(Quick Reference)

8 The Web Layer - Reference Documentation

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

Version: 3.1.1

Table of Contents

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.

Public Methods as Actions

In earlier versions of Grails actions were implemented with Closures. This is still supported, but the preferred approach is to use methods.

Leveraging methods instead of Closure properties has some advantages:

  • Memory efficient
  • Allow use of stateless controllers (singleton scope)
  • You can override actions from subclasses and call the overridden superclass method with super.actionName()
  • Methods can be intercepted with standard proxying mechanisms, something that is complicated to do with Closures since they're fields.

If you prefer the Closure syntax or have older controller classes created in earlier versions of Grails and still want the advantages of using methods, you can set the grails.compile.artefacts.closures.convert property to true in application.yml:

                convert: true

and a compile-time AST transformation will convert your Closures to methods in the generated bytecode.

If a controller class extends some other class which is not defined under the grails-app/controllers/ directory, methods inherited from that class are not converted to controller actions. If the intent is to expose those inherited methods as controller actions the methods may be overridden in the subclass and the subclass method may invoke the method in the super class.

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}"
    … // 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:

        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


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:

  • Another closure within the same controller class:

// Call the login action within the same class
redirect(action: login)
  • 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")

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".


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"])

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)

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:


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

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 grials-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:


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:


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"
          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 books0 and books1 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"
          noSelection="['null': '']"/>

Will render a select box that will remove the association at books0 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"
          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:


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.


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:


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 org.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.

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 org.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 org.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 default formats that are used are "yyyy-MM-dd HH:mm:ss.S", "yyyy-MM-dd'T'hh:mm:ss'Z'" and "yyyy-MM-dd HH:mm:ss.S z".

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 org.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 org.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 org.grails.databinding.BindingFormat

class Person { @BindingFormat(code='date.formats.birthdays') Date birthDate }

# grails-app/conf/i18n/messages.properties

# grails-app/conf/i18n/messages_es.properties

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/groovy/databinding/Gadget.groovy
package databinding

class Gadget { Shape expandedShape Shape compressedShape }

// src/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 org.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/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 org.grails.databinding.events;

import org.grails.databinding.errors.BindingError;

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.

The Grails data binder has limited support for the older BindEventListener style listeners. BindEventListener looks like this:

package org.codehaus.groovy.grails.web.binding;

import org.springframework.beans.MutablePropertyValues; import org.springframework.beans.TypeConverter;

public interface BindEventListener {

/** * @param target The target to bind to * @param source The source of the binding, typically a Map * @param typeConverter The type converter to be used */ void doBind(Object target, MutablePropertyValues source, TypeConverter typeConverter); }

Support for BindEventListener is disabled by default. To enable support assign a value of true to the grails.databinding.enableSpringEventAdapter property in grails-app/conf/application.groovy.

// grails-app/conf/application.groovy


With enableSpringEventAdapter set to true instances of BindEventListener which are in the Spring application context will automatically be registered with the data binder. Notice that the MutablePropertyValues and TypeConverter arguments to the doBind method in BindEventListener are Spring specific classes and are not relevant to the current data binder. The event adapter will pass null values for those arguments. The only real value passed into the doBind method will be the object being bound to. This limited support is provided for backward compatibility and will be useful for a subset of scenarios. Developers are encouraged to migrate their BindEventListener beans to the newer DataBindingListener model.

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/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 org.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']])

Note that 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 XML and JSON Responses

Using the render method to output XML

Grails supports a few different ways to produce XML and JSON responses. The first is the render method.

The render method can be passed a block of code to do mark-up building in XML:

def list() {

def results = Book.list()

render(contentType: "text/xml") { books { for (b in results) { book(title: b.title) } } } }

The result of this code would be something like:

    <book title="The Stand" />
    <book title="The Shining" />

Be careful to avoid naming conflicts when using mark-up building. For example this code would produce an error:

def list() {

def books = Book.list() // naming conflict here

render(contentType: "text/xml") { books { for (b in results) { book(title: b.title) } } } }

This is because there is local variable books which Groovy attempts to invoke as a method.

Using the render method to output JSON

The render method can also be used to output JSON:

def list() {

def results = Book.list()

render(contentType: "application/json") { books = array { for (b in results) { book title: b.title } } } }

In this case the result would be something along the lines of:

    {"title":"The Stand"},
    {"title":"The Shining"}

The same dangers with naming conflicts described above for XML also apply to JSON building.

Automatic XML Marshalling

Grails also supports automatic marshalling of domain classes to XML using special converters.

To start off with, import the grails.converters package into your controller:

import grails.converters.*

Now you can use the following highly readable syntax to automatically convert domain classes to XML:

render Book.list() as XML

The resulting output would look something like the following::

<?xml version="1.0" encoding="ISO-8859-1"?>
  <book id="1">
    <author>Stephen King</author>
    <title>The Stand</title>
  <book id="2">
    <author>Stephen King</author>
    <title>The Shining</title>

For more information on XML marshalling see the section on REST

Automatic JSON Marshalling

Grails also supports automatic marshalling to JSON using the same mechanism. Simply substitute XML with JSON:

render Book.list() as JSON

The resulting output would look something like the following:

     "author":"Stephen King",
     "title":"The Stand"},
     "author":"Stephen King",
     "releaseDate":new Date(1194127343161),
     "title":"The Shining"}

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, the JSON builder is a custom implementation specific to Grails.

JSONBuilder and Grails versions

JSONBuilder behaves different depending on the version of Grails you use. For version below 1.2 the deprecated grails.web.JSONBuilder class is used. This section covers the usage of the Grails 1.2 JSONBuilder

For backwards compatibility the old JSONBuilder class is used with the render method for older applications; to use the newer/better JSONBuilder class set the following in application.groovy:

grails.json.legacy.builder = false

Rendering Simple Objects

To render a simple JSON object just set properties within the context of the Closure:

render(contentType: "application/json") {
    hello = "world"

The above will produce the JSON:


Rendering JSON Arrays

To render a list of objects simple assign a list:

render(contentType: "application/json") {
    categories = ['a', 'b', 'c']

This will produce:


You can also render lists of complex objects, for example:

render(contentType: "application/json") {
    categories = [ { a = "A" }, { b = "B" } ]

This will produce:

{"categories":[ {"a":"A"} , {"b":"B"}] }

Use the special element method to return a list as the root:

render(contentType: "application/json") {
    element 1
    element 2
    element 3

The above code produces:


Rendering Complex Objects

Rendering complex objects can be done with Closures. For example:

render(contentType: "application/json") {
    categories = ['a', 'b', 'c']
    title = "Hello JSON"
    information = {
        pages = 10

The above will produce the JSON:

{"categories":["a","b","c"],"title":"Hello JSON","information":{"pages":10}}

Arrays of Complex Objects

As mentioned previously you can nest complex objects within arrays using Closures:

render(contentType: "application/json") {
    categories = [ { a = "A" }, { b = "B" } ]

You can use the array method to build them up dynamically:

def results = Book.list()
render(contentType: "application/json") {
    books = array {
        for (b in results) {
            book title: b.title

Direct JSONBuilder API Access

If you don't have access to the render method, but still want to produce JSON you can use the API directly:

def builder = new JSONBuilder()

def result = builder.build { categories = ['a', 'b', 'c'] title = "Hello JSON" information = { pages = 10 } }

// prints the JSON text println result.toString()

def sw = new StringWriter() result.render sw

8.1.8 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" />

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')

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

8.1.9 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.

Note: 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","size":"42"}' 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
~ $

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

// ...

} }

8.1.10 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!

The withForm tag makes use of the session and hence requires session affinity or clustered sessions if used in a cluster.

8.1.11 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

8.1.12 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/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.

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:


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):

     <% out << "Hello GSP!" %>

You can also use the <%= %> syntax to output values:

     <%="Hello GSP!" %>

GSP also supports JSP-style server-side comments (which are not rendered in the HTML response) as the following example demonstrates:

     <%-- This is my comment --%>
     <%="Hello GSP!" %>

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. Variables and Scopes

Within the <% %> brackets you can declare variables:

<% now = new Date() %>

and then access those variables later in the page:


Within the scope of a GSP there are a number of pre-defined variables, including: Logic and Iteration

Using the <% %> syntax you can embed loops and so on using this syntax:

      <% [1,2,3,4].each { num -> %>
         <p><%="Hello ${num}!" %></p>

As well as logical branching:

      <% if (params.hello == 'true')%>
      <% else %>
</html> 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. 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}:

    Hello ${params.name}

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:

   Hello world

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

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

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

With the basic syntax out the way, the next sections look at the tags that are built into Grails by default. 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()}

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" /> 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 --%>
   <%-- show basic functions --%>

Use the each and while tags for iteration:

<g:each in="${[1,2,3]}" var="num">
   <p>Number ${num}</p>

<g:set var="num" value="${1}" /> <g:while test="${num < 5 }"> <p>Number ${num++}</p> </g:while> 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>

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>

Or using a regular expression:

<g:grep in="${books.title}" filter="~/.*?Groovy.*?/">
     <p>Title: ${it}</p>

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! 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> 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" /> 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>

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:

        <title><g:layoutTitle default="An example decorator" /></title>
        <g:layoutHead />
    <body onload="${pageProperty(name:'body.onload')}">
        <div class="menu"><!--my common menu goes here--></div>
        <div class="body">
            <g:layoutBody />

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:

        <title>An Example Page</title>
        <meta name="layout" content="main" />
    <body>This is my content!</body>

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:

        <title>An Example Page</title>
    <body onload="">
        <div class="menu"><!--my common menu goes here--></div>
        <div class="body">
            This is my content!

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" />

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 2.0 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 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:

        <div id="header">
            <g:applyLayout name="headerLayout">
                <g:pageProