    def buy(Person buyer, Product product) {
        // ...
    }
}

class Person {
    String name
    Address address
}

class Address {
    String city
}

class Product {
    String name
}

If there is a request parameter named name it isn’t clear if that should represent the name of the Product or the name of the Person. Another version of the problem can come up if a controller action accepts 2 command objects of the same type as shown below.

class StoreController {
    def buy(Person buyer, Person seller, Product product) {
        // ...
    }
}

class Person {
    String name
    Address address
}

class Address {
    String city
}

class Product {
    String name
}

To help deal with this the framework imposes special rules for mapping parameter names to command object types. The command object data binding will treat all parameters that begin with the controller action parameter name as belonging to the corresponding command object.

For example, the product.name request parameter will be bound to the name property in the product argument, the buyer.name request parameter will be bound to the name property in the buyer argument the seller.address.city request parameter will be bound to the city property of the address property of the seller argument, etc…​

Command Objects and Dependency Injection

Command objects can participate in dependency injection. This is useful if your command object has some custom validation logic which uses a Grails service:

class LoginCommand implements grails.validation.Validateable {

    def loginService

    String username
    String password

    static constraints = {
        username validator: { val, obj ->
            obj.loginService.canLogin(obj.username, obj.password)
        }
    }
}

In this example the command object interacts with the loginService bean which is injected by name from the Spring ApplicationContext.

Binding The Request Body To Command Objects

When a request is made to a controller action which accepts a command object and the request contains a body, Grails will attempt to parse the body of the request based on the request content type and use the body to do data binding on the command object. See the following example.

grails-app/controllers/bindingdemo/DemoController.groovy
package bindingdemo

class DemoController {

    def createWidget(Widget w) {
        render "Name: ${w?.name}, Size: ${w?.size}"
    }
}

class Widget {
    String name
    Integer size
}
$ curl -H "Content-Type: application/json" -d '{"name":"Some Widget","42"}'[size] localhost:8080/demo/createWidget
 Name: Some Widget, Size: 42

$ curl -H "Content-Type: application/xml" -d '<widget><name>Some Other Widget</name><size>2112</size></widget>' localhost:8080/bodybind/demo/createWidget
 Name: Some Other Widget, Size: 2112
The request body will not be parsed under the following conditions: * The request method is GET * The request method is DELETE * The content length is 0

Note that the body of the request is being parsed to make that work. Any attempt to read the body of the request after that will fail since the corresponding input stream will be empty. The controller action can either use a command object or it can parse the body of the request on its own (either directly, or by referring to something like request.JSON), but cannot do both.

grails-app/controllers/bindingdemo/DemoController.groovy
package bindingdemo

class DemoController {

    def createWidget(Widget w) {
        // this will fail because it requires reading the body,
        // which has already been read.
        def json = request.JSON

        // ...

    }
}

Working with Lists of Command Objects

A common use case for command objects is a Command Object that contains a collection of another:

class DemoController {

    def createAuthor(AuthorCommand command) {
        // ...

    }

    class AuthorCommand {
        String fullName
        List<BookCommand> books
    }

    class BookCommand {
        String title
        String isbn
    }
}

On this example, we want to create an Author with multiple Books.

In order to make this work from the UI layer, you can do the following in your GSP:

<g:form name="submit-author-books" controller="demo" action="createAuthor">
    <g:fieldValue name="fullName" value=""/>
    <ul>
        <li>
            <g:fieldValue name="books[0].title" value=""/>
            <g:fieldValue name="books[0].isbn" value=""/>
        </li>

        <li>
            <g:fieldValue name="books[1].title" value=""/>
            <g:fieldValue name="books[1].isbn" value=""/>
        </li>
    </ul>
</g:form>

There is also support for JSON, so you can submit the following with correct databinding

{
    "fullName": "Graeme Rocher",
    "books": [{
        "title": "The Definitive Guide to Grails",
        "isbn": "1111-343455-1111"
    }, {
        "title": "The Definitive Guide to Grails 2",
        "isbn": "1111-343455-1112"
    }],
}

8.1.11 Handling Duplicate Form Submissions

Grails has built-in support for handling duplicate form submissions using the "Synchronizer Token Pattern". To get started you define a token on the form tag:

<g:form useToken="true" ...>

Then in your controller code you can use the withForm method to handle valid and invalid requests:

withForm {
   // good request
}.invalidToken {
   // bad request
}

If you only provide the withForm method and not the chained invalidToken method then by default Grails will store the invalid token in a flash.invalidToken variable and redirect the request back to the original page. This can then be checked in the view:

<g:if test="${flash.invalidToken}">
  Don't click the button twice!
</g:if>
The withForm tag makes use of the session and hence requires session affinity or clustered sessions if used in a cluster.

8.1.12 Simple Type Converters

Type Conversion Methods

If you prefer to avoid the overhead of data binding and simply want to convert incoming parameters (typically Strings) into another more appropriate type the params object has a number of convenience methods for each type:

def total = params.int('total')

The above example uses the int method, and there are also methods for boolean, long, char, short and so on. Each of these methods is null-safe and safe from any parsing errors, so you don’t have to perform any additional checks on the parameters.

Each of the conversion methods allows a default value to be passed as an optional second argument. The default value will be returned if a corresponding entry cannot be found in the map or if an error occurs during the conversion. Example:

def total = params.int('total', 42)

These same type conversion methods are also available on the attrs parameter of GSP tags.

Handling Multi Parameters

A common use case is dealing with multiple request parameters of the same name. For example you could get a query string such as ?name=Bob&name=Judy.

In this case dealing with one parameter and dealing with many has different semantics since Groovy’s iteration mechanics for String iterate over each character. To avoid this problem the params object provides a list method that always returns a list:

for (name in params.list('name')) {
    println name
}

8.1.13 Declarative Controller Exception Handling

Grails controllers support a simple mechanism for declarative exception handling. If a controller declares a method that accepts a single argument and the argument type is java.lang.Exception or some subclass of java.lang.Exception, that method will be invoked any time an action in that controller throws an exception of that type. See the following example.

grails-app/controllers/demo/DemoController.groovy
package demo

class DemoController {

    def someAction() {
        // do some work
    }

    def handleSQLException(SQLException e) {
        render 'A SQLException Was Handled'
    }

    def handleBatchUpdateException(BatchUpdateException e) {
        redirect controller: 'logging', action: 'batchProblem'
    }

    def handleNumberFormatException(NumberFormatException nfe) {
        [problemDescription: 'A Number Was Invalid']
    }
}

That controller will behave as if it were written something like this…​

grails-app/controllers/demo/DemoController.groovy
package demo

class DemoController {

    def someAction() {
        try {
            // do some work
        } catch (BatchUpdateException e) {
            return handleBatchUpdateException(e)
        } catch (SQLException e) {
            return handleSQLException(e)
        } catch (NumberFormatException e) {
            return handleNumberFormatException(e)
        }
    }

    def handleSQLException(SQLException e) {
        render 'A SQLException Was Handled'
    }

    def handleBatchUpdateException(BatchUpdateException e) {
        redirect controller: 'logging', action: 'batchProblem'
    }

    def handleNumberFormatException(NumberFormatException nfe) {
        [problemDescription: 'A Number Was Invalid']
    }
}

The exception handler method names can be any valid method name. The name is not what makes the method an exception handler, the Exception argument type is the important part.

The exception handler methods can do anything that a controller action can do including invoking render, redirect, returning a model, etc.

One way to share exception handler methods across multiple controllers is to use inheritance. Exception handler methods are inherited into subclasses so an application could define the exception handlers in an abstract class that multiple controllers extend from. Another way to share exception handler methods across multiple controllers is to use a trait, as shown below…​

src/main/groovy/com/demo/DatabaseExceptionHandler.groovy
package com.demo

trait DatabaseExceptionHandler {
    def handleSQLException(SQLException e) {
        // handle SQLException
    }

    def handleBatchUpdateException(BatchUpdateException e) {
        // handle BatchUpdateException
    }
}
grails-app/controllers/com/demo/DemoController.groovy
package com.demo

class DemoController implements DatabaseExceptionHandler {

    // all of the exception handler methods defined
    // in DatabaseExceptionHandler will be added to
    // this class at compile time
}

Exception handler methods must be present at compile time. Specifically, exception handler methods which are runtime metaprogrammed onto a controller class are not supported.

8.2 Groovy Server Pages

Groovy Servers Pages (or GSP for short) is Grails' view technology. It is designed to be familiar for users of technologies such as ASP and JSP, but to be far more flexible and intuitive.

Although GSP can render any format, not just HTML, it is more designed around rendering markup. If you are looking for a way to simplify JSON responses take a look at JSON Views.

GSPs live in the grails-app/views directory and are typically rendered automatically (by convention) or with the render method such as:

render(view: "index")

A GSP is typically a mix of mark-up and GSP tags which aid in view rendering.

Although it is possible to have Groovy logic embedded in your GSP and doing this will be covered in this document, the practice is strongly discouraged. Mixing mark-up and code is a bad thing and most GSP pages contain no code and needn’t do so.

A GSP typically has a "model" which is a set of variables that are used for view rendering. The model is passed to the GSP view from a controller. For example consider the following controller action:

def show() {
    [book: Book.get(params.id)]
}

This action will look up a Book instance and create a model that contains a key called book. This key can then be referenced within the GSP view using the name book:

${book.title}
Embedding data received from user input has the risk of making your application vulnerable to an Cross Site Scripting (XSS) attack. Please read the documentation on XSS prevention for information on how to prevent XSS attacks.

8.2.1 GSP Basics

In the next view sections we’ll go through the basics of GSP and what is available to you. First off let’s cover some basic syntax that users of JSP and ASP should be familiar with.

GSP supports the usage of <% %> scriptlet blocks to embed Groovy code (again this is discouraged):

<html>
   <body>
     <% out << "Hello GSP!" %>
   </body>
</html>

You can also use the <%= %> syntax to output values:

<html>
   <body>
     <%="Hello GSP!" %>
   </body>
</html>

GSP also supports JSP-style server-side comments (which are not rendered in the HTML response) as the following example demonstrates:

<html>
   <body>
     <%-- This is my comment --%>
     <%="Hello GSP!" %>
   </body>
</html>
Embedding data received from user input has the risk of making your application vulnerable to an Cross Site Scripting (XSS) attack. Please read the documentation on XSS prevention for information on how to prevent XSS attacks.

8.2.1.1 Variables and Scopes

Within the <% %> brackets you can declare variables:

<% now = new Date() %>

and then access those variables later in the page:

<%=now%>

Within the scope of a GSP there are a number of pre-defined variables, including:

8.2.1.2 Logic and Iteration

Using the <% %> syntax you can embed loops and so on using this syntax:

<html>
   <body>
      <% [1,2,3,4].each { num -> %>
         <p><%="Hello ${num}!" %></p>
      <%}%>
   </body>
</html>

As well as logical branching:

<html>
   <body>
      <% if (params.hello == 'true')%>
      <%="Hello!"%>
      <% else %>
      <%="Goodbye!"%>
   </body>
</html>

8.2.1.3 Page Directives

GSP also supports a few JSP-style page directives.

The import directive lets you import classes into the page. However, it is rarely needed due to Groovy’s default imports and GSP Tags:

<%@ page import="java.awt.*" %>

GSP also supports the contentType directive:

<%@ page contentType="application/json" %>

The contentType directive allows using GSP to render other formats.

8.2.1.4 Expressions

In GSP the <%= %> syntax introduced earlier is rarely used due to the support for GSP expressions. A GSP expression is similar to a JSP EL expression or a Groovy GString and takes the form ${expr}:

<html>
  <body>
    Hello ${params.name}
  </body>
</html>

However, unlike JSP EL you can have any Groovy expression within the ${..} block.

Embedding data received from user input has the risk of making your application vulnerable to an Cross Site Scripting (XSS) attack. Please read the documentation on XSS prevention for information on how to prevent XSS attacks.

8.2.2 GSP Tags

Now that the less attractive JSP heritage has been set aside, the following sections cover GSP’s built-in tags, which are the preferred way to define GSP pages.

The section on Tag Libraries covers how to add your own custom tag libraries.

All built-in GSP tags start with the prefix g:. Unlike JSP, you don’t specify any tag library imports. If a tag starts with g: it is automatically assumed to be a GSP tag. An example GSP tag would look like:

<g:example />

GSP tags can also have a body such as:

<g:example>
   Hello world
</g:example>

Expressions can be passed into GSP tag attributes, if an expression is not used it will be assumed to be a String value:

<g:example attr="${new Date()}">
   Hello world
</g:example>

Maps can also be passed into GSP tag attributes, which are often used for a named parameter style syntax:

<g:example attr="${new Date()}" attr2="[one:1, two:2, three:3]">
   Hello world
</g:example>

Note that within the values of attributes you must use single quotes for Strings:

<g:example attr="${new Date()}" attr2="[one:'one', two:'two']">
   Hello world
</g:example>

With the basic syntax out the way, the next sections look at the tags that are built into Grails by default.

8.2.2.1 Variables and Scopes

Variables can be defined within a GSP using the set tag:

<g:set var="now" value="${new Date()}" />

Here we assign a variable called now to the result of a GSP expression (which simply constructs a new java.util.Date instance). You can also use the body of the <g:set> tag to define a variable:

<g:set var="myHTML">
   Some re-usable code on: ${new Date()}
</g:set>

The assigned value can also be a bean from the applicationContext:

<g:set var="bookService" bean="bookService" />

Variables can also be placed in one of the following scopes:

  • page - Scoped to the current page (default)

  • request - Scoped to the current request

  • flash - Placed within flash scope and hence available for the next request

  • session - Scoped for the user session

  • application - Application-wide scope.

To specify the scope, use the scope attribute:

<g:set var="now" value="${new Date()}" scope="request" />

8.2.2.2 Logic and Iteration

GSP also supports logical and iterative tags out of the box. For logic there are if, else and elseif tags for use with branching:

<g:if test="${session.role == 'admin'}">
   <%-- show administrative functions --%>
</g:if>
<g:else>
   <%-- show basic functions --%>
</g:else>

Use the each and while tags for iteration:

<g:each in="${[1,2,3]}" var="num">
   <p>Number ${num}</p>
</g:each>

<g:set var="num" value="${1}" />
<g:while test="${num < 5 }">
    <p>Number ${num++}</p>
</g:while>

8.2.2.3 Search and Filtering

If you have collections of objects you often need to sort and filter them. Use the findAll and grep tags for these tasks:

Stephen King's Books:
<g:findAll in="${books}" expr="it.author == 'Stephen King'">
     <p>Title: ${it.title}</p>
</g:findAll>

The expr attribute contains a Groovy expression that can be used as a filter. The grep tag does a similar job, for example filtering by class:

<g:grep in="${books}" filter="NonFictionBooks.class">
     <p>Title: ${it.title}</p>
</g:grep>

Or using a regular expression:

<g:grep in="${books.title}" filter="~/.*?Groovy.*?/">
     <p>Title: ${it}</p>
</g:grep>

The above example is also interesting due to its usage of GPath. GPath is an XPath-like language in Groovy. The books variable is a collection of Book instances. Since each Book has a title, you can obtain a list of Book titles using the expression books.title. Groovy will auto-magically iterate the collection, obtain each title, and return a new list!

8.2.2.4 Links and Resources

GSP also features tags to help you manage linking to controllers and actions. The link tag lets you specify controller and action name pairing and it will automatically work out the link based on the URL Mappings, even if you change them! For example:

<g:link action="show" id="1">Book 1</g:link>

<g:link action="show" id="${currentBook.id}">${currentBook.name}</g:link>

<g:link controller="book">Book Home</g:link>

<g:link controller="book" action="list">Book List</g:link>

<g:link url="[action: 'list', controller: 'book']">Book List</g:link>

<g:link params="[sort: 'title', order: 'asc', author: currentBook.author]"
        action="list">Book List</g:link>

8.2.2.5 Forms and Fields

Form Basics

GSP supports many different tags for working with HTML forms and fields, the most basic of which is the form tag. This is a controller/action aware version of the regular HTML form tag. The url attribute lets you specify which controller and action to map to:

<g:form name="myForm" url="[controller:'book',action:'list']">...</g:form>

In this case we create a form called myForm that submits to the BookController’s `list action. Beyond that all of the usual HTML attributes apply.

Form Fields

In addition to easy construction of forms, GSP supports custom tags for dealing with different types of fields, including:

  • textField - For input fields of type 'text'

  • passwordField - For input fields of type 'password'

  • checkBox - For input fields of type 'checkbox'

  • radio - For input fields of type 'radio'

  • hiddenField - For input fields of type 'hidden'

  • select - For dealing with HTML select boxes

Each of these allows GSP expressions for the value:

<g:textField name="myField" value="${myValue}" />

GSP also contains extended helper versions of the above tags such as radioGroup (for creating groups of radio tags), localeSelect, currencySelect and timeZoneSelect (for selecting locales, currencies and time zones respectively).

Multiple Submit Buttons

The age old problem of dealing with multiple submit buttons is also handled elegantly with Grails using the actionSubmit tag. It is just like a regular submit, but lets you specify an alternative action to submit to:

<g:actionSubmit value="Some update label" action="update" />

8.2.2.6 Tags as Method Calls

One major different between GSP tags and other tagging technologies is that GSP tags can be called as either regular tags or as method calls from controllers, tag libraries or GSP views.

Tags as method calls from GSPs

Tags return their results as a String-like object (a StreamCharBuffer which has all of the same methods as String) instead of writing directly to the response when called as methods. For example:

Static Resource: ${createLinkTo(dir: "images", file: "logo.jpg")}

This is particularly useful for using a tag within an attribute:

<img src="${createLinkTo(dir: 'images', file: 'logo.jpg')}" />

In view technologies that don’t support this feature you have to nest tags within tags, which becomes messy quickly and often has an adverse effect of WYSIWYG tools such as Dreamweaver that attempt to render the mark-up as it is not well-formed:

<img src="<g:createLinkTo dir="images" file="logo.jpg" />" />

Tags as method calls from Controllers and Tag Libraries

You can also invoke tags from controllers and tag libraries. Tags within the default g: namespace can be invoked without the prefix and a StreamCharBuffer result is returned:

def imageLocation = createLinkTo(dir:"images", file:"logo.jpg").toString()

Prefix the namespace to avoid naming conflicts:

def imageLocation = g.createLinkTo(dir:"images", file:"logo.jpg").toString()

For tags that use a custom namespace, use that prefix for the method call. For example (from the FCK Editor plugin):

def editor = fckeditor.editor(name: "text", width: "100%", height: "400")

8.2.3 Views and Templates

Grails also has the concept of templates. These are useful for partitioning your views into maintainable chunks, and combined with Layouts provide a highly re-usable mechanism for structured views.

Template Basics

Grails uses the convention of placing an underscore before the name of a view to identify it as a template. For example, you might have a template that renders Books located at grails-app/views/book/_bookTemplate.gsp:

<div class="book" id="${book?.id}">
   <div>Title: ${book?.title}</div>
   <div>Author: ${book?.author?.name}</div>
</div>

Use the render tag to render this template from one of the views in grails-app/views/book:

<g:render template="bookTemplate" model="[book: myBook]" />

Notice how we pass into a model to use using the model attribute of the render tag. If you have multiple Book instances you can also render the template for each Book using the render tag with a collection attribute:

<g:render template="bookTemplate" var="book" collection="${bookList}" />

Shared Templates

In the previous example we had a template that was specific to the BookController and its views at grails-app/views/book. However, you may want to share templates across your application.

In this case you can place them in the root views directory at grails-app/views or any subdirectory below that location, and then with the template attribute use an absolute location starting with / instead of a relative location. For example if you had a template called grails-app/views/shared/_mySharedTemplate.gsp, you would reference it as:

<g:render template="/shared/mySharedTemplate" />

You can also use this technique to reference templates in any directory from any view or controller:

<g:render template="/book/bookTemplate" model="[book: myBook]" />

The Template Namespace

Since templates are used so frequently there is template namespace, called tmpl, available that makes using templates easier. Consider for example the following usage pattern:

<g:render template="bookTemplate" model="[book:myBook]" />

This can be expressed with the tmpl namespace as follows:

<tmpl:bookTemplate book="${myBook}" />

Templates in Controllers and Tag Libraries

You can also render templates from controllers using the render controller method. This is useful for JavaScript heavy applications where you generate small HTML or data responses to partially update the current page instead of performing new request:

def bookData() {
    def b = Book.get(params.id)
    render(template:"bookTemplate", model:[book:b])
}

The render controller method writes directly to the response, which is the most common behaviour. To instead obtain the result of template as a String you can use the render tag:

def bookData() {
    def b = Book.get(params.id)
    String content = g.render(template:"bookTemplate", model:[book:b])
    render content
}

Notice the usage of the g namespace which tells Grails we want to use the tag as method call instead of the render method.

8.2.4 Layouts with Sitemesh

Creating Layouts

Grails leverages Sitemesh, a decorator engine, to support view layouts. Layouts are located in the grails-app/views/layouts directory. A typical layout can be seen below:

<html>
    <head>
        <title><g:layoutTitle default="An example decorator" /></title>
        <g:layoutHead />
    </head>
    <body onload="${pageProperty(name:'body.onload')}">
        <div class="menu"><!--my common menu goes here--></div>
        <div class="body">
            <g:layoutBody />
        </div>
    </body>
</html>

The key elements are the layoutHead, layoutTitle and layoutBody tag invocations:

  • layoutTitle - outputs the target page’s title

  • layoutHead - outputs the target page’s head tag contents

  • layoutBody - outputs the target page’s body tag contents

The previous example also demonstrates the pageProperty tag which can be used to inspect and return aspects of the target page.

Triggering Layouts

There are a few ways to trigger a layout. The simplest is to add a meta tag to the view:

<html>
    <head>
        <title>An Example Page</title>
        <meta name="layout" content="main" />
    </head>
    <body>This is my content!</body>
</html>

In this case a layout called grails-app/views/layouts/main.gsp will be used to layout the page. If we were to use the layout from the previous section the output would resemble this:

<html>
    <head>
        <title>An Example Page</title>
    </head>
    <body onload="">
        <div class="menu"><!--my common menu goes here--></div>
        <div class="body">
            This is my content!
        </div>
    </body>
</html>

Specifying A Layout In A Controller

Another way to specify a layout is to specify the name of the layout by assigning a value to the "layout" property in a controller. For example, if you have a controller such as:

class BookController {
    static layout = 'customer'

    def list() { ... }
}

You can create a layout called grails-app/views/layouts/customer.gsp which will be applied to all views that the BookController delegates to. The value of the "layout" property may contain a directory structure relative to the grails-app/views/layouts/ directory. For example:

class BookController {
    static layout = 'custom/customer'

    def list() { ... }
}

Views rendered from that controller would be decorated with the grails-app/views/layouts/custom/customer.gsp template.

Layout by Convention

Another way to associate layouts is to use "layout by convention". For example, if you have this controller:

class BookController {
    def list() { ... }
}

You can create a layout called grails-app/views/layouts/book.gsp, which will be applied to all views that the BookController delegates to.

Alternatively, you can create a layout called grails-app/views/layouts/book/list.gsp which will only be applied to the list action within the BookController.

If you have both the above mentioned layouts in place the layout specific to the action will take precedence when the list action is executed.

If a layout may not be located using any of those conventions, the convention of last resort is to look for the application default layout which is grails-app/views/layouts/application.gsp. The name of the application default layout may be changed by defining a property in grails-app/conf/application.groovy as follows:

grails.sitemesh.default.layout = 'myLayoutName'

With that property in place, the application default layout will be grails-app/views/layouts/myLayoutName.gsp.

Inline Layouts

Grails' also supports Sitemesh’s concept of inline layouts with the applyLayout tag. This can be used to apply a layout to a template, URL or arbitrary section of content. This lets you even further modularize your view structure by "decorating" your template includes.

Some examples of usage can be seen below:

<g:applyLayout name="myLayout" template="bookTemplate" collection="${books}" />

<g:applyLayout name="myLayout" url="http://www.google.com" />

<g:applyLayout name="myLayout">
The content to apply a layout to
</g:applyLayout>

Server-Side Includes

While the applyLayout tag is useful for applying layouts to external content, if you simply want to include external content in the current page you use the include tag:

<g:include controller="book" action="list" />

You can even combine the include tag and the applyLayout tag for added flexibility:

<g:applyLayout name="myLayout">
   <g:include controller="book" action="list" />
</g:applyLayout>

Finally, you can also call the include tag from a controller or tag library as a method:

def content = include(controller:"book", action:"list")

The resulting content will be provided via the return value of the include tag.

8.2.5 Static Resources

Grails 3 integrates with the Asset Pipeline plugin to provide sophisticated static asset management. This plugin is installed by default in new Grails applications.

The basic way to include a link to a static asset in your application is to use the resource tag. This simple approach creates a URI pointing to the file.

However modern applications with dependencies on multiple JavaScript and CSS libraries and frameworks (as well as dependencies on multiple Grails plugins) require something more powerful.

The issues that the Asset-Pipeline plugin tackles are:

  • Reduced Dependence - The plugin has compression, minification, and cache-digests built in.

  • Easy Debugging - Makes for easy debugging by keeping files separate in development mode.

  • Asset Bundling using require directives.

  • Web application performance tuning is difficult.

  • The need for a standard way to expose static assets in plugins and applications.

  • The need for extensible processing to make languages like LESS or Coffee first class citizens.

The asset-pipeline allows you to define your javascript or css requirements right at the top of the file and they get compiled on War creation.

Take a look at the documentation for the asset-pipeline to get started.

If you do not want to use the Asset-Pipeline plugin, you can serve the static assets from directories src/main/resources/public or src/main/webapp, but the latter one only gets included in WAR packaging but not in JAR packaging.

8.2.6 Sitemesh Content Blocks

Although it is useful to decorate an entire page sometimes you may find the need to decorate independent sections of your site. To do this you can use content blocks. To get started, partition the page to be decorated using the <content> tag:

<content tag="navbar">
... draw the navbar here...
</content>

<content tag="header">
... draw the header here...
</content>

<content tag="footer">
... draw the footer here...
</content>

<content tag="body">
... draw the body here...
</content>

Then within the layout you can reference these components and apply individual layouts to each:

<html>
    <body>
        <div id="header">
            <g:applyLayout name="headerLayout">
                <g:pageProperty name="page.header" />
            </g:applyLayout>
        </div>
        <div id="nav">
            <g:applyLayout name="navLayout">
                <g:pageProperty name="page.navbar" />
            </g:applyLayout>
        </div>
        <div id="body">
            <g:applyLayout name="bodyLayout">
                <g:pageProperty name="page.body" />
            </g:applyLayout>
        </div>
        <div id="footer">
            <g:applyLayout name="footerLayout">
                <g:pageProperty name="page.footer" />
            </g:applyLayout>
        </div>
    </body>
</html>

8.2.7 Making Changes to a Deployed Application

One of the main issues with deploying a Grails application (or typically any servlet-based one) is that any change to the views requires that you redeploy your whole application. If all you want to do is fix a typo on a page, or change an image link, it can seem like a lot of unnecessary work. For such simple requirements, Grails does have a solution: the grails.gsp.view.dir configuration setting.

How does this work? The first step is to decide where the GSP files should go. Let’s say we want to keep them unpacked in a /var/www/grails/my-app directory. We add these two lines to grails-app/conf/application.groovy :

grails.gsp.enable.reload = true
grails.gsp.view.dir = "/var/www/grails/my-app/"

The first line tells Grails that modified GSP files should be reloaded at runtime. If you don’t have this setting, you can make as many changes as you like but they won’t be reflected in the running application until you restart. The second line tells Grails where to load the views and layouts from.

The trailing slash on the grails.gsp.view.dir value is important! Without it, Grails will look for views in the parent directory.

Setting "grails.gsp.view.dir" is optional. If it’s not specified, you can update files directly to the application server’s deployed war directory. Depending on the application server, these files might get overwritten when the server is restarted. Most application servers support "exploded war deployment" which is recommended in this case.

With those settings in place, all you need to do is copy the views from your web application to the external directory. On a Unix-like system, this would look something like this:

mkdir -p /var/www/grails/my-app/grails-app/views
cp -R grails-app/views/* /var/www/grails/my-app/grails-app/views

The key point here is that you must retain the view directory structure, including the grails-app/views bit. So you end up with the path /var/www/grails/my-app/grails-app/views/…​ .

One thing to bear in mind with this technique is that every time you modify a GSP, it uses up permgen space. So at some point you will eventually hit "out of permgen space" errors unless you restart the server. So this technique is not recommended for frequent or large changes to the views.

There are also some System properties to control GSP reloading:

Name Description Default

grails.gsp.enable.reload

alternative system property for enabling the GSP reload mode without changing application.groovy

grails.gsp.reload.interval

interval between checking the lastmodified time of the gsp source file

unit is milliseconds

5000

grails.gsp.reload.granularity

the number of milliseconds leeway to give before deciding a file is out of date. this is needed because different roundings usually cause a 1000ms difference in lastmodified times

1000

GSP reloading is supported for precompiled GSPs since Grails 1.3.5 .

8.2.8 GSP Debugging

Viewing the generated source code

  • Adding "?showSource=true" or "&showSource=true" to the url shows the generated Groovy source code for the view instead of rendering it. It won’t show the source code of included templates. This only works in development mode

  • The saving of all generated source code can be activated by setting the property "grails.views.gsp.keepgenerateddir" (in application.groovy) . It must point to a directory that exists and is writable.

  • During "grails war" gsp pre-compilation, the generated source code is stored in grails.project.work.dir/gspcompile (usually in ~/.grails/(grails_version)/projects/(project name)/gspcompile).

Debugging GSP code with a debugger

Viewing information about templates used to render a single url

GSP templates are reused in large web applications by using the g:render taglib. Several small templates can be used to render a single page. It might be hard to find out what GSP template actually renders the html seen in the result. The debug templates -feature adds html comments to the output. The comments contain debug information about gsp templates used to render the page.

Usage is simple: append "?debugTemplates" or "&debugTemplates" to the url and view the source of the result in your browser. "debugTemplates" is restricted to development mode. It won’t work in production.

Here is an example of comments added by debugTemplates :

<!-- GSP #2 START template: /home/.../views/_carousel.gsp
     precompiled: false lastmodified: ... -->
.
.
.
<!-- GSP #2 END template: /home/.../views/_carousel.gsp
     rendering time: 115 ms -->

Each comment block has a unique id so that you can find the start & end of each template call.

8.3 Tag Libraries

Like Java Server Pages (JSP), GSP supports the concept of custom tag libraries. Unlike JSP, Grails' tag library mechanism is simple, elegant and completely reloadable at runtime.

Quite simply, to create a tag library create a Groovy class that ends with the convention TagLib and place it within the grails-app/taglib directory:

class SimpleTagLib {

}

Now to create a tag create a Closure property that takes two arguments: the tag attributes and the body content:

class SimpleTagLib {
    def simple = { attrs, body ->

    }
}

The attrs argument is a Map of the attributes of the tag, whilst the body argument is a Closure that returns the body content when invoked:

class SimpleTagLib {
    def emoticon = { attrs, body ->
       out << body() << (attrs.happy == 'true' ? " :-)" : " :-(")
    }
}

As demonstrated above there is an implicit out variable that refers to the output Writer which you can use to append content to the response. Then you can reference the tag inside your GSP; no imports are necessary:

<g:emoticon happy="true">Hi John</g:emoticon>
To help IDEs like Spring Tool Suite (STS) and others autocomplete tag attributes, you should add Javadoc comments to your tag closures with @attr descriptions. Since taglibs use Groovy code it can be difficult to reliably detect all usable attributes.

For example:

class SimpleTagLib {

    /**
     * Renders the body with an emoticon.
     *
     * @attr happy whether to show a happy emoticon ('true') or
     * a sad emoticon ('false')
     */
    def emoticon = { attrs, body ->
       out << body() << (attrs.happy == 'true' ? " :-)" : " :-(")
    }
}

and any mandatory attributes should include the REQUIRED keyword, e.g.

class SimpleTagLib {

    /**
     * Creates a new password field.
     *
     * @attr name REQUIRED the field name
     * @attr value the field value
     */
    def passwordField = { attrs ->
        attrs.type = "password"
        attrs.tagName = "passwordField"
        fieldImpl(out, attrs)
    }
}

8.3.1 Variables and Scopes

Within the scope of a tag library there are a number of pre-defined variables including:

  • actionName - The currently executing action name

  • controllerName - The currently executing controller name

  • flash - The flash object

  • grailsApplication - The GrailsApplication instance

  • out - The response writer for writing to the output stream

  • pageScope - A reference to the pageScope object used for GSP rendering (i.e. the binding)

  • params - The params object for retrieving request parameters

  • pluginContextPath - The context path to the plugin that contains the tag library

  • request - The HttpServletRequest instance

  • response - The HttpServletResponse instance

  • servletContext - The ServletContext instance

  • session - The HttpSession instance

8.3.2 Simple Tags

As demonstrated in the previous example it is easy to write simple tags that have no body and just output content. Another example is a dateFormat style tag:

def dateFormat = { attrs, body ->
    out << new java.text.SimpleDateFormat(attrs.format).format(attrs.date)
}

The above uses Java’s SimpleDateFormat class to format a date and then write it to the response. The tag can then be used within a GSP as follows:

<g:dateFormat format="dd-MM-yyyy" date="${new Date()}" />

With simple tags sometimes you need to write HTML mark-up to the response. One approach would be to embed the content directly:

def formatBook = { attrs, body ->
    out << "<div id=\"${attrs.book.id}\">"
    out << "Title : ${attrs.book.title}"
    out << "</div>"
}

Although this approach may be tempting it is not very clean. A better approach would be to reuse the render tag:

def formatBook = { attrs, body ->
    out << render(template: "bookTemplate", model: [book: attrs.book])
}

And then have a separate GSP template that does the actual rendering.

8.3.3 Logical Tags

You can also create logical tags where the body of the tag is only output once a set of conditions have been met. An example of this may be a set of security tags:

def isAdmin = { attrs, body ->
    def user = attrs.user
    if (user && checkUserPrivs(user)) {
        out << body()
    }
}

The tag above checks if the user is an administrator and only outputs the body content if he/she has the correct set of access privileges:

<g:isAdmin user="${myUser}">
    // some restricted content
</g:isAdmin>

8.3.4 Iterative Tags

Iterative tags are easy too, since you can invoke the body multiple times:

def repeat = { attrs, body ->
    attrs.times?.toInteger()?.times { num ->
        out << body(num)
    }
}

In this example we check for a times attribute and if it exists convert it to a number, then use Groovy’s times method to iterate the specified number of times:

<g:repeat times="3">
<p>Repeat this 3 times! Current repeat = ${it}</p>
</g:repeat>

Notice how in this example we use the implicit it variable to refer to the current number. This works because when we invoked the body we passed in the current value inside the iteration:

out << body(num)

That value is then passed as the default variable it to the tag. However, if you have nested tags this can lead to conflicts, so you should instead name the variables that the body uses:

def repeat = { attrs, body ->
    def var = attrs.var ?: "num"
    attrs.times?.toInteger()?.times { num ->
        out << body((var):num)
    }
}

Here we check if there is a var attribute and if there is use that as the name to pass into the body invocation on this line:

out << body((var):num)
Take notice to the usage of the parenthesis around the variable name. If you omit these Groovy assumes you are using a String key and not referring to the variable itself.

Now we can change the usage of the tag as follows:

<g:repeat times="3" var="j">
<p>Repeat this 3 times! Current repeat = ${j}</p>
</g:repeat>

Notice how we use the var attribute to define the name of the variable j and then we are able to reference that variable within the body of the tag.

8.3.5 Tag Namespaces

By default, tags are added to the default Grails namespace and are used with the g: prefix in GSP pages. However, you can specify a different namespace by adding a static property to your TagLib class:

class SimpleTagLib {
    static namespace = "my"

    def example = { attrs ->
        ...
    }
}

Here we have specified a namespace of my and hence the tags in this tag lib must then be referenced from GSP pages like this:

<my:example name="..." />

where the prefix is the same as the value of the static namespace property. Namespaces are particularly useful for plugins.

Tags within namespaces can be invoked as methods using the namespace as a prefix to the method call:

out << my.example(name:"foo")

This works from GSP, controllers or tag libraries

8.3.6 Using JSP Tag Libraries

In addition to the simplified tag library mechanism provided by GSP, you can also use JSP tags from GSP. To do so simply declare the JSP to use with the taglib directive:

<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>

Besides this you have to configure Grails to scan for the JSP tld files. This is configured with the grails.gsp.tldScanPattern setting. It accepts a comma separated String value. Spring’s PathMatchingResourcePatternResolver is used to resolve the patterns.

For example you could scan for all available tld files by adding this to application.groovy:

grails.gsp.tldScanPattern='classpath*:/META-INF/*.tld,/WEB-INF/tld/*.tld'

JSTL standard library is no more added as a dependency by default. In case you are using JSTL, you should also add these dependencies to build.gradle:

runtime 'javax.servlet:jstl:1.1.2'
        runtime 'taglibs:standard:1.1.2'

Then you can use JSP tags like any other tag:

<fmt:formatNumber value="${10}" pattern=".00"/>

With the added bonus that you can invoke JSP tags like methods:

${fmt.formatNumber(value:10, pattern:".00")}

8.3.7 Tag return value

A taglib can be used in a GSP as an ordinary tag or it might be used as a function in other taglibs or GSP expressions.

Internally Grails intercepts calls to taglib closures. The "out" that is available in a taglib is mapped to a java.io.Writer implementation that writes to a buffer that "captures" the output of the taglib call. This buffer is the return value of a tag library call when it’s used as a function.

If the tag is listed in the library’s static returnObjectForTags array, then its return value will written to the output when it’s used as a normal tag. The return value of the tag lib closure will be returned as-is if it’s used as a function in GSP expressions or other taglibs.

If the tag is not included in the returnObjectForTags array, then its return value will be discarded. Using "out" to write output in returnObjectForTags is not supported.

Example:

class ObjectReturningTagLib {
    static namespace = "cms"
    static returnObjectForTags = ['content']

    def content = { attrs, body ->
        CmsContent.findByCode(attrs.code)?.content
    }
}

Given this example cms.content(code:'something') call in another taglib or GSP expression would return the value "CmsContent.content" directly to the caller without wrapping the return value in a buffer. It might be worth doing so also because of performance optimization reasons. There is no need to wrap the tag return value in an output buffer in such cases.

8.4 URL Mappings

Throughout the documentation so far the convention used for URLs has been the default of /controller/action/id. However, this convention is not hard wired into Grails and is in fact controlled by a URL Mappings class located at grails-app/controllers/mypackage/UrlMappings.groovy.

The UrlMappings class contains a single property called mappings that has been assigned a block of code:

package mypackage

class UrlMappings {
    static mappings = {
    }
}

8.4.1 Mapping to Controllers and Actions

To create a simple mapping simply use a relative URL as the method name and specify named parameters for the controller and action to map to:

"/product"(controller: "product", action: "list")

In this case we’ve mapped the URL /product to the list action of the ProductController. Omit the action definition to map to the default action of the controller:

"/product"(controller: "product")

An alternative syntax is to assign the controller and action to use within a block passed to the method:

"/product" {
    controller = "product"
    action = "list"
}

Which syntax you use is largely dependent on personal preference.

If you have mappings that all fall under a particular path you can group mappings with the group method:

group "/product", {
    "/apple"(controller:"product", id:"apple")
    "/htc"(controller:"product", id:"htc")
}

You can also create nested group url mappings:

group "/store", {
    group "/product", {
        "/$id"(controller:"product")
    }
}

To rewrite one URI onto another explicit URI (rather than a controller/action pair) do something like this:

"/hello"(uri: "/hello.dispatch")

Rewriting specific URIs is often useful when integrating with other frameworks.

8.4.2 Mapping to REST resources

Since Grails 2.3, it possible to create RESTful URL mappings that map onto controllers by convention. The syntax to do so is as follows:

"/books"(resources:'book')

You define a base URI and the name of the controller to map to using the resources parameter. The above mapping will result in the following URLs:

HTTP Method URI Grails Action

GET

/books

index

GET

/books/create

create

POST

/books

save

GET

/books/${id}

show

GET

/books/${id}/edit

edit

PUT

/books/${id}

update

DELETE

/books/${id}

delete

If you are not sure which mapping will be generated for your case just run the command url-mappings-report in your grails console. It will give you a really neat report for all the url mappings.

If you wish to include or exclude any of the generated URL mappings you can do so with the includes or excludes parameter, which accepts the name of the Grails action to include or exclude:

"/books"(resources:'book', excludes:['delete', 'update'])

or

"/books"(resources:'book', includes:['index', 'show'])

Explicit REST Mappings

As of Grails 3.1, if you prefer not to rely on a resources mapping to define your mappings then you can prefix any URL mapping with the HTTP method name (in lower case) to indicate the HTTP method it applies to. The following URL mapping:

"/books"(resources:'book')

Is equivalent to:

get "/books"(controller:"book", action:"index")
get "/books/create"(controller:"book", action:"create")
post "/books"(controller:"book", action:"save")
get "/books/$id"(controller:"book", action:"show")
get "/books/$id/edit"(controller:"book", action:"edit")
put "/books/$id"(controller:"book", action:"update")
delete "/books/$id"(controller:"book", action:"delete")

Notice how the HTTP method name is prefixed prior to each URL mapping definition.

Single resources

A single resource is a resource for which there is only one (possibly per user) in the system. You can create a single resource using the resource parameter (as opposed to resources):

"/book"(resource:'book')

This results in the following URL mappings:

HTTP Method URI Grails Action

GET

/book/create

create

POST

/book

save

GET

/book

show

GET

/book/edit

edit

PUT

/book

update

DELETE

/book

delete

The main difference is that the id is not included in the URL mapping.

Nested Resources

You can nest resource mappings to generate child resources. For example:

"/books"(resources:'book') {
  "/authors"(resources:"author")
}

The above will result in the following URL mappings:

HTTP Method URL Grails Action

GET

/books/${bookId}/authors

index

GET

/books/${bookId}/authors/create

create

POST

/books/${bookId}/authors

save

GET

/books/${bookId}/authors/${id}

show

GET

/books/${bookId}/authors/edit/${id}

edit

PUT

/books/${bookId}/authors/${id}

update

DELETE

/books/${bookId}/authors/${id}

delete

You can also nest regular URL mappings within a resource mapping:

"/books"(resources: "book") {
    "/publisher"(controller:"publisher")
}

This will result in the following URL being available:

HTTP Method URL Grails Action

GET

/books/1/publisher

index

To map a URI directly below a resource then use a collection:

"/books"(resources: "book") {
    collection {
        "/publisher"(controller:"publisher")
    }
}

This will result in the following URL being available (without the ID):

HTTP Method URL Grails Action

GET

/books/publisher

index

Linking to RESTful Mappings

You can link to any URL mapping created with the g:link tag provided by Grails simply by referencing the controller and action to link to:

<g:link controller="book" action="index">My Link</g:link>

As a convenience you can also pass a domain instance to the resource attribute of the link tag:

<g:link resource="${book}">My Link</g:link>

This will automatically produce the correct link (in this case "/books/1" for an id of "1").

The case of nested resources is a little different as they typically required two identifiers (the id of the resource and the one it is nested within). For example given the nested resources:

"/books"(resources:'book') {
  "/authors"(resources:"author")
}

If you wished to link to the show action of the author controller, you would write:

// Results in /books/1/authors/2
<g:link controller="author" action="show" method="GET" params="[bookId:1]" id="2">The Author</g:link>

However, to make this more concise there is a resource attribute to the link tag which can be used instead:

// Results in /books/1/authors/2
<g:link resource="book/author" action="show" bookId="1" id="2">My Link</g:link>

The resource attribute accepts a path to the resource separated by a slash (in this case "book/author"). The attributes of the tag can be used to specify the necessary bookId parameter.

8.4.3 Redirects In URL Mappings

Since Grails 2.3, it is possible to define URL mappings which specify a redirect. When a URL mapping specifies a redirect, any time that mapping matches an incoming request, a redirect is initiated with information provided by the mapping.

When a URL mapping specifies a redirect the mapping must either supply a String representing a URI to redirect to or must provide a Map representing the target of the redirect. That Map is structured just like the Map that may be passed as an argument to the redirect method in a controller.

"/viewBooks"(redirect: '/books/list')
"/viewAuthors"(redirect: [controller: 'author', action: 'list'])
"/viewPublishers"(redirect: [controller: 'publisher', action: 'list', permanent: true])

Request parameters that were part of the original request will be included in the redirect.

8.4.4 Embedded Variables

Simple Variables

The previous section demonstrated how to map simple URLs with concrete "tokens". In URL mapping speak tokens are the sequence of characters between each slash, '/'. A concrete token is one which is well defined such as as /product. However, in many circumstances you don’t know what the value of a particular token will be until runtime. In this case you can use variable placeholders within the URL for example:

static mappings = {
  "/product/$id"(controller: "product")
}

In this case by embedding a $id variable as the second token Grails will automatically map the second token into a parameter (available via the params object) called id. For example given the URL /product/MacBook, the following code will render "MacBook" to the response:

class ProductController {
     def index() { render params.id }
}

You can of course construct more complex examples of mappings. For example the traditional blog URL format could be mapped as follows:

static mappings = {
   "/$blog/$year/$month/$day/$id"(controller: "blog", action: "show")
}

The above mapping would let you do things like:

/graemerocher/2007/01/10/my_funky_blog_entry

The individual tokens in the URL would again be mapped into the params object with values available for year, month, day, id and so on.

Dynamic Controller and Action Names

Variables can also be used to dynamically construct the controller and action name. In fact the default Grails URL mappings use this technique:

static mappings = {
    "/$controller/$action?/$id?"()
}

Here the name of the controller, action and id are implicitly obtained from the variables controller, action and id embedded within the URL.

You can also resolve the controller name and action name to execute dynamically using a closure:

static mappings = {
    "/$controller" {
        action = { params.goHere }
    }
}

Optional Variables

Another characteristic of the default mapping is the ability to append a ? at the end of a variable to make it an optional token. In a further example this technique could be applied to the blog URL mapping to have more flexible linking:

static mappings = {
    "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show")
}

With this mapping all of these URLs would match with only the relevant parameters being populated in the params object:

/graemerocher/2007/01/10/my_funky_blog_entry
/graemerocher/2007/01/10
/graemerocher/2007/01
/graemerocher/2007
/graemerocher

Optional File Extensions

If you wish to capture the extension of a particular path, then a special case mapping exists:

"/$controller/$action?/$id?(.$format)?"()

By adding the (.$format)? mapping you can access the file extension using the response.format property in a controller:

def index() {
    render "extension is ${response.format}"
}

Arbitrary Variables

You can also pass arbitrary parameters from the URL mapping into the controller by just setting them in the block passed to the mapping:

"/holiday/win" {
     id = "Marrakech"
     year = 2007
}

This variables will be available within the params object passed to the controller.

Dynamically Resolved Variables

The hard coded arbitrary variables are useful, but sometimes you need to calculate the name of the variable based on runtime factors. This is also possible by assigning a block to the variable name:

"/holiday/win" {
     id = { params.id }
     isEligible = { session.user != null } // must be logged in
}

In the above case the code within the blocks is resolved when the URL is actually matched and hence can be used in combination with all sorts of logic.

8.4.5 Mapping to Views

You can resolve a URL to a view without a controller or action involved. For example to map the root URL / to a GSP at the location grails-app/views/index.gsp you could use:

static mappings = {
    "/"(view: "/index")  // map the root URL
}

Alternatively if you need a view that is specific to a given controller you could use:

static mappings = {
   "/help"(controller: "site", view: "help") // to a view for a controller
}

8.4.6 Mapping to Response Codes

Grails also lets you map HTTP response codes to controllers, actions or views. Just use a method name that matches the response code you are interested in:

static mappings = {
   "403"(controller: "errors", action: "forbidden")
   "404"(controller: "errors", action: "notFound")
   "500"(controller: "errors", action: "serverError")
}

Or you can specify custom error pages:

static mappings = {
   "403"(view: "/errors/forbidden")
   "404"(view: "/errors/notFound")
   "500"(view: "/errors/serverError")
}

Declarative Error Handling

In addition you can configure handlers for individual exceptions:

static mappings = {
   "403"(view: "/errors/forbidden")
   "404"(view: "/errors/notFound")
   "500"(controller: "errors", action: "illegalArgument",
         exception: IllegalArgumentException)
   "500"(controller: "errors", action: "nullPointer",
         exception: NullPointerException)
   "500"(controller: "errors", action: "customException",
         exception: MyException)
   "500"(view: "/errors/serverError")
}

With this configuration, an IllegalArgumentException will be handled by the illegalArgument action in ErrorsController, a NullPointerException will be handled by the nullPointer action, and a MyException will be handled by the customException action. Other exceptions will be handled by the catch-all rule and use the /errors/serverError view.

You can access the exception from your custom error handing view or controller action using the request’s exception attribute like so:

class ErrorController {
    def handleError() {
        def exception = request.exception
        // perform desired processing to handle the exception
    }
}
If your error-handling controller action throws an exception as well, you’ll end up with a StackOverflowException.

8.4.7 Mapping to HTTP methods

URL mappings can also be configured to map based on the HTTP method (GET, POST, PUT or DELETE). This is very useful for RESTful APIs and for restricting mappings based on HTTP method.

As an example the following mappings provide a RESTful API URL mappings for the ProductController:

static mappings = {
   "/product/$id"(controller:"product", action: "update", method: "PUT")
}

Note that if you specify a HTTP method other than GET in your URL mapping, you also have to specify it when creating the corresponding link by passing the method argument to g:link or g:createLink to get a link of the desired format.

8.4.8 Mapping Wildcards

Grails' URL mappings mechanism also supports wildcard mappings. For example consider the following mapping:

static mappings = {
    "/images/*.jpg"(controller: "image")
}

This mapping will match all paths to images such as /image/logo.jpg. Of course you can achieve the same effect with a variable:

static mappings = {
    "/images/$name.jpg"(controller: "image")
}

However, you can also use double wildcards to match more than one level below:

static mappings = {
    "/images/**.jpg"(controller: "image")
}

In this cases the mapping will match /image/logo.jpg as well as /image/other/logo.jpg. Even better you can use a double wildcard variable:

static mappings = {
    // will match /image/logo.jpg and /image/other/logo.jpg
    "/images/$name**.jpg"(controller: "image")
}

In this case it will store the path matched by the wildcard inside a name parameter obtainable from the params object:

def name = params.name
println name // prints "logo" or "other/logo"

If you use wildcard URL mappings then you may want to exclude certain URIs from Grails' URL mapping process. To do this you can provide an excludes setting inside the UrlMappings.groovy class:

class UrlMappings {
    static excludes = ["/images/*", "/css/*"]
    static mappings = {
        ...
    }
}

In this case Grails won’t attempt to match any URIs that start with /images or /css.

8.4.9 Automatic Link Re-Writing

Another great feature of URL mappings is that they automatically customize the behaviour of the link tag so that changing the mappings don’t require you to go and change all of your links.

This is done through a URL re-writing technique that reverse engineers the links from the URL mappings. So given a mapping such as the blog one from an earlier section:

static mappings = {
   "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show")
}

If you use the link tag as follows:

<g:link controller="blog" action="show"
        params="[blog:'fred', year:2007]">
    My Blog
</g:link>

<g:link controller="blog" action="show"
        params="[blog:'fred', year:2007, month:10]">
    My Blog - October 2007 Posts
</g:link>

Grails will automatically re-write the URL in the correct format:

<a href="/fred/2007">My Blog</a>
<a href="/fred/2007/10">My Blog - October 2007 Posts</a>

8.4.10 Applying Constraints

URL Mappings also support Grails' unified validation constraints mechanism, which lets you further "constrain" how a URL is matched. For example, if we revisit the blog sample code from earlier, the mapping currently looks like this:

static mappings = {
   "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show")
}

This allows URLs such as:

/graemerocher/2007/01/10/my_funky_blog_entry

However, it would also allow:

/graemerocher/not_a_year/not_a_month/not_a_day/my_funky_blog_entry

This is problematic as it forces you to do some clever parsing in the controller code. Luckily, URL Mappings can be constrained to further validate the URL tokens:

"/$blog/$year?/$month?/$day?/$id?" {
     controller = "blog"
     action = "show"
     constraints {
          year(matches:/\\\d{4}/)
          month(matches:/\\\d{2}/)
          day(matches:/\\\d{2}/)
     }
}

In this case the constraints ensure that the year, month and day parameters match a particular valid pattern thus relieving you of that burden later on.

8.4.11 Named URL Mappings

URL Mappings also support named mappings, that is mappings which have a name associated with them. The name may be used to refer to a specific mapping when links are generated.

The syntax for defining a named mapping is as follows:

static mappings = {
   name <mapping name>: <url pattern> {
      // ...
   }
}

For example:

static mappings = {
    name personList: "/showPeople" {
        controller = 'person'
        action = 'list'
    }
    name accountDetails: "/details/$acctNumber" {
        controller = 'product'
        action = 'accountDetails'
    }
}

The mapping may be referenced in a link tag in a GSP.

<g:link mapping="personList">List People</g:link>

That would result in:

<a href="/showPeople">List People</a>

Parameters may be specified using the params attribute.

<g:link mapping="accountDetails" params="[acctNumber:'8675309']">
    Show Account
</g:link>

That would result in:

<a href="/details/8675309">Show Account</a>

Alternatively you may reference a named mapping using the link namespace.

<link:personList>List People</link:personList>

That would result in:

<a href="/showPeople">List People</a>

The link namespace approach allows parameters to be specified as attributes.

<link:accountDetails acctNumber="8675309">Show Account</link:accountDetails>

That would result in:

<a href="/details/8675309">Show Account</a>

To specify attributes that should be applied to the generated href, specify a Map value to the attrs attribute. These attributes will be applied directly to the href, not passed through to be used as request parameters.

<link:accountDetails attrs="[class: 'fancy']" acctNumber="8675309">
    Show Account
</link:accountDetails>

That would result in:

<a href="/details/8675309" class="fancy">Show Account</a>

8.4.12 Customizing URL Formats

The default URL Mapping mechanism supports camel case names in the URLs. The default URL for accessing an action named addNumbers in a controller named MathHelperController would be something like /mathHelper/addNumbers. Grails allows for the customization of this pattern and provides an implementation which replaces the camel case convention with a hyphenated convention that would support URLs like /math-helper/add-numbers. To enable hyphenated URLs assign a value of "hyphenated" to the grails.web.url.converter property in grails-app/conf/application.groovy.

grails-app/conf/application.groovy
grails.web.url.converter = 'hyphenated'

Arbitrary strategies may be plugged in by providing a class which implements the UrlConverter interface and adding an instance of that class to the Spring application context with the bean name of grails.web.UrlConverter.BEAN_NAME. If Grails finds a bean in the context with that name, it will be used as the default converter and there is no need to assign a value to the grails.web.url.converter config property.

src/main/groovy/com/myapplication/MyUrlConverterImpl.groovy
package com.myapplication

class MyUrlConverterImpl implements grails.web.UrlConverter {

    String toUrlElement(String propertyOrClassName) {
        // return some representation of a property or class name that should be used in URLs...
    }
}
grails-app/conf/spring/resources.groovy
beans = {
    "${grails.web.UrlConverter.BEAN_NAME}"(com.myapplication.MyUrlConverterImpl)
}

8.4.13 Namespaced Controllers

If an application defines multiple controllers with the same name in different packages, the controllers must be defined in a namespace. The way to define a namespace for a controller is to define a static property named namespace in the controller and assign a String to the property that represents the namespace.

grails-app/controllers/com/app/reporting/AdminController.groovy
package com.app.reporting

class AdminController {

    static namespace = 'reports'

    // ...
}
grails-app/controllers/com/app/security/AdminController.groovy
package com.app.security

class AdminController {

    static namespace = 'users'

    // ...
}

When defining url mappings which should be associated with a namespaced controller, the namespace variable needs to be part of the URL mapping.

grails-app/controllers/UrlMappings.groovy
class UrlMappings {

    static mappings = {
        '/userAdmin' {
            controller = 'admin'
            namespace = 'users'
        }

        '/reportAdmin' {
            controller = 'admin'
            namespace = 'reports'
        }

        "/$namespace/$controller/$action?"()
    }
}

Reverse URL mappings also require that the namespace be specified.

<g:link controller="admin" namespace="reports">Click For Report Admin</g:link>
<g:link controller="admin" namespace="users">Click For User Admin</g:link>

When resolving a URL mapping (forward or reverse) to a namespaced controller, a mapping will only match if the namespace has been provided. If the application provides several controllers with the same name in different packages, at most 1 of them may be defined without a namespace property. If there are multiple controllers with the same name that do not define a namespace property, the framework will not know how to distinguish between them for forward or reverse mapping resolutions.

It is allowed for an application to use a plugin which provides a controller with the same name as a controller provided by the application and for neither of the controllers to define a namespace property as long as the controllers are in separate packages. For example, an application may include a controller named com.accounting.ReportingController and the application may use a plugin which provides a controller named com.humanresources.ReportingController. The only issue with that is the URL mapping for the controller provided by the plugin needs to be explicit in specifying that the mapping applies to the ReportingController which is provided by the plugin.

See the following example.

static mappings = {
    "/accountingReports" {
        controller = "reporting"
    }
    "/humanResourceReports" {
        controller = "reporting"
        plugin = "humanResources"
    }
}

With that mapping in place, a request to /accountingReports will be handled by the ReportingController which is defined in the application. A request to /humanResourceReports will be handled by the ReportingController which is provided by the humanResources plugin.

There could be any number of ReportingController controllers provided by any number of plugins but no plugin may provide more than one ReportingController even if they are defined in separate packages.

Assigning a value to the plugin variable in the mapping is only required if there are multiple controllers with the same name available at runtime provided by the application and/or plugins. If the humanResources plugin provides a ReportingController and there is no other ReportingController available at runtime, the following mapping would work.

static mappings = {
    "/humanResourceReports" {
        controller = "reporting"
    }
}

It is best practice to be explicit about the fact that the controller is being provided by a plugin.

8.5 CORS

Spring Boot provides CORS support out of the box, but it is difficult to configure in a Grails application due to the way UrlMappings are used instead of annotations that define URLs. Starting with Grails 3.2.1, we have added a way to configure CORS that makes sense in a Grails application.

Once enabled, the default setting is "wide open".

application.yml
grails:
    cors:
        enabled: true

That will produce a mapping to all urls /** with:

allowedOrigins

['*']

allowedMethods

['*']

allowedHeaders

['*']

exposedHeaders

null

maxAge

1800

allowCredentials

true

Some of these settings come directly from Spring Boot and can change in future versions.

All of those settings can be easily overridden.

application.yml
grails:
    cors:
        enabled: true
        allowedOrigins: [http://localhost:5000]

In the example above, the allowedOrigins setting will replace [*].

You can also configure different URLs.

application.yml
grails:
    cors:
        enabled: true
        allowedHeaders: [Content-Type]
        mappings:
            /api/**:
                allowedOrigins: [http://localhost:5000]
                //Other configurations not specified default to the global config
Specifying at least one mapping will disable the creation of the global mapping (/**). If you wish to keep that setting, you should specify it along with your other mappings.

The settings above will produce a single mapping of /api/** with the following settings:

allowedOrigins

['http://localhost:5000']

allowedMethods

['*']

allowedHeaders

['Content-Type']

exposedHeaders

null

maxAge

1800

allowCredentials

true

If you don’t wish to override any of the default settings, but only want to specify URLs, you can do so like this example:

application.yml
grails:
    cors:
        enabled: true
        mappings:
            /api/**: {}

8.6 Interceptors

Grails provides standalone Interceptors using the create-interceptor command:

$ grails create-interceptor MyInterceptor

The above command will create an Interceptor in the grails-app/controllers directory with the following default contents:

class MyInterceptor {

  boolean before() { true }

  boolean after() { true }

  void afterView() {
    // no-op
  }

}

Interceptors vs Filters

In versions of Grails prior to Grails 3.0, Grails supported the notion of filters. These are still supported for backwards compatibility but are considered deprecated.

The new interceptors concept in Grails 3.0 is superior in a number of ways, most significantly interceptors can use Groovy’s CompileStatic annotation to optimize performance (something which is often critical as interceptors can be executed for every request.)

8.6.1 Defining Interceptors

By default interceptors will match the controllers with the same name. For example if you have an interceptor called BookInterceptor then all requests to the actions of the BookController will trigger the interceptor.

An Interceptor implements the Interceptor trait and provides 3 methods that can be used to intercept requests:

/**
 * Executed before a matched action
 *
 * @return Whether the action should continue and execute
 */
boolean before() { true }

/**
 * Executed after the action executes but prior to view rendering
 *
 * @return True if view rendering should continue, false otherwise
 */
boolean after() { true }

/**
 * Executed after view rendering completes
 */
void afterView() {}

As described above the before method is executed prior to an action and can cancel the execution of the action by returning false.

The after method is executed after an action executes and can halt view rendering if it returns false. The after method can also modify the view or model using the view and model properties respectively:

boolean after() {
  model.foo = "bar" // add a new model attribute called 'foo'
  view = 'alternate' // render a different view called 'alternate'
  true
}

The afterView method is executed after view rendering completes. If an exception occurs, the exception is available using the throwable property of the Interceptor trait.

8.6.2 Matching Requests with Inteceptors

As mention in the previous section, by default an interceptor will match only requests to the associated controller by convention. However you can configure the interceptor to match any request using the match or matchAll methods defined in the Interceptor API.

The matching methods return a Matcher instance which can be used to configure how the interceptor matches the request.

For example the following interceptor will match all requests except those to the login controller:

class AuthInterceptor {
  AuthInterceptor() {
    matchAll()
    .excludes(controller:"login")
  }

  boolean before() {
    // perform authentication
  }
}

You can also perform matching using named argument:

class LoggingInterceptor {
  LoggingInterceptor() {
    match(controller:"book", action:"show") // using strings
    match(controller: ~/(author|publisher)/) // using regex
  }

  boolean before() {
    ...
  }
}

You can use any number of matchers defined in your interceptor. They will be executed in the order in which they have been defined. For example the above interceptor will match for all of the following:

  • when the show action of BookController is called

  • when AuthorController or PublisherController is called

All named arguments except for uri accept either a String or a Regex expression. The uri argument supports a String path that is compatible with Spring’s AntPathMatcher. The possible named arguments are:

  • namespace - The namespace of the controller

  • controller - The name of the controller

  • action - The name of the action

  • method - The HTTP method

  • uri - The URI of the request. If this argument is used then all other arguments will be ignored and only this will be used.

8.6.3 Ordering Interceptor Execution

Interceptors can be ordered by defining an order property that defines a priority.

For example:

class AuthInterceptor {

  int order = HIGHEST_PRECEDENCE

  ...
}

The default value of the order property is 0.

The values HIGHEST_PRECEDENCE and LOWEST_PRECEDENCE can be used to define filters that should should run first or last respectively.

Note that if you write an interceptor that is to be used by others it is better increment or decrement the HIGHEST_PRECEDENCE and LOWEST_PRECEDENCE to allow other interceptors to be inserted before or after the interceptor you are authoring:

int order = HIGHEST_PRECEDENCE + 50

// or

int order = LOWEST_PRECEDENCE - 50

To find out the computed order of interceptors you can add a debug logger to logback.groovy as follows:

logger 'grails.artefact.Interceptor', DEBUG, ['STDOUT'], false

You can override any interceptors default order by using bean override configuration in grails-app/conf/application.yml:

beans:
  authInterceptor:
    order: 50

Or in grails-app/conf/application.groovy:

beans {
  authInterceptor {
    order = 50
  }
}

Thus giving you complete control over interceptor execution order.

8.7 Content Negotiation

Grails has built in support for Content negotiation using either the HTTP Accept header, an explicit format request parameter or the extension of a mapped URI.

Configuring Mime Types

Before you can start dealing with content negotiation you need to tell Grails what content types you wish to support. By default Grails comes configured with a number of different content types within grails-app/conf/application.yml using the grails.mime.types setting:

grails:
    mime:
        types:
            all: '*/*'
            atom: application/atom+xml
            css: text/css
            csv: text/csv
            form: application/x-www-form-urlencoded
            html:
              - text/html
              - application/xhtml+xml
            js: text/javascript
            json:
              - application/json
              - text/json
            multipartForm: multipart/form-data
            rss: application/rss+xml
            text: text/plain
            hal:
              - application/hal+json
              - application/hal+xml
            xml:
              - text/xml
              - application/xml

The setting can also be done in grails-app/conf/application.groovy as shown below:

grails.mime.types = [ // the first one is the default format
    all:           '*/*', // 'all' maps to '*' or the first available format in withFormat
    atom:          'application/atom+xml',
    css:           'text/css',
    csv:           'text/csv',
    form:          'application/x-www-form-urlencoded',
    html:          ['text/html','application/xhtml+xml'],
    js:            'text/javascript',
    json:          ['application/json', 'text/json'],
    multipartForm: 'multipart/form-data',
    rss:           'application/rss+xml',
    text:          'text/plain',
    hal:           ['application/hal+json','application/hal+xml'],
    xml:           ['text/xml', 'application/xml']
]

The above bit of configuration allows Grails to detect to format of a request containing either the 'text/xml' or 'application/xml' media types as simply 'xml'. You can add your own types by simply adding new entries into the map. The first one is the default format.

Content Negotiation using the format Request Parameter

Let’s say a controller action can return a resource in a variety of formats: HTML, XML, and JSON. What format will the client get? The easiest and most reliable way for the client to control this is through a format URL parameter.

So if you, as a browser or some other client, want a resource as XML, you can use a URL like this:

http://my.domain.org/books?format=xml

The result of this on the server side is a format property on the response object with the value xml .

You can also define this parameter in the URL Mappings definition:

"/book/list"(controller:"book", action:"list") {
    format = "xml"
}

You could code your controller action to return XML based on this property, but you can also make use of the controller-specific withFormat() method:

import grails.converters.JSON
import grails.converters.XML

class BookController {

    def list() {
        def books = Book.list()

        withFormat {
            html bookList: books
            json { render books as JSON }
            xml { render books as XML }
            '*' { render books as JSON }
        }
    }
}

In this example, Grails will only execute the block inside withFormat() that matches the requested content type. So if the preferred format is html then Grails will execute the html() call only. Each 'block' can either be a map model for the corresponding view (as we are doing for 'html' in the above example) or a closure. The closure can contain any standard action code, for example it can return a model or render content directly.

When no format matches explicitly, a * (wildcard) block can be used to handle all other formats.

There is a special format, "all", that is handled differently from the explicit formats. If "all" is specified (normally this happens through the Accept header - see below), then the first block of withFormat() is executed when there isn’t a * (wildcard) block available.

You should not add an explicit "all" block. In this example, a format of "all" will trigger the html handler (html is the first block and there is no * block).

withFormat {
    html bookList: books
    json { render books as JSON }
    xml { render books as XML }
}
When using withFormat make sure it is the last call in your controller action as the return value of the withFormat method is used by the action to dictate what happens next.

Using the Accept header

Every incoming HTTP request has a special Accept header that defines what media types (or mime types) a client can "accept". In older browsers this is typically:

*/*

which simply means anything. However, newer browsers send more interesting values such as this one sent by Firefox:

text/xml, application/xml, application/xhtml+xml, text/html;q=0.9, \
    text/plain;q=0.8, image/png, */*;q=0.5

This particular accept header is unhelpful because it indicates that XML is the preferred response format whereas the user is really expecting HTML. That’s why Grails ignores the accept header by default for browsers. However, non-browser clients are typically more specific in their requirements and can send accept headers such as

application/json

As mentioned the default configuration in Grails is to ignore the accept header for browsers. This is done by the configuration setting grails.mime.disable.accept.header.userAgents, which is configured to detect the major rendering engines and ignore their ACCEPT headers. This allows Grails' content negotiation to continue to work for non-browser clients:

grails.mime.disable.accept.header.userAgents = ['Gecko', 'WebKit', 'Presto', 'Trident']

For example, if it sees the accept header above ('application/json') it will set format to json as you’d expect. And of course this works with the withFormat() method in just the same way as when the format URL parameter is set (although the URL parameter takes precedence).

An accept header of '*/\*' results in a value of all for the format property.

If the accept header is used but contains no registered content types, Grails will assume a broken browser is making the request and will set the HTML format - note that this is different from how the other content negotiation modes work as those would activate the "all" format!

Request format vs. Response format

As of Grails 2.0, there is a separate notion of the request format and the response format. The request format is dictated by the CONTENT_TYPE header and is typically used to detect if the incoming request can be parsed into XML or JSON, whilst the response format uses the file extension, format parameter or ACCEPT header to attempt to deliver an appropriate response to the client.

The withFormat available on controllers deals specifically with the response format. If you wish to add logic that deals with the request format then you can do so using a separate withFormat method available on the request:

request.withFormat {
    xml {
        // read XML
    }
    json {
        // read JSON
    }
}

Content Negotiation with URI Extensions

Grails also supports content negotiation using URI extensions. For example given the following URI:

/book/list.xml

This works as a result of the default URL Mapping definition which is:

"/$controller/$action?/$id?(.$format)?"{

Note the inclusion of the format variable in the path. If you do not wish to use content negotiation via the file extension then simply remove this part of the URL mapping:

"/$controller/$action?/$id?"{

Testing Content Negotiation

To test content negotiation in a unit or integration test (see the section on Testing) you can either manipulate the incoming request headers:

void testJavascriptOutput() {
    def controller = new TestController()
    controller.request.addHeader "Accept",
              "text/javascript, text/html, application/xml, text/xml, */*"

    controller.testAction()
    assertEquals "alert('hello')", controller.response.contentAsString
}

Or you can set the format parameter to achieve a similar effect:

void testJavascriptOutput() {
    def controller = new TestController()
    controller.params.format = 'js'

    controller.testAction()
    assertEquals "alert('hello')", controller.response.contentAsString
}

9 Traits

Overview

Grails provides a number of traits which provide access to properties and behavior that may be accessed from various Grails artefacts as well as arbitrary Groovy classes which are part of a Grails project. Many of these traits are automatically added to Grails artefact classes (like controllers and taglibs, for example) and are easy to add to other classes.

9.1 Traits Provided by Grails

Grails artefacts are automatically augmented with certain traits at compile time.

Domain Class Traits

Interceptor Trait

Tag Library Trait

Service Trait

Below is a list of other traits provided by the framework. The javadocs provide more detail about methods and properties related to each trait.

Trait Brief Description

grails.web.api.WebAttributes

Common Web Attributes

grails.web.api.ServletAttributes

Servlet API Attributes

grails.web.databinding.DataBinder

Data Binding API

grails.artefact.controller.support.RequestForwarder

Request Forwarding API

grails.artefact.controller.support.ResponseRedirector

Response Redirecting API

grails.artefact.controller.support.ResponseRenderer

Response Rendering API

grails.validation.Validateable

Validation API

9.1.1 WebAttributes Trait Example

WebAttributes is one of the traits provided by the framework. Any Groovy class may implement this trait to inherit all of the properties and behaviors provided by the trait.

src/main/groovy/demo/Helper.groovy
package demo

import grails.web.api.WebAttributes

class Helper implements WebAttributes {

    List<String> getControllerNames() {
        // There is no need to pass grailsApplication as an argument
        // or otherwise inject the grailsApplication property.  The
        // WebAttributes trait provides access to grailsApplication.
        grailsApplication.getArtefacts('Controller')*.name
    }
}

The traits are compatible with static compilation…​

src/main/groovy/demo/Helper.groovy
package demo

import grails.web.api.WebAttributes
import groovy.transform.CompileStatic

@CompileStatic
class Helper implements WebAttributes {

    List<String> getControllerNames() {
        // There is no need to pass grailsApplication as an argument
        // or otherwise inject the grailsApplication property.  The
        // WebAttributes trait provides access to grailsApplication.
        grailsApplication.getArtefacts('Controller')*.name
    }
}

10 Web Services

Web Services are all about providing a web API onto your web application and are typically implemented in either REST or SOAP.

10.1 REST

REST is not really a technology in itself, but more an architectural pattern. REST is very simple and just involves using plain XML or JSON as a communication medium, combined with URL patterns that are "representational" of the underlying system, and HTTP methods such as GET, PUT, POST and DELETE.

Each HTTP method maps to an action type. For example GET for retrieving data, POST for creating data, PUT for updating and so on.

Grails includes flexible features that make it easy to create RESTful APIs. Creating a RESTful resource can be as simple as one line of code, as demonstrated in the next section.

10.1.1 Domain classes as REST resources

The easiest way to create a RESTful API in Grails is to expose a domain class as a REST resource. This can be done by adding the grails.rest.Resource transformation to any domain class:

import grails.rest.*

@Resource(uri='/books')
class Book {

    String title

    static constraints = {
        title blank:false
    }
}

Simply by adding the Resource transformation and specifying a URI, your domain class will automatically be available as a REST resource in either XML or JSON formats. The transformation will automatically register the necessary RESTful URL mapping and create a controller called BookController.

You can try it out by adding some test data to BootStrap.groovy:

def init = { servletContext ->
    new Book(title:"The Stand").save()
    new Book(title:"The Shining").save()
}

And then hitting the URL http://localhost:8080/books/1, which will render the response like:

<?xml version="1.0" encoding="UTF-8"?>
<book id="1">
    <title>The Stand</title>
</book>

If you change the URL to http://localhost:8080/books/1.json you will get a JSON response such as:

{"id":1,"title":"The Stand"}

If you wish to change the default to return JSON instead of XML, you can do this by setting the formats attribute of the Resource transformation:

import grails.rest.*

@Resource(uri='/books', formats=['json', 'xml'])
class Book {
    ...
}

With the above example JSON will be prioritized. The list that is passed should contain the names of the formats that the resource should expose. The names of formats are defined in the grails.mime.types setting of application.groovy:

grails.mime.types = [
    ...
    json:          ['application/json', 'text/json'],
    ...
    xml:           ['text/xml', 'application/xml']
]

See the section on Configuring Mime Types in the user guide for more information.

Instead of using the file extension in the URI, you can also obtain a JSON response using the ACCEPT header. Here’s an example using the Unix curl tool:

$ curl -i -H "Accept: application/json" localhost:8080/books/1
{"id":1,"title":"The Stand"}

This works thanks to Grails' Content Negotiation features.

You can create a new resource by issuing a POST request:

$ curl -i -X POST -H "Content-Type: application/json" -d '{"title":"Along Came A Spider"}' localhost:8080/books
HTTP/1.1 201 Created
Server: Apache-Coyote/1.1
...

Updating can be done with a PUT request:

$ curl -i -X PUT -H "Content-Type: application/json" -d '{"title":"Along Came A Spider"}' localhost:8080/books/1
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
...

Finally a resource can be deleted with DELETE request:

$ curl -i -X DELETE localhost:8080/books/1
HTTP/1.1 204 No Content
Server: Apache-Coyote/1.1
...

As you can see, the Resource transformation enables all of the HTTP method verbs on the resource. You can enable only read-only capabilities by setting the readOnly attribute to true:

import grails.rest.*

@Resource(uri='/books', readOnly=true)
class Book {
    ...
}

In this case POST, PUT and DELETE requests will be forbidden.

10.1.2 Mapping to REST resources

If you prefer to keep the declaration of the URL mapping in your UrlMappings.groovy file then simply removing the uri attribute of the Resource transformation and adding the following line to UrlMappings.groovy will suffice:

"/books"(resources:"book")

Extending your API to include more end points then becomes trivial:

"/books"(resources:"book") {
    "/publisher"(controller:"publisher", method:"GET")
}

The above example will expose the URI /books/1/publisher.

A more detailed explanation on creating RESTful URL mappings can be found in the URL Mappings section of the user guide.

10.1.3 Linking to REST resources from GSP pages

The link tag offers an easy way to link to any domain class resource:

<g:link resource="${book}">My Link</g:link>

However, currently you cannot use g:link to link to the DELETE action and most browsers do not support sending the DELETE method directly.

The best way to accomplish this is to use a form submit:

<form action="/book/2" method="post">
         <input type="hidden" name="_method" value="DELETE"/>
</form>

Grails supports overriding the request method via the hidden _method parameter. This is for browser compatibility purposes. This is useful when using restful resource mappings to create powerful web interfaces. To make a link fire this type of event, perhaps capture all click events for links with a data-method attribute and issue a form submit via JavaScript.

10.1.4 Versioning REST resources

A common requirement with a REST API is to expose different versions at the same time. There are a few ways this can be achieved in Grails.

Versioning using the URI

A common approach is to use the URI to version APIs (although this approach is discouraged in favour of Hypermedia). For example, you can define the following URL mappings:

"/books/v1"(resources:"book", namespace:'v1')
"/books/v2"(resources:"book", namespace:'v2')

That will match the following controllers:

package myapp.v1

class BookController {
    static namespace = 'v1'
}

package myapp.v2

class BookController {
    static namespace = 'v2'
}

This approach has the disadvantage of requiring two different URI namespaces for your API.

Versioning with the Accept-Version header

As an alternative Grails supports the passing of an Accept-Version header from clients. For example you can define the following URL mappings:

"/books"(version:'1.0', resources:"book", namespace:'v1')
"/books"(version:'2.0', resources:"book", namespace:'v2')

Then in the client simply pass which version you need using the Accept-Version header:

$ curl -i -H "Accept-Version: 1.0" -X GET http://localhost:8080/books

Versioning using Hypermedia / Mime Types

Another approach to versioning is to use Mime Type definitions to declare the version of your custom media types (see the section on "Hypermedia as the Engine of Application State" for more information about Hypermedia concepts). For example, in application.groovy you can declare a custom Mime Type for your resource that includes a version parameter (the 'v' parameter):

grails.mime.types = [
    all: '*/*',
    book: "application/vnd.books.org.book+json;v=1.0",
    bookv2: "application/vnd.books.org.book+json;v=2.0",
    ...
}
It is critical that place your new mime types after the 'all' Mime Type because if the Content Type of the request cannot be established then the first entry in the map is used for the response. If you have your new Mime Type at the top then Grails will always try and send back your new Mime Type if the requested Mime Type cannot be established.

Then override the renderer (see the section on "Customizing Response Rendering" for more information on custom renderers) to send back the custom Mime Type in grails-app/conf/spring/resourses.groovy:

import grails.rest.render.json.*
import grails.web.mime.*

beans = {
    bookRendererV1(JsonRenderer, myapp.v1.Book, new MimeType("application/vnd.books.org.book+json", [v:"1.0"]))
    bookRendererV2(JsonRenderer, myapp.v2.Book, new MimeType("application/vnd.books.org.book+json", [v:"2.0"]))
}

Then update the list of acceptable response formats in your controller:

class BookController extends RestfulController {
    static responseFormats = ['json', 'xml', 'book', 'bookv2']

    // ...
}

Then using the Accept header you can specify which version you need using the Mime Type:

$ curl -i -H "Accept: application/vnd.books.org.book+json;v=1.0" -X GET http://localhost:8080/books

10.1.5 Implementing REST controllers

The Resource transformation is a quick way to get started, but typically you’ll want to customize the controller logic, the rendering of the response or extend the API to include additional actions.

10.1.5.1 Extending the RestfulController super class

The easiest way to get started doing so is to create a new controller for your resource that extends the grails.rest.RestfulController super class. For example:

class BookController extends RestfulController {
    static responseFormats = ['json', 'xml']
    BookController() {
        super(Book)
    }
}

To customize any logic you can just override the appropriate action. The following table provides the names of the action names and the URIs they map to:

HTTP Method URI Controller Action

GET

/books

index

GET

/books/create

create

POST

/books

save

GET

/books/${id}

show

GET

/books/${id}/edit

edit

PUT

/books/${id}

update

DELETE

/books/${id}

delete

The create and edit actions are only needed if the controller exposes an HTML interface.

As an example, if you have a nested resource then you would typically want to query both the parent and the child identifiers. For example, given the following URL mapping:

"/authors"(resources:'author') {
    "/books"(resources:'book')
}

You could implement the nested controller as follows:

class BookController extends RestfulController {
    static responseFormats = ['json', 'xml']
    BookController() {
        super(Book)
    }

    @Override
    protected Book queryForResource(Serializable id) {
        Book.where {
            id == id && author.id = params.authorId
        }.find()
    }

}

The example above subclasses RestfulController and overrides the protected queryForResource method to customize the query for the resource to take into account the parent resource.

Customizing Data Binding In A RestfulController Subclass

The RestfulController class contains code which does data binding for actions like save and update. The class defines a getObjectToBind() method which returns a value which will be used as the source for data binding. For example, the update action does something like this…​

class RestfulController<T> {

    def update() {
        T instance = // retrieve instance from the database...

        instance.properties = getObjectToBind()

        // ...
    }

    // ...
}

By default the getObjectToBind() method returns the request object. When the request object is used as the binding source, if the request has a body then the body will be parsed and its contents will be used to do the data binding, otherwise the request parameters will be used to do the data binding. Subclasses of RestfulController may override the getObjectToBind() method and return anything that is a valid binding source, including a Map or a DataBindingSource. For most use cases binding the request is appropriate but the getObjectToBind() method allows for changing that behavior where desired.

Using custom subclass of RestfulController with Resource annotation

You can also customize the behaviour of the controller that backs the Resource annotation.

The class must provide a constructor that takes a domain class as it’s argument. The second constructor is required for supporting Resource annotation with readOnly=true.

This is a template that can be used for subclassed RestfulController classes used in Resource annotations:

class SubclassRestfulController<T> extends RestfulController<T> {
    SubclassRestfulController(Class<T> domainClass) {
        this(domainClass, false)
    }

    SubclassRestfulController(Class<T> domainClass, boolean readOnly) {
        super(domainClass, readOnly)
    }
}

You can specify the super class of the controller that backs the Resource annotation with the superClass attribute.

import grails.rest.*

@Resource(uri='/books', superClass=SubclassRestfulController)
class Book {

    String title

    static constraints = {
        title blank:false
    }
}

10.1.5.2 Implementing REST Controllers Step by Step

If you don’t want to take advantage of the features provided by the RestfulController super class, then you can implement each HTTP verb yourself manually. The first step is to create a controller:

$ grails create-controller book

Then add some useful imports and enable readOnly by default:

import grails.transaction.*
import static org.springframework.http.HttpStatus.*
import static org.springframework.http.HttpMethod.*

@Transactional(readOnly = true)
class BookController {
    ...
}

Recall that each HTTP verb matches a particular Grails action according to the following conventions:

HTTP Method URI Controller Action

GET

/books

index

GET

/books/${id}

show

GET

/books/create

create

GET

/books/${id}/edit

edit

POST

/books

save

PUT

/books/${id}

update

DELETE

/books/${id}

delete

The create and edit actions are already required if you plan to implement an HTML interface for the REST resource. They are there in order to render appropriate HTML forms to create and edit a resource. They can be discarded if that is not a requirement.

The key to implementing REST actions is the respond method introduced in Grails 2.3. The respond method tries to produce the most appropriate response for the requested content type (JSON, XML, HTML etc.)

Implementing the 'index' action

For example, to implement the index action, simply call the respond method passing the list of objects to respond with:

def index(Integer max) {
    params.max = Math.min(max ?: 10, 100)
    respond Book.list(params), model:[bookCount: Book.count()]
}

Note that in the above example we also use the model argument of the respond method to supply the total count. This is only required if you plan to support pagination via some user interface.

The respond method will, using Content Negotiation, attempt to reply with the most appropriate response given the content type requested by the client (via the ACCEPT header or file extension).

If the content type is established to be HTML then a model will be produced such that the action above would be the equivalent of writing:

def index(Integer max) {
    params.max = Math.min(max ?: 10, 100)
    [bookList: Book.list(params), bookCount: Book.count()]
}

By providing an index.gsp file you can render an appropriate view for the given model. If the content type is something other than HTML then the respond method will attempt to lookup an appropriate grails.rest.render.Renderer instance that is capable of rendering the passed object. This is done by inspecting the grails.rest.render.RendererRegistry.

By default there are already renderers configured for JSON and XML, to find out how to register a custom renderer see the section on "Customizing Response Rendering".

Implementing the 'show' action

The show action, which is used to display and individual resource by id, can be implemented in one line of Groovy code (excluding the method signature):

def show(Book book) {
    respond book
}

By specifying the domain instance as a parameter to the action Grails will automatically attempt to lookup the domain instance using the id parameter of the request. If the domain instance doesn’t exist, then null will be passed into the action. The respond method will return a 404 error if null is passed otherwise once again it will attempt to render an appropriate response. If the format is HTML then an appropriate model will produced. The following action is functionally equivalent to the above action:

def show(Book book) {
    if(book == null) {
        render status:404
    }
    else {
        return [book: book]
    }
}

Implementing the 'save' action

The save action creates new resource representations. To start off, simply define an action that accepts a resource as the first argument and mark it as Transactional with the grails.transaction.Transactional transform:

@Transactional
def save(Book book) {
    ...
}

Then the first thing to do is check whether the resource has any validation errors and if so respond with the errors:

if(book.hasErrors()) {
    respond book.errors, view:'create'
}
else {
    ...
}

In the case of HTML the 'create' view will be rendered again so the user can correct the invalid input. In the case of other formats (JSON, XML etc.), the errors object itself will be rendered in the appropriate format and a status code of 422 (UNPROCESSABLE_ENTITY) returned.

If there are no errors then the resource can be saved and an appropriate response sent:

book.save flush:true
    withFormat {
        html {
            flash.message = message(code: 'default.created.message', args: [message(code: 'book.label', default: 'Book'), book.id])
            redirect book
        }
        '*' { render status: CREATED }
    }

In the case of HTML a redirect is issued to the originating resource and for other formats a status code of 201 (CREATED) is returned.

Implementing the 'update' action

The update action updates an existing resource representation and is largely similar to the save action. First define the method signature:

@Transactional
def update(Book book) {
    ...
}

If the resource exists then Grails will load the resource, otherwise null is passed. In the case of null, you should return a 404:

if(book == null) {
        render status: NOT_FOUND
    }
    else {
        ...
    }

Then once again check for errors validation errors and if so respond with the errors:

if(book.hasErrors()) {
    respond book.errors, view:'edit'
}
else {
    ...
}

In the case of HTML the 'edit' view will be rendered again so the user can correct the invalid input. In the case of other formats (JSON, XML etc.) the errors object itself will be rendered in the appropriate format and a status code of 422 (UNPROCESSABLE_ENTITY) returned.

If there are no errors then the resource can be saved and an appropriate response sent:

book.save flush:true
withFormat {
    html {
        flash.message = message(code: 'default.updated.message', args: [message(code: 'book.label', default: 'Book'), book.id])
        redirect book
    }
    '*' { render status: OK }
}

In the case of HTML a redirect is issued to the originating resource and for other formats a status code of 200 (OK) is returned.

Implementing the 'delete' action

The delete action deletes an existing resource. The implementation is largely similar to the update action, except the delete() method is called instead:

book.delete flush:true
withFormat {
    html {
        flash.message = message(code: 'default.deleted.message', args: [message(code: 'Book.label', default: 'Book'), book.id])
        redirect action:"index", method:"GET"
    }
    '*'{ render status: NO_CONTENT }
}

Notice that for an HTML response a redirect is issued back to the index action, whilst for other content types a response code 204 (NO_CONTENT) is returned.

10.1.5.3 Generating a REST controller using scaffolding

To see some of these concepts in action and help you get going, the Scaffolding plugin, version 2.0 and above, can generate a REST ready controller for you, simply run the command:

$ grails generate-controller <<Domain Class Name>>

10.1.6 The REST Profile

Since Grails 3.1, Grails supports a tailored profile for creating REST applications that provides a more focused set of dependencies and commands.

To get started with the REST profile, create an application specifying rest-api as the name of the profile:

$ grails create-app my-api --profile rest-api

This will create a new REST application that provides the following features:

  • Default set of commands for creating and generating REST endpoints

  • Defaults to using JSON views for rendering responses (see the next section)

  • Few plugins than the default Grails plugin (no GSP, no Asset Pipeline, nothing HTML related)

You will notice for example in the grails-app/views directory that there are *.gson files for rendering the default index page and as well as any 404 and 500 errors.

If you issue the following set of commands:

$ grails create-domain-class book
$ grails generate-all my.api.Book

Instead of CRUD HTML interface a REST endpoint is generated that produces JSON responses. In addition, the generated functional and unit tests by default test the REST endpoint.

10.1.7 The Angular Profile

Since Grails 3.1, Grails supports a profile for creating applications with AngularJS that provides a more focused set of dependencies and commands. The angular profile inherits from the REST profile and therefore has all of the commands and properties that the REST profile has.

To get started with the Angular profile, create an application specifying angular as the name of the profile:

$ grails create-app my-api --profile angular

This will create a new Grails application that provides the following features:

  • Default set of commands for creating Angular artefacts

  • Gradle plugin to manage client side dependencies

  • Gradle plugin to execute client side unit tests

  • Asset Pipeline plugins to ease development

By default the Angular profile includes GSP support in order to render the index page. This is necessary because the profile is designed around asset pipeline.

The new commands are:

  • create-ng-component

  • create-ng-controller

  • create-ng-directive

  • create-ng-domain

  • create-ng-module

  • create-ng-service

Project structure

The Angular profile is designed around a specific project structure. The create-ng commands will automatically create modules where they do not exist.

Example:

$ grails create-ng-controller foo

This will produce a fooController.js file in grails-app/assets/javascripts/${default package name}/controllers.

By default the angular profile will create files in the javascripts directory. You can change that behavior in your configuration with the key grails.codegen.angular.assetDir.
$ grails create-ng-domain foo.bar

This will produce a Bar.js file in grails-app/assets/javascripts/foo/domains. It will also create the "foo" module if it does not already exist.

$ grails create-ng-module foo.bar

This will produce a foo.bar.js file in grails-app/assets/javascripts/foo/bar. Note the naming convention for modules is different than other artefacts.

$ grails create-ng-service foo.bar --type constant

This will produce a bar.js file in grails-app/assets/javascripts/foo/services. It will also create the "foo" module if it does not already exist. The create-ng-service command accepts a flag -type. The types that can be used are:

  • service

  • factory default

  • value

  • provider

  • constant

Along with the artefacts themselves, the profile will also produce a skeleton unit test file under src/test/javascripts for each create command.

Client side dependencies

The Gradle Bower Plugin is used to manage dependencies with bower. Visit the plugin documentation to learn how to use the plugin.

Unit Testing

The Gradle Karma Plugin is used to execute client side unit tests. All generated tests are written with Jasmine. Visit the plugin documentation to learn how to use the plugin.

Asset Pipeline

The Angular profile includes several asset pipeline plugins to make development easier.

10.1.8 The Angular 2 Profile

Since Grails 3.2.1, Grails supports a profile for creating applications with Angular that provides a more future facing setup.

The biggest change in this profile is that the profile creates a multi project gradle build. This is the first profile to have done so. The Angular 2 profile relies on the Angular CLI to manage the client side application. The server side application is the same as an application created with the rest-api profile.

To get started with the Angular 2 profile, create an application specifying angular2 as the name of the profile:

$ grails create-app my-app --profile angular2

This will create a my-app directory with the following contents:

client/
gradle/
gradlew
gradlew.bat
server/
settings.gradle

The entire client application lives in the client folder and the entire server application lives in the server folder.

Prerequisites

To use this profile, you should have Node, NPM, and the Angular CLI installed. Node should be at least version 5 and NPM should be at least version 3.

Project Structure

The Angular 2 profile is designed to be used with the Angular CLI. The CLI was used to create the client application side of the profile to start with. The CLI provides commands to do most of the things you would want to do with the client application, including creating components or services. Because of that, the profile itself provides no commands to do those same things.

Running The App

To execute the server side application only, you can execute the bootRun task in the server project:

./gradlew server:bootRun

The same can be done for the client application:

./gradlew client:bootRun

To execute both, you must do so in parallel:

./gradlew bootRun --parallel
It is necessary to do so in parallel because by default Gradle executes tasks synchronously, and neither of the bootRun tasks will "finish".

Testing

The default client application that comes with the profile provides some tests that can be executed. To execute tests in the application:

./gradlew test

The test task will execute unit tests with Karma and Jasmine.

./gradlew integrationTest

The integrationTest task will execute e2e tests with Protractor.

You can execute the test and integrationTest tasks on each of the sub-projects the same as you would bootRun.

CORS

Because the client side and server side will be running on separate ports, CORS configuration is required. By default the profile will configure the server side to allow CORS from all hosts via the following config:

server/grails-app/conf/application.yml
grails:
    cors:
        enabled: true

See the section on CORS in the user guide for information on configuring this feature for your needs.

10.1.9 JSON Views

As mentioned in the previous section the REST profile by default uses JSON views to render JSON responses. These play a similar role to GSP, but instead are optimized for outputing JSON responses instead of HTML.

You can continue to separate your application in terms of MVC, with the logic of your application residing in controllers and services, whilst view related matters are handled by JSON views.

JSON views also provide the flexibility to easily customize the JSON presented to clients without having to resort to relatively complex marshalling libraries like Jackson or Grails' marshaller API.

Since Grails 3.1, JSON views are considered by the Grails team the best way to present JSON output for the client, the section on writing custom marshallers has been removed from the user guide. If you are looking for information on that topic, see the Grails 3.0.x guide.

10.1.9.1 Getting Started

If you are using the REST or AngularJS profiles then the JSON views plugin will already be included and you can skip the remainder of this section. Otherwise you will need to modify your build.gradle to include the necessary plugin to activate JSON views:

compile 'org.grails.plugins:views-json:1.0.0' // or whatever is the latest version
The source code repository for JSON views can be found on Github if you are looking for more documentation and contributions

In order to compile JSON views for production deployment you should also activate the Gradle plugin by first modifying the buildscript block:

buildscript {
    ...
    dependencies {
        ...
        classpath "org.grails.plugins:views-gradle:1.0.0"
    }
}

Then apply the org.grails.plugins.views-json Gradle plugin after any Grails core gradle plugins:

...
apply plugin: "org.grails.grails-web"
apply plugin: "org.grails.plugins.views-json"

This will add a compileGsonViews task to Gradle, which is invoked prior to creating the production JAR or WAR file.

10.1.9.2 Creating JSON Views

JSON views go into the grails-app/views directory and end with the .gson suffix. They are regular Groovy scripts and can be opened in any Groovy editor.

Example JSON view:

json.person {
    name "bob"
}
To open them in the Groovy editor in Intellij IDEA, double click on the file and when asked which file to associate it with, choose "Groovy"

The above JSON view produces:

{"person":{"name":"bob"}}

There is an implicit json variable which is an instance of StreamingJsonBuilder.

Example usages:

json(1,2,3) == "[1,2,3]"
json { name "Bob" } == '{"name":"Bob"}'
json([1,2,3]) { n it } == '[{"n":1},{"n":2},{"n":3}]'

Refer to the API documentation on StreamingJsonBuilder for more information about what is possible.

10.1.9.3 JSON View Templates

You can define templates starting with underscore _. For example given the following template called _person.gson:

model {
    Person person
}
json {
    name person.name
    age person.age
}

You can render it with a view as follows:

model {
    Family family
}
json {
    name family.father.name
    age family.father.age
    oldestChild g.render(template:"person", model:[person: family.children.max { Person p -> p.age } ])
    children g.render(template:"person", collection: family.children, var:'person')
}

Alternatively for a more concise way to invoke templates, using the tmpl variable:

model {
    Family family
}
json {
    name family.father.name
    age family.father.age
    oldestChild tmpl.person( family.children.max { Person p -> p.age } ] )
    children tmpl.person( family.children )
}

10.1.9.4 Rendering Domain Classes with JSON Views

Typically your model may involve one or many domain instances. JSON views provide a render method for rendering these.

For example given the following domain class:

class Book {
    String title
}

And the following template:

model {
    Book book
}

json g.render(book)

The resulting output is:

{id:1, title:"The Stand"}

You can customize the rendering by including or excluding properties:

json g.render(book, [includes:['title']])

Or by providing a closure to add additional JSON output:

json g.render(book) {
    pages 1000
}

10.1.9.5 JSON Views by Convention

There are a few useful conventions you can follow when creating JSON views. For example if you have a domain class called Book, then creating a template located at grails-app/views/book/_book.gson and using the respond method will result in rendering the template:

def show(Long id) {
    respond Book.get(id)
}

In addition if an error occurs during validation by default Grails will try to render a template called grails-app/views/book/_errors.gson, otherwise it will try to render grails-app/views/errors/_errors.gson if the former doesn’t exist.

This is useful because when persisting objects you can respond with validation errors to render these aforementioned templates:

@Transactional
def save(Book book) {
    if (book.hasErrors()) {
        transactionStatus.setRollbackOnly()
        respond book.errors
    }
    else {
        // valid object
    }
}

If a validation error occurs in the above example the grails-app/views/book/_errors.gson template will be rendered.

For more information on JSON views (and Markup views), see the JSON Views user guide.

10.1.10 Customizing Response Rendering

If you are looking for a more low-level API and JSON or Markup views don’t suite your needs then you may want to consider implementing a custom renderer.

10.1.10.1 Customizing the Default Renderers

The default renderers for XML and JSON can be found in the grails.rest.render.xml and grails.rest.render.json packages respectively. These use the Grails converters (grails.converters.XML and grails.converters.JSON) by default for response rendering.

You can easily customize response rendering using these default renderers. A common change you may want to make is to include or exclude certain properties from rendering.

Including or Excluding Properties from Rendering

As mentioned previously, Grails maintains a registry of grails.rest.render.Renderer instances. There are some default configured renderers and the ability to register or override renderers for a given domain class or even for a collection of domain classes. To include a particular property from rendering you need to register a custom renderer by defining a bean in grails-app/conf/spring/resources.groovy:

import grails.rest.render.xml.*

beans = {
    bookRenderer(XmlRenderer, Book) {
        includes = ['title']
    }
}
The bean name is not important (Grails will scan the application context for all registered renderer beans), but for organizational and readability purposes it is recommended you name it something meaningful.

To exclude a property, the excludes property of the XmlRenderer class can be used:

import grails.rest.render.xml.*

beans = {
    bookRenderer(XmlRenderer, Book) {
        excludes = ['isbn']
    }
}

Customizing the Converters

As mentioned previously, the default renders use the grails.converters package under the covers. In other words, under the covers they essentially do the following:

import grails.converters.*

...
render book as XML

// or render book as JSON

Why the separation between converters and renderers? Well a renderer has more flexibility to use whatever rendering technology you chose. When implementing a custom renderer you could use Jackson, Gson or any Java library to implement the renderer. Converters on the other hand are very much tied to Grails' own marshalling implementation.

10.1.10.2 Implementing a Custom Renderer

If you want even more control of the rendering or prefer to use your own marshalling techniques then you can implement your own Renderer instance. For example below is a simple implementation that customizes the rendering of the Book class:

package myapp
import grails.rest.render.*
import grails.web.mime.MimeType

class BookXmlRenderer extends AbstractRenderer<Book> {
    BookXmlRenderer() {
        super(Book, [MimeType.XML,MimeType.TEXT_XML] as MimeType[])
    }

    void render(Book object, RenderContext context) {
        context.contentType = MimeType.XML.name

        def xml = new groovy.xml.MarkupBuilder(context.writer)
        xml.book(id: object.id, title:object.title)
    }
}

The AbstractRenderer super class has a constructor that takes the class that it renders and the MimeType(s) that are accepted (via the ACCEPT header or file extension) for the renderer.

To configure this renderer, simply add it is a bean to grails-app/conf/spring/resources.groovy:

beans = {
    bookRenderer(myapp.BookXmlRenderer)
}

The result will be that all Book instances will be rendered in the following format:

<book id="1" title="The Stand"/>
If you change the rendering to a completely different format like the above, then you also need to change the binding if you plan to support POST and PUT requests. Grails will not automatically know how to bind data from a custom XML format to a domain class otherwise. See the section on "Customizing Binding of Resources" for further information.

Container Renderers

A grails.rest.render.ContainerRenderer is a renderer that renders responses for containers of objects (lists, maps, collections etc.). The interface is largely the same as the Renderer interface except for the addition of the getComponentType() method, which should return the "contained" type. For example:

class BookListRenderer implements ContainerRenderer<List, Book> {
    Class<List> getTargetType() { List }
    Class<Book> getComponentType() { Book }
    MimeType[] getMimeTypes() { [ MimeType.XML] as MimeType[] }
    void render(List object, RenderContext context) {
        ....
    }
}

10.1.10.3 Using GSP to Customize Rendering

You can also customize rendering on a per action basis using Groovy Server Pages (GSP). For example given the show action mentioned previously:

def show(Book book) {
    respond book
}

You could supply a show.xml.gsp file to customize the rendering of the XML:

<%@page contentType="application/xml"%>
<book id="${book.id}" title="${book.title}"/>

10.1.11 Hypermedia as the Engine of Application State

HATEOAS, an abbreviation for Hypermedia as the Engine of Application State, is a common pattern applied to REST architectures that uses hypermedia and linking to define the REST API.

Hypermedia (also called Mime or Media Types) are used to describe the state of a REST resource, and links tell clients how to transition to the next state. The format of the response is typically JSON or XML, although standard formats such as Atom and/or HAL are frequently used.

10.1.11.1 HAL Support

HAL is a standard exchange format commonly used when developing REST APIs that follow HATEOAS principals. An example HAL document representing a list of orders can be seen below:

{
    "_links": {
        "self": { "href": "/orders" },
        "next": { "href": "/orders?page=2" },
        "find": {
            "href": "/orders{?id}",
            "templated": true
        },
        "admin": [{
            "href": "/admins/2",
            "title": "Fred"
        }, {
            "href": "/admins/5",
            "title": "Kate"
        }]
    },
    "currentlyProcessing": 14,
    "shippedToday": 20,
    "_embedded": {
        "order": [{
            "_links": {
                "self": { "href": "/orders/123" },
                "basket": { "href": "/baskets/98712" },
                "customer": { "href": "/customers/7809" }
            },
            "total": 30.00,
            "currency": "USD",
            "status": "shipped"
        }, {
            "_links": {
                "self": { "href": "/orders/124" },
                "basket": { "href": "/baskets/97213" },
                "customer": { "href": "/customers/12369" }
            },
            "total": 20.00,
            "currency": "USD",
            "status": "processing"
        }]
    }
}

Exposing Resources Using HAL

To return HAL instead of regular JSON for a resource you can simply override the renderer in grails-app/conf/spring/resources.groovy with an instance of grails.rest.render.hal.HalJsonRenderer (or HalXmlRenderer for the XML variation):

import grails.rest.render.hal.*
beans = {
    halBookRenderer(HalJsonRenderer, rest.test.Book)
}

You will also need to update the acceptable response formats for the resource so that the HAL format is included. Not doing so will result in a 406 - Not Acceptable response being returned from the server.

This can be done by setting the formats attribute of the Resource transformation:

import grails.rest.*

@Resource(uri='/books', formats=['json', 'xml', 'hal'])
class Book {
    ...
}

Or by updating the responseFormats in the controller:

class BookController extends RestfulController {
    static responseFormats = ['json', 'xml', 'hal']

    // ...
}

With the bean in place requesting the HAL content type will return HAL:

$ curl -i -H "Accept: application/hal+json" http://localhost:8080/books/1

HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/hal+json;charset=ISO-8859-1

{
  "_links": {
    "self": {
      "href": "http://localhost:8080/books/1",
      "hreflang": "en",
      "type": "application/hal+json"
    }
  },
  "title": "\"The Stand\""
}

To use HAL XML format simply change the renderer:

import grails.rest.render.hal.*
beans = {
    halBookRenderer(HalXmlRenderer, rest.test.Book)
}

Rendering Collections Using HAL

To return HAL instead of regular JSON for a list of resources you can simply override the renderer in grails-app/conf/spring/resources.groovy with an instance of grails.rest.render.hal.HalJsonCollectionRenderer:

import grails.rest.render.hal.*
beans = {
    halBookCollectionRenderer(HalJsonCollectionRenderer, rest.test.Book)
}

With the bean in place requesting the HAL content type will return HAL:

$ curl -i -H "Accept: application/hal+json" http://localhost:8080/books
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/hal+json;charset=UTF-8
Transfer-Encoding: chunked
Date: Thu, 17 Oct 2013 02:34:14 GMT

{
  "_links": {
    "self": {
      "href": "http://localhost:8080/books",
      "hreflang": "en",
      "type": "application/hal+json"
    }
  },
  "_embedded": {
    "book": [
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/books/1",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "The Stand"
      },
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/books/2",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "Infinite Jest"
      },
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/books/3",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "Walden"
      }
    ]
  }
}

Notice that the key associated with the list of Book objects in the rendered JSON is book which is derived from the type of objects in the collection, namely Book. In order to customize the value of this key assign a value to the collectionName property on the HalJsonCollectionRenderer bean as shown below:

import grails.rest.render.hal.*
beans = {
    halBookCollectionRenderer(HalCollectionJsonRenderer, rest.test.Book) {
        collectionName = 'publications'
    }
}

With that in place the rendered HAL will look like the following:

$ curl -i -H "Accept: application/hal+json" http://localhost:8080/books
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/hal+json;charset=UTF-8
Transfer-Encoding: chunked
Date: Thu, 17 Oct 2013 02:34:14 GMT

{
  "_links": {
    "self": {
      "href": "http://localhost:8080/books",
      "hreflang": "en",
      "type": "application/hal+json"
    }
  },
  "_embedded": {
    "publications": [
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/books/1",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "The Stand"
      },
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/books/2",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "Infinite Jest"
      },
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/books/3",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "Walden"
      }
    ]
  }
}

Using Custom Media / Mime Types

If you wish to use a custom Mime Type then you first need to declare the Mime Types in grails-app/conf/application.groovy:

grails.mime.types = [
    all:      "*/*",
    book:     "application/vnd.books.org.book+json",
    bookList: "application/vnd.books.org.booklist+json",
    ...
]
It is critical that place your new mime types after the 'all' Mime Type because if the Content Type of the request cannot be established then the first entry in the map is used for the response. If you have your new Mime Type at the top then Grails will always try and send back your new Mime Type if the requested Mime Type cannot be established.

Then override the renderer to return HAL using the custom Mime Types:

import grails.rest.render.hal.*
import grails.web.mime.*

beans = {
    halBookRenderer(HalJsonRenderer, rest.test.Book, new MimeType("application/vnd.books.org.book+json", [v:"1.0"]))
    halBookListRenderer(HalJsonCollectionRenderer, rest.test.Book, new MimeType("application/vnd.books.org.booklist+json", [v:"1.0"]))
}

In the above example the first bean defines a HAL renderer for a single book instance that returns a Mime Type of application/vnd.books.org.book+json. The second bean defines the Mime Type used to render a collection of books (in this case application/vnd.books.org.booklist+json).

application/vnd.books.org.booklist+json is an example of a media-range (http://www.w3.org/Protocols/rfc2616/rfc2616.html - Header Field Definitions). This example uses entity (book) and operation (list) to form the media-range values but in reality, it may not be necessary to create a separate Mime type for each operation. Further, it may not be necessary to create Mime types at the entity level. See the section on "Versioning REST resources" for further information about how to define your own Mime types.

With this in place issuing a request for the new Mime Type returns the necessary HAL:

$ curl -i -H "Accept: application/vnd.books.org.book+json" http://localhost:8080/books/1

HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/vnd.books.org.book+json;charset=ISO-8859-1

{
  "_links": {
    "self": {
      "href": "http://localhost:8080/books/1",
      "hreflang": "en",
      "type": "application/vnd.books.org.book+json"
    }
  },
  "title": "\"The Stand\""
}

An important aspect of HATEOAS is the usage of links that describe the transitions the client can use to interact with the REST API. By default the HalJsonRenderer will automatically create links for you for associations and to the resource itself (using the "self" relationship).

However you can customize link rendering using the link method that is added to all domain classes annotated with grails.rest.Resource or any class annotated with grails.rest.Linkable. For example, the show action can be modified as follows to provide a new link in the resulting output:

def show(Book book) {
    book.link rel:'publisher', href: g.createLink(absolute: true, resource:"publisher", params:[bookId: book.id])
    respond book
}

Which will result in output such as:

{
  "_links": {
    "self": {
      "href": "http://localhost:8080/books/1",
      "hreflang": "en",
      "type": "application/vnd.books.org.book+json"
    }
    "publisher": {
        "href": "http://localhost:8080/books/1/publisher",
        "hreflang": "en"
    }
  },
  "title": "\"The Stand\""
}

The link method can be passed named arguments that match the properties of the grails.rest.Link class.

10.1.11.2 Atom Support

Atom is another standard interchange format used to implement REST APIs. An example of Atom output can be seen below:

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">

 <title>Example Feed</title>
 <link href="http://example.org/"/>
 <updated>2003-12-13T18:30:02Z</updated>
 <author>
   <name>John Doe</name>
 </author>
 <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>

 <entry>
   <title>Atom-Powered Robots Run Amok</title>
   <link href="http://example.org/2003/12/13/atom03"/>
   <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
   <updated>2003-12-13T18:30:02Z</updated>
   <summary>Some text.</summary>
 </entry>

</feed>

To use Atom rendering again simply define a custom renderer:

import grails.rest.render.atom.*
beans = {
    halBookRenderer(AtomRenderer, rest.test.Book)
    halBookListRenderer(AtomCollectionRenderer, rest.test.Book)
}

10.1.11.3 Vnd.Error Support

Vnd.Error is a standardised way of expressing an error response.

By default when a validation error occurs when attempting to POST new resources then the errors object will be sent back allow with a 422 respond code:

$ curl -i -H "Accept: application/json"  -H "Content-Type: application/json" -X POST -d "" http://localhost:8080/books

HTTP/1.1 422 Unprocessable Entity
Server: Apache-Coyote/1.1
Content-Type: application/json;charset=ISO-8859-1

{
  "errors": [
    {
      "object": "rest.test.Book",
      "field": "title",
      "rejected-value": null,
      "message": "Property [title] of class [class rest.test.Book] cannot be null"
    }
  ]
}

If you wish to change the format to Vnd.Error then simply register grails.rest.render.errors.VndErrorJsonRenderer bean in grails-app/conf/spring/resources.groovy:

beans = {
    vndJsonErrorRenderer(grails.rest.render.errors.VndErrorJsonRenderer)
    // for Vnd.Error XML format
    vndXmlErrorRenderer(grails.rest.render.errors.VndErrorXmlRenderer)
}

Then if you alter the client request to accept Vnd.Error you get an appropriate response:

$ curl -i -H "Accept: application/vnd.error+json,application/json" -H "Content-Type: application/json" -X POST -d "" http://localhost:8080/books
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/vnd.error+json;charset=ISO-8859-1

[
    {
        "logref": "book.nullable,
        "message": "Property [title] of class [class rest.test.Book] cannot be null",
        "_links": {
            "resource": {
                "href": "http://localhost:8080/rest-test/books"
            }
        }
    }
]

10.1.12 Customizing Binding of Resources

The framework provides a sophisticated but simple mechanism for binding REST requests to domain objects and command objects. One way to take advantage of this is to bind the request property in a controller the properties of a domain class. Given the following XML as the body of the request, the createBook action will create a new Book and assign "The Stand" to the title property and "Stephen King" to the authorName property.

<?xml version="1.0" encoding="UTF-8"?>
<book>
    <title>The Stand</title>
    <authorName>Stephen King</authorName>
</book>
class BookController {

    def createBook() {
        def book = new Book()
        book.properties = request

        // ...
    }
}

Command objects will automatically be bound with the body of the request:

class BookController {
    def createBook(BookCommand book) {

        // ...
    }
}

class BookCommand {
    String title
    String authorName
}

If the command object type is a domain class and the root element of the XML document contains an id attribute, the id value will be used to retrieve the corresponding persistent instance from the database and then the rest of the document will be bound to the instance. If no corresponding record is found in the database, the command object reference will be null.

<?xml version="1.0" encoding="UTF-8"?>
<book id="42">
    <title>Walden</title>
    <authorName>Henry David Thoreau</authorName>
</book>
class BookController {
    def updateBook(Book book) {
        // The book will have been retrieved from the database and updated
        // by doing something like this:
        //
        // book == Book.get('42')
        // if(book != null) {
        //    book.properties = request
        // }
        //
        // the code above represents what the framework will
        // have done. There is no need to write that code.

        // ...

    }
}

The data binding depends on an instance of the DataBindingSource interface created by an instance of the DataBindingSourceCreator interface. The specific implementation of DataBindingSourceCreator will be selected based on the contentType of the request. Several implementations are provided to handle common content types. The default implementations will be fine for most use cases. The following table lists the content types which are supported by the core framework and which DataBindingSourceCreator implementations are used for each. All of the implementation classes are in the org.grails.databinding.bindingsource package.

Content Type(s) Bean Name DataBindingSourceCreator Impl.

application/xml, text/xml

xmlDataBindingSourceCreator

XmlDataBindingSourceCreator

application/json, text/json

jsonDataBindingSourceCreator

JsonDataBindingSourceCreator

application/hal+json

halJsonDataBindingSourceCreator

HalJsonDataBindingSourceCreator

application/hal+xml

halXmlDataBindingSourceCreator

HalXmlDataBindingSourceCreator

In order to provide your own DataBindingSourceCreator for any of those content types, write a class which implements DataBindingSourceCreator and register an instance of that class in the Spring application context. If you are replacing one of the existing helpers, use the corresponding bean name from above. If you are providing a helper for a content type other than those accounted for by the core framework, the bean name may be anything that you like but you should take care not to conflict with one of the bean names above.

The DataBindingSourceCreator interface defines just 2 methods:

package org.grails.databinding.bindingsource

import grails.web.mime.MimeType
import grails.databinding.DataBindingSource

/**
 * A factory for DataBindingSource instances
 *
 * @since 2.3
 * @see DataBindingSourceRegistry
 * @see DataBindingSource
 *
 */
interface DataBindingSourceCreator {

    /**
     * `return All of the {`link MimeType} supported by this helper
     */
    MimeType[] getMimeTypes()

    /**
     * Creates a DataBindingSource suitable for binding bindingSource to bindingTarget
     *
     * @param mimeType a mime type
     * @param bindingTarget the target of the data binding
     * @param bindingSource the value being bound
     * @return a DataBindingSource
     */
    DataBindingSource createDataBindingSource(MimeType mimeType, Object bindingTarget, Object bindingSource)
}

AbstractRequestBodyDataBindingSourceCreator is an abstract class designed to be extended to simplify writing custom DataBindingSourceCreator classes. Classes which extend AbstractRequestbodyDatabindingSourceCreator need to implement a method named createBindingSource which accepts an InputStream as an argument and returns a DataBindingSource as well as implementing the getMimeTypes method described in the DataBindingSourceCreator interface above. The InputStream argument to createBindingSource provides access to the body of the request.

The code below shows a simple implementation.

src/main/groovy/com/demo/myapp/databinding/MyCustomDataBindingSourceCreator.groovy
package com.demo.myapp.databinding

import grails.web.mime.MimeType
import grails.databinding.DataBindingSource
import org...databinding.SimpleMapDataBindingSource
import org...databinding.bindingsource.AbstractRequestBodyDataBindingSourceCreator

/**
 * A custom DataBindingSourceCreator capable of parsing key value pairs out of
 * a request body containing a comma separated list of key:value pairs like:
 *
 * name:Herman,age:99,town:STL
 *
 */
class MyCustomDataBindingSourceCreator extends AbstractRequestBodyDataBindingSourceCreator {

    @Override
    public MimeType[] getMimeTypes() {
        [new MimeType('text/custom+demo+csv')] as MimeType[]
    }

    @Override
    protected DataBindingSource createBindingSource(InputStream inputStream) {
        def map = [:]

        def reader = new InputStreamReader(inputStream)

        // this is an obviously naive parser and is intended
        // for demonstration purposes only.

        reader.eachLine { line ->
            def keyValuePairs = line.split(',')
            keyValuePairs.each { keyValuePair ->
                if(keyValuePair?.trim()) {
                    def keyValuePieces = keyValuePair.split(':')
                    def key = keyValuePieces[0].trim()
                    def value = keyValuePieces[1].trim()
                    map<<key>> = value
                }
            }
        }

        // create and return a DataBindingSource which contains the parsed data
        new SimpleMapDataBindingSource(map)
    }
}

An instance of MyCustomDataSourceCreator needs to be registered in the spring application context.

grails-app/conf/spring/resources.groovy
beans = {

    myCustomCreator com.demo.myapp.databinding.MyCustomDataBindingSourceCreator

    // ...
}

With that in place the framework will use the myCustomCreator bean any time a DataBindingSourceCreator is needed to deal with a request which has a contentType of "text/custom+demo+csv".

10.2 RSS and Atom

No direct support is provided for RSS or Atom within Grails. You could construct RSS or ATOM feeds with the render method’s XML capability.

11 Asynchronous Programming

With modern hardware featuring multiple cores, many programming languages have been adding asynchronous, parallel programming APIs, Groovy being no exception.

The excellent GPars project features a whole range of different APIs for asynchronous programming techniques including actors, promises, STM and data flow concurrency.

Added Grails 2.3, the Async features of Grails aim to simplify concurrent programming within the framework and include the concept of Promises and a unified event model.

11.1 Promises

A Promise is a concept being embraced by many concurrency frameworks. They are similar to java.util.concurrent.Future instances, but include a more user friendly exception handling model, useful features like chaining and the ability to attach listeners.

Promise Basics

In Grails the grails.async.Promises class provides the entry point to the Promise API:

import static grails.async.Promises.*

To create promises you can use the task method, which returns an instance of the grails.async.Promise interface:

def p1 = task { 2 * 2 }
def p2 = task { 4 * 4 }
def p3 = task { 8 * 8 }
assert [4,16,64] == waitAll(p1, p2, p3)

The waitAll method waits synchronously, blocking the current thread, for all of the concurrent tasks to complete and returns the results.

If you prefer not to block the current thread you can use the onComplete method:

onComplete([p1,p2,p3]) { List results ->
   assert [4,16,64] == results
}

The waitAll method will throw an exception if an error occurs executing one of the promises. The originating exception will be thrown. The onComplete method, however, will simply not execute the passed closure if an exception occurs. You can register an onError listener if you wish to handle exceptions without blocking:

onError([p1,p2,p3]) { Throwable t ->
   println "An error occured ${t.message}"
}

If you have just a single long running promise then the grails.async.Promise interface provides a similar API on the promise itself. For example:

import static java.util.concurrent.TimeUnit.*
import static grails.async.Promises.*

Promise p = task {
        // Long running task
}
p.onError { Throwable err ->
        println "An error occured ${err.message}"
}
p.onComplete { result ->
    println "Promise returned $result"
}
// block until result is called
def result = p.get()
// block for the specified time
def result = p.get(1,MINUTES)

Promise Chaining

It is possible to chain several promises and wait for the chain to complete using the then method:

final polish = { ... }
final transform = { ... }
final save = { ... }
final notify = { ... }

Promise promise = task {
    // long running task
}
promise.then polish then transform then save then {
        // notify end result
}

If an exception occurs at any point in the chain it will be propagated back to the caller and the next step in the chain will not be called.

Promise Lists and Maps

Grails' async API also features the concept of a promise lists and maps. These are represented by the grails.async.PromiseList and grails.async.PromiseMap classes respectively.

The easiest way to create a promise list or map is via the tasks method of the Promises class:

import static grails.async.Promises.*

def promiseList = tasks([{ 2 * 2 }, { 4 * 4}, { 8 * 8 }])

assert [4,16,64] == promiseList.get()

The tasks method, when passed a list of closures, returns a PromiseList. You can also construct a PromiseList manually:

import grails.async.*

def list = new PromiseList()
list << { 2 * 2 }
list << { 4 * 4 }
list << { 8 * 8 }
list.onComplete { List results ->
  assert [4,16,64] == results
}
The PromiseList class does not implement the java.util.List interface, but instead returns a java.util.List from the get() method

Working with PromiseMap instances is largely similar. Again you can either use the tasks method:

import static grails.async.Promises.*

def promiseList = tasks one:{ 2 * 2 },
                        two:{ 4 * 4},
                        three:{ 8 * 8 }

assert [one:4,two:16,three:64] == promiseList.get()

Or construct a PromiseMap manually:

import grails.async.*

def map = new PromiseMap()
map['one'] = { 2 * 2 }
map['two'] = { 4 * 4 }
map['three'] = { 8 * 8 }
map.onComplete { Map results ->
  assert [one:4,two:16,three:64] == results
}

Promise Factories

The Promises class uses a grails.async.PromiseFactory instance to create Promise instances.

The default implementation uses Project Reactor and is called org.grails.async.factory.reactor.ReactorPromiseFactory, however it is possible to swap implementations by setting the Promises.promiseFactory variable.

One common use case for this is unit testing, typically you do not want promises to execute asynchronously during unit tests, as this makes tests harder to write. For this purpose Grails ships with a org.grails.async.factory.SynchronousPromiseFactory instance that makes it easier to test promises:

import org.grails.async.factory.*
import grails.async.*

Promises.promiseFactory = new SynchronousPromiseFactory()

Using the PromiseFactory mechanism it is theoretically possible to plug in other concurrency libraries into the Grails framework. For this you need to override the two interfaces grails.async.Promise and grails.async.PromiseFactory.

DelegateAsync Transformation

It is quite common to require both synchronous and asynchronous versions of the same API. Developing both can result in a maintenance problem as typically the asynchronous API would simply delegate to the synchronous version.

The DelegateAsync transformation is designed to mitigate this problem by transforming any synchronous API into an asynchronous one.

For example, consider the following service:

class BookService {
    List<Book> findBooks(String title) {
      // implementation
    }
}

The findBooks method executes synchronously in the same thread as the caller. To make an asynchronous version of this API you can define another class as follows:

import grails.async.*

class AsyncBookService {
   @DelegateAsync BookService bookService
}

The DelegateAsync transformation will automatically add a new method that looks like the following to the AsyncBookService class:

Promise<List<Book>> findBooks(String title) {
    Promises.task {
       bookService.findBooks(title)
    }
}

As you see the transform adds equivalent methods that return a Promise and execute asynchronously.

The AsyncBookService can then be injected into other controllers and services and used as follows:

AsyncBookService asyncBookService
def findBooks(String title) {
    asyncBookService.findBooks(title)
       .onComplete { List results ->
          println "Books = ${results}"
       }
}

11.2 Reactive Programming with RxJava

Since Grails 3.2, you can use RxJava to write reactive logic in your Grails controllers that leverages the underlying containers asynchronous processing capabilities.

To get started simply declare a dependency on the plugin in build.gradle:

build.gradle
dependencies {
    ...
    compile 'org.grails.plugins:rxjava'
}

You can then return rx.Observable as a return value from any controller and Grails will automatically apply the following steps:

  1. Create a new asynchronous request

  2. Spawn a new thread that subscribes to the observable

  3. When the observable omits a result process the result using the respond method.

For more detailed instructions on how to use the RxJava plugin see the user guide documentation for the plugin.

11.2.1 Server Sent Events

Server-sent events (SSE) is a technology where a browser receives automatic updates from a server via HTTP connection. The Server-Sent Events EventSource API is standardized as part of HTML5 by the W3C.

The RxJava plugin adds support for SSE to Grails making it simple to write controllers that maintain continuous non-blocking communication with a JavaScript client.

For example:

def index() {

    rx.stream { Subscriber subscriber -> (1)
       for(i in (0..5)) {
           if(i % 2 == 0) {
               subscriber.onNext(
                   rx.render("Tick") (2)
               )
           }
           else {
               subscriber.onNext(
                   rx.render("Tock")
               )

           }
           sleep 1000 (3)
       }
       subscriber.onCompleted() (4)
   }
}
1 Call the stream method passing a closure that accepts an rx.Subscriber to start sending events
2 Emit a one or many items using onNext
3 Call sleep to simulate a slow request
4 Call onCompleted to complete the request

For more detailed instructions on how to use SSE and the RxJava plugin see the user guide documentation for the plugin.

11.2.2 RxGORM

RxGORM is new implementation of GORM that has the following goals:

  • Reactive

  • Non-blocking

  • Stateless

  • Simple

RxGORM, unlike the Asynchronous GORM implementation, aims to be truly non-blocking, down to the driver level.

The following is an example of RxGORM in action:

Book.get(id)
    .subscribe { Book it ->
    println "Title = ${it.title}"
}

You can combine RxGORM with the RxJava plugin to implement reactive responses from Grails controllers. For example:

def show() {
        // returns an rx.Observable
    Book.get(params.id?.toString())
}

For more information on how to use RxGORM, see the RxGORM user guide.

11.3 Events

Grails 3.0 introduces a new Events API based on Reactor.

All services and controllers in Grails 3.0 implement the Events trait.

The Events trait allows the ability to consume and publish events that are handled by Reactor.

The default Reactor configuration utilises a thread pool backed event bus. You can however configure Reactor within application.yml, for example:

reactor:
    dispatchers:
        default: myExecutor
        myExecutor:
            type: threadPoolExecutor
            size: 5
            backlog: 2048

11.3.1 Consuming Events

There are several ways to consume an event. As mentioned previously services and controllers implement the Events trait.

The Events trait provides several methods to register event consumers. For example:

on("myEvent") {
    println "Event fired!"
}

Note that if you wish a class (other than a controller or service) to be an event consumer you simply have to implement the Events trait and ensure the class is registered as a Spring bean.

For example given the following class:

import grails.events.*
import javax.annotation.*

class MyClass implements Events {

        @PostConstruct
        void init() {
                on("myEvent") {
                    println "Event fired!"
                }
        }
}

You can override doWithSpring in your Application class to register it as a Spring bean (or annotate it with Component):

Closure doWithSpring() {
        {->
                    myClass(MyClass)
        }
    }

11.3.2 Event Notification

The Events trait also provides methods for notifying of events. For example:

notify "myEvent", "myData"
sendAndReceive "myEvent", "myData", {
    println "Got response!"
}

11.3.3 Reactor Spring Annotations

Reactor provides a few useful annotations that can be used for declaratively consuming events in a Grails service.

To declare an event consumer use the Consumer annotation:

import reactor.spring.context.annotation.*

@Consumer
class MyService {

}

Then to register to listen for an event use the Selector annotation:

import reactor.spring.context.annotation.*

@Consumer
class MyService {
        @Selector('myEvent')
        void myEventListener(Object data) {
                println "GOT EVENT $data"
        }
}

11.3.4 Events from GORM

GORM defines a number of useful events that you can listen for.

Each event is translated into a key that starts with gorm:. For example:

import org.grails.datastore.mapping.engine.event.*
...

on("gorm:preInsert") { PreInsertEvent event ->
        println "GOT EVENT $event"
}
These events are triggered asynchronously, and so cannot cancel or manipulate the persistence operations. If you want to do that see the section on Events & Auto Timestamping in the GORM docs

11.3.5 Events from Spring

Spring also fires a number of useful events. All events in the org.springframework package are prefixed with spring:.

For example:

import org.springframework.web.context.support.*
import org.springframework.boot.context.event.*
...

on("spring:applicationStarted") { ApplicationStartedEvent event ->
        // fired when the application starts
}

on("spring:servletRequestHandled") { RequestHandledEvent event ->
        // fired each time a request is handled
}

11.4 Asynchronous GORM

Since Grails 2.3, GORM features an asynchronous programming model that works across all supported datastores (Hibernate, MongoDB etc.).

Although GORM executes persistence operations asynchronously, these operations still block as the underlying database drivers are not asynchronous. Asynchornous GORM is designed to allow you to isolate these blocking operations onto a separate thread you can scale and control allowing your controller layer to remain non-blocking.

Async Namespace

The Asynchronous GORM API is available on every domain class via the async namespace.

For example, the following code listing reads 3 objects from the database asynchronously:

import static grails.async.Promises.*

def p1 = Person.async.get(1L)
def p2 = Person.async.get(2L)
def p3 = Person.async.get(3L)
def results = waitAll(p1, p2, p3)

Using the async namespace, all the regular GORM methods are available (even dynamic finders), but instead of executing synchronously, the query is run in the background and a Promise instance is returned.

The following code listing shows a few common examples of GORM queries executed asynchronously:

import static grails.async.Promises.*

Person.async.list().onComplete { List results ->
  println "Got people = ${results}"
}
def p = Person.async.getAll(1L, 2L, 3L)
List results = p.get()

def p1 = Person.async.findByFirstName("Homer")
def p2 = Person.async.findByFirstName("Bart")
def p3 = Person.async.findByFirstName("Barney")
results = waitAll(p1, p2, p3)

Async and the Session

When using GORM async each promise is executed in a different thread. Since the Hibernate session is not concurrency safe, a new session is bound per thread.

This is an important consideration when using GORM async (particularly with Hibernate as the persistence engine). The objects returned from asynchronous queries will be detached entities.

This means you cannot save objects returned from asynchronous queries without first merging them back into session. For example the following will not work:

def promise = Person.async.findByFirstName("Homer")
def person = promise.get()
person.firstName = "Bart"
person.save()

Instead you need to merge the object with the session bound to the calling thread. The above code needs to be written as:

def promise = Person.async.findByFirstName("Homer")
def person = promise.get()
person.merge()
person.firstName = "Bart"

Note that merge() is called first because it may refresh the object from the cache or database, which would result in the change being lost. In general it is not recommended to read and write objects in different threads and you should avoid this technique unless absolutely necessary.

Finally, another issue with detached objects is that association lazy loading will not work and you will encounter LazyInitializationException errors if you do so. If you plan to access the associated objects of those returned from asynchronous queries you should use eager queries (which is recommended anyway to avoid N+1 problems).

