(Quick Reference)

3 Upgrading

Version: 3.2.4

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



Build time configuration is now defined in a Gradle build file



Renamed for consistency with Spring Boot



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



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



Moved for consistency with Gradle



Moved for consistency with Gradle


src/main/groovy (yes, groovy!)

Moved for consistency with Gradle



Moved for consistency with Gradle



Moved for consistency with Gradle


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

Moved for consistency with Gradle



The plugin descriptor moved to a source directory

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


The Gradle build descriptor located in the root of the project


Properties file defining the Grails and Gradle versions


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


Configuration can now also be defined using YAML


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


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


Merged together into application.yml


Dependency resolution should be used to resolve JAR files


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


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


Removed. Sitemesh filter no longer present.


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 = {



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:

        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


import grails.transaction.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:

            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:

                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:

            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:


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 {

  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:

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.