Multiple Asynchronous GORM calls

As discussed in the previous section you should avoid reading and writing objects in different threads as merging tends to be inefficient.

However, if you wish to do more complex GORM work asynchronously then the GORM async namespace provides a task method that makes this possible. For example:

def promise = Person.async.task {
    withTransaction {
       def person = findByFirstName("Homer")
       person.firstName = "Bart"
       person.save(flush:true)
    }
}

Person updatedPerson = promise.get()

Note that the GORM task method differs from the static Promises.task method in that it deals with binding a new session to the asynchronous thread for you. If you do not use the GORM version and do asynchronous work with GORM then you need to do this manually. Example:

import static grails.async.Promises.*

def promise = task {
    Person.withNewSession {
            // your logic here
    }
}

Async DetachedCriteria

The DetachedCriteria class also supports the async namespace. For example you can do the following:

DetachedCriteria query = Person.where {
    lastName == "Simpson"
}

def promise = query.async.list()

11.5 Asynchronous Request Handling

If you are deploying to a Servlet 3.0 container such as Tomcat 7 and above then it is possible to deal with responses asynchronously.

In general for controller actions that execute quickly there is little benefit in handling requests asynchronously. However, for long running controller actions it is extremely beneficial.

The reason being that with an asynchronous / non-blocking response, the one thread == one request == one response relationship is broken. The container can keep a client response open and active, and at the same time return the thread back to the container to deal with another request, improving scalability.

For example, if you have 70 available container threads and an action takes a minute to complete, if the actions are not executed in a non-blocking fashion the likelihood of all 70 threads being occupied and the container not being able to respond is quite high and you should consider asynchronous request processing.

Since Grails 2.3, Grails features a simplified API for creating asynchronous responses built on the Promise mechanism discussed previously.

Async Models

A typical activity in a Grails controller is to produce a model (a map of key/value pairs) that can be rendered by a view.

If the model takes a while to produce then the server could arrive at a blocking state, impacting scalability. You tell Grails to build the model asynchronously by returning a grails.async.PromiseMap via the Promises.tasks method:

import static grails.async.Promises.*
...
def index() {
   tasks books: Book.async.list(),
         totalBooks: Book.async.count(),
         otherValue: {
           // do hard work
         }
}

Grails will handle the response asynchronously, waiting for the promises to complete before rendering the view. The equivalent synchronous action of the above is:

def index() {
    def otherValue = ...
        [ books: Book.list() ,
          totalBooks: Book.count(),
          otherValue: otherValue  ]
}

You can even render different view by passing the PromiseMap to the model attribute of the render method:

import static grails.async.Promises.*
...
def index() {
   render view:"myView", model: tasks( one:{ 2 * 2 },
                                       two:{ 3 * 3 } )
}

Async Response Rendering

You can also write to the response asynchronously using promises in Grails 2.3 and above:

import static grails.async.Promises.*
class StockController {

    def stock(String ticker) {
       task {
           ticker = ticker ?: 'GOOG'
           def url = new URL("http://download.finance.yahoo.com/d/quotes.csv?s=${ticker}&f=nsl1op&e=.csv")
           Double price = url.text.split(',')[-1] as Double
           render "ticker: $ticker, price: \$price"
       }
    }
}

The above example using Yahoo Finance to query stock prices, executing asynchronously and only rendering the response once the result has been obtained. This is done by returning a Promise instance from the controller action.

If the Yahoo URL is unresponsive the original request thread will not be blocked and the container will not become unresponsive.

11.6 Servlet 3.0 Async

In addition to the higher level async features discussed earlier in the section, you can access the raw Servlet 3.0 asynchronous API from a Grails application.

Servlet 3.0 Asynchronous Rendering

You can render content (templates, binary data etc.) in an asynchronous manner by calling the startAsync method which returns an instance of the Servlet 3.0 AsyncContext. Once you have a reference to the AsyncContext you can use Grails' regular render method to render content:

def index() {
    def ctx = startAsync()
    ctx.start {
        new Book(title:"The Stand").save()
        render template:"books", model:[books:Book.list()]
        ctx.complete()
    }
}

Note that you must call the complete() method to terminate the connection.

Resuming an Async Request

You resume processing of an async request (for example to delegate to view rendering) by using the dispatch method of the AsyncContext class:

def index() {
    def ctx = startAsync()
    ctx.start {
        // do working
        ...
        // render view
        ctx.dispatch()
    }
}

12 Validation

Grails validation capability is built on Spring’s Validator API and data binding capabilities. However Grails takes this further and provides a unified way to define validation "constraints" with its constraints mechanism.

Constraints in Grails are a way to declaratively specify validation rules. Most commonly they are applied to domain classes, however URL Mappings and Command Objects also support constraints.

12.1 Declaring Constraints

Within a domain class constraints are defined with the constraints property that is assigned a code block:

class User {
    String login
    String password
    String email
    Integer age

    static constraints = {
      ...
    }
}

You then use method calls that match the property name for which the constraint applies in combination with named parameters to specify constraints:

class User {
    ...

    static constraints = {
        login size: 5..15, blank: false, unique: true
        password size: 5..15, blank: false
        email email: true, blank: false
        age min: 18
    }
}

In this example we’ve declared that the login property must be between 5 and 15 characters long, it cannot be blank and must be unique. We’ve also applied other constraints to the password, email and age properties.

By default, all domain class properties are not nullable (i.e. they have an implicit nullable: false constraint).

A complete reference for the available constraints can be found in the Quick Reference section under the Constraints heading.

Note that constraints are only evaluated once which may be relevant for a constraint that relies on a value like an instance of java.util.Date.

class User {
    ...

    static constraints = {
        // this Date object is created when the constraints are evaluated, not
        // each time an instance of the User class is validated.
        birthDate max: new Date()
    }
}

A word of warning - referencing domain class properties from constraints

It’s very easy to attempt to reference instance variables from the static constraints block, but this isn’t legal in Groovy (or Java). If you do so, you will get a MissingPropertyException for your trouble. For example, you may try

class Response {
    Survey survey
    Answer answer

    static constraints = {
        survey blank: false
        answer blank: false, inList: survey.answers
    }
}

See how the inList constraint references the instance property survey? That won’t work. Instead, use a custom validator:

class Response {
    ...
    static constraints = {
        survey blank: false
        answer blank: false, validator: { val, obj -> val in obj.survey.answers }
    }
}

In this example, the obj argument to the custom validator is the domain instance that is being validated, so we can access its survey property and return a boolean to indicate whether the new value for the answer property, val, is valid.

12.2 Validating Constraints

Validation Basics

Call the validate method to validate a domain class instance:

def user = new User(params)

if (user.validate()) {
    // do something with user
}
else {
    user.errors.allErrors.each {
        println it
    }
}

The errors property on domain classes is an instance of the Spring Errors interface. The Errors interface provides methods to navigate the validation errors and also retrieve the original values.

Validation Phases

Within Grails there are two phases of validation, the first one being data binding which occurs when you bind request parameters onto an instance such as:

def user = new User(params)

At this point you may already have errors in the errors property due to type conversion (such as converting Strings to Dates). You can check these and obtain the original input value using the Errors API:

if (user.hasErrors()) {
    if (user.errors.hasFieldErrors("login")) {
        println user.errors.getFieldError("login").rejectedValue
    }
}

The second phase of validation happens when you call validate or save. This is when Grails will validate the bound values against the constraints you defined. For example, by default the save method calls validate before executing, allowing you to write code like:

if (user.save()) {
    return user
}
else {
    user.errors.allErrors.each {
        println it
    }
}

12.3 Sharing Constraints Between Classes

A common pattern in Grails is to use Command Objects for validating user-submitted data and then copy the properties of the command object to the relevant domain classes. This often means that your command objects and domain classes share properties and their constraints. You could manually copy and paste the constraints between the two, but that’s a very error-prone approach. Instead, make use of Grails' global constraints and import mechanism.

Global Constraints

In addition to defining constraints in domain classes, command objects and other validateable classes, you can also define them in grails-app/conf/application.groovy:

grails.gorm.default.constraints = {
    '*'(nullable: true, size: 1..20)
    myShared(nullable: false, blank: false)
}

These constraints are not attached to any particular classes, but they can be easily referenced from any validateable class:

class User {
    ...

    static constraints = {
        login shared: "myShared"
    }
}

Note the use of the shared argument, whose value is the name of one of the constraints defined in grails.gorm.default.constraints. Despite the name of the configuration setting, you can reference these shared constraints from any validateable class, such as command objects.

The '*' constraint is a special case: it means that the associated constraints ('nullable' and 'size' in the above example) will be applied to all properties in all validateable classes. These defaults can be overridden by the constraints declared in a validateable class.

Importing Constraints

Grails 2 introduced an alternative approach to sharing constraints that allows you to import a set of constraints from one class into another.

Let’s say you have a domain class like so:

class User {
    String firstName
    String lastName
    String passwordHash

    static constraints = {
        firstName blank: false, nullable: false
        lastName blank: false, nullable: false
        passwordHash blank: false, nullable: false
    }
}

You then want to create a command object, UserCommand, that shares some of the properties of the domain class and the corresponding constraints. You do this with the importFrom() method:

class UserCommand {
    String firstName
    String lastName
    String password
    String confirmPassword

    static constraints = {
        importFrom User

        password blank: false, nullable: false
        confirmPassword blank: false, nullable: false
    }
}

This will import all the constraints from the User domain class and apply them to UserCommand. The import will ignore any constraints in the source class (User) that don’t have corresponding properties in the importing class (UserCommand). In the above example, only the 'firstName' and 'lastName' constraints will be imported into UserCommand because those are the only properties shared by the two classes.

If you want more control over which constraints are imported, use the include and exclude arguments. Both of these accept a list of simple or regular expression strings that are matched against the property names in the source constraints. So for example, if you only wanted to import the 'lastName' constraint you would use:

...
static constraints = {
    importFrom User, include: ["lastName"]
    ...
}

or if you wanted all constraints that ended with 'Name':

...
static constraints = {
    importFrom User, include: [/.*Name/]
    ...
}

Of course, exclude does the reverse, specifying which constraints should not be imported.

12.4 Validation on the Client

Displaying Errors

Typically if you get a validation error you redirect back to the view for rendering. Once there you need some way of displaying errors. Grails supports a rich set of tags for dealing with errors. To render the errors as a list you can use renderErrors:

<g:renderErrors bean="${user}" />

If you need more control you can use hasErrors and eachError:

<g:hasErrors bean="${user}">
  <ul>
   <g:eachError var="err" bean="${user}">
       <li>${err}</li>
   </g:eachError>
  </ul>
</g:hasErrors>

Highlighting Errors

It is often useful to highlight using a red box or some indicator when a field has been incorrectly input. This can also be done with the hasErrors by invoking it as a method. For example:

<div class='value ${hasErrors(bean:user,field:'login','errors')}'>
   <input type="text" name="login" value="${fieldValue(bean:user,field:'login')}"/>
</div>

This code checks if the login field of the user bean has any errors and if so it adds an errors CSS class to the div, allowing you to use CSS rules to highlight the div.

Retrieving Input Values

Each error is actually an instance of the FieldError class in Spring, which retains the original input value within it. This is useful as you can use the error object to restore the value input by the user using the fieldValue tag:

<input type="text" name="login" value="${fieldValue(bean:user,field:'login')}"/>

This code will check for an existing FieldError in the User bean and if there is obtain the originally input value for the login field.

12.5 Validation and Internationalization

Another important thing to note about errors in Grails is that error messages are not hard coded anywhere. The FieldError class in Spring resolves messages from message bundles using Grails' i18n support.

Constraints and Message Codes

The codes themselves are dictated by a convention. For example consider the constraints we looked at earlier:

package com.mycompany.myapp

class User {
    ...

    static constraints = {
        login size: 5..15, blank: false, unique: true
        password size: 5..15, blank: false
        email email: true, blank: false
        age min: 18
    }
}

If a constraint is violated Grails will by convention look for a message code of the form:

<<Class Name>>.<<Property Name>>.<<Constraint Code>>

In the case of the blank constraint this would be user.login.blank so you would need a message such as the following in your grails-app/i18n/messages.properties file:

user.login.blank=Your login name must be specified!

The class name is looked for both with and without a package, with the packaged version taking precedence. So for example, com.mycompany.myapp.User.login.blank will be used before user.login.blank. This allows for cases where your domain class message codes clash with a plugin’s.

For a reference on what codes are for which constraints refer to the reference guide for each constraint (e.g. blank).

Displaying Messages

The renderErrors tag will automatically look up messages for you using the message tag. If you need more control of rendering you can handle this yourself:

<g:hasErrors bean="${user}">
  <ul>
   <g:eachError var="err" bean="${user}">
       <li><g:message error="${err}" /></li>
   </g:eachError>
  </ul>
</g:hasErrors>

In this example within the body of the eachError tag we use the message tag in combination with its error argument to read the message for the given error.

12.6 Applying Validation to Other Classes

Domain classes and Command Objects support validation by default. Other classes may be made validateable by defining the static constraints property in the class (as described above) and then telling the framework about them. It is important that the application register the validateable classes with the framework. Simply defining the constraints property is not sufficient.

The Validateable Trait

Classes which define the static constraints property and implement the Validateable trait will be validateable. Consider this example:

src/main/groovy/com/mycompany/myapp/User.groovy
package com.mycompany.myapp

import grails.validation.Validateable

class User implements Validateable {
    ...

    static constraints = {
        login size: 5..15, blank: false, unique: true
        password size: 5..15, blank: false
        email email: true, blank: false
        age min: 18
    }
}

13 The Service Layer

Grails defines the notion of a service layer. The Grails team discourages the embedding of core application logic inside controllers, as it does not promote reuse and a clean separation of concerns.

Services in Grails are the place to put the majority of the logic in your application, leaving controllers responsible for handling request flow with redirects and so on.

Creating a Service

You can create a Grails service by running the create-service command from the root of your project in a terminal window:

grails create-service helloworld.simple
If no package is specified with the create-service script, Grails automatically uses the application name as the package name.

The above example will create a service at the location grails-app/services/helloworld/SimpleService.groovy. A service’s name ends with the convention Service, other than that a service is a plain Groovy class:

package helloworld

class SimpleService {
}

13.1 Declarative Transactions

Declarative Transactions

Services are typically involved with coordinating logic between domain classes, and hence often involved with persistence that spans large operations. Given the nature of services, they frequently require transactional behaviour. You can use programmatic transactions with the withTransaction method, however this is repetitive and doesn’t fully leverage the power of Spring’s underlying transaction abstraction.

Services enable transaction demarcation, which is a declarative way of defining which methods are to be made transactional. To enable transactions on a service use the Transactional transform:

import grails.transaction.*

@Transactional
class CountryService {

}

The result is that all methods are wrapped in a transaction and automatic rollback occurs if a method throws a runtime exception (i.e. one that extends RuntimeException) or an Error. The propagation level of the transaction is by default set to PROPAGATION_REQUIRED.

Checked exceptions do not roll back transactions. Even though Groovy blurs the distinction between checked and unchecked exceptions, Spring isn’t aware of this and its default behaviour is used, so it’s important to understand the distinction between checked and unchecked exceptions.
Warning: dependency injection is the only way that declarative transactions work. You will not get a transactional service if you use the new operator such as new BookService()

The Transactional annotation vs the transactional property

In versions of Grails prior to Grails 3.1, Grails created Spring proxies and used the transactional property to enable and disable proxy creation. These proxies are disabled by default in applications created with Grails 3.1 and above in favor of the @Transactional transformation.

If you wish to renable this feature (not recommended) then you must set grails.spring.transactionManagement to true or remove the configuration in grails-app/conf/application.yml or grails-app/conf/application.groovy

In addition, prior to Grails 3.1 services were transactional by default, as of Grails 3.1 they are only transactional if the @Transactional transformation is applied.

Custom Transaction Configuration

Grails also provides @Transactional and @NotTransactional annotations for cases where you need more fine-grained control over transactions at a per-method level or need to specify an alternative propagation level. For example, the @NotTransactional annotation can be used to mark a particular method to be skipped when a class is annotated with @Transactional.

The grails.transaction.Transactional annotation was first introduced in Grails 2.3. Prior to 2.3, Spring’s @Transactional annotation was used.
Annotating a service method with Transactional disables the default Grails transactional behavior for that service (in the same way that adding transactional=false does) so if you use any annotations you must annotate all methods that require transactions.

In this example listBooks uses a read-only transaction, updateBook uses a default read-write transaction, and deleteBook is not transactional (probably not a good idea given its name).

import grails.transaction.Transactional

class BookService {

    @Transactional(readOnly = true)
    def listBooks() {
        Book.list()
    }

    @Transactional
    def updateBook() {
        // ...
    }

    def deleteBook() {
        // ...
    }
}

You can also annotate the class to define the default transaction behavior for the whole service, and then override that default per-method:

import grails.transaction.Transactional

@Transactional
class BookService {

    def listBooks() {
        Book.list()
    }

    def updateBook() {
        // ...
    }

    def deleteBook() {
        // ...
    }
}

This version defaults to all methods being read-write transactional (due to the class-level annotation), but the listBooks method overrides this to use a read-only transaction:

import grails.transaction.Transactional

@Transactional
class BookService {

    @Transactional(readOnly = true)
    def listBooks() {
        Book.list()
    }

    def updateBook() {
        // ...
    }

    def deleteBook() {
        // ...
    }
}

Although updateBook and deleteBook aren’t annotated in this example, they inherit the configuration from the class-level annotation.

For more information refer to the section of the Spring user guide on Using @Transactional.

Unlike Spring you do not need any prior configuration to use Transactional; just specify the annotation as needed and Grails will detect them up automatically.

Transaction status

An instance of TransactionStatus is available by default in Grails transactional service methods.

Example:

import grails.transaction.Transactional

@Transactional
class BookService {

    def deleteBook() {
        transactionStatus.setRollbackOnly()
    }
}

13.1.1 Transactions Rollback and the Session

Understanding Transactions and the Hibernate Session

When using transactions there are important considerations you must take into account with regards to how the underlying persistence session is handled by Hibernate. When a transaction is rolled back the Hibernate session used by GORM is cleared. This means any objects within the session become detached and accessing uninitialized lazy-loaded collections will lead to a LazyInitializationException.

To understand why it is important that the Hibernate session is cleared. Consider the following example:

class Author {
    String name
    Integer age

    static hasMany = [books: Book]
}

If you were to save two authors using consecutive transactions as follows:

Author.withTransaction { status ->
    new Author(name: "Stephen King", age: 40).save()
    status.setRollbackOnly()
}

Author.withTransaction { status ->
    new Author(name: "Stephen King", age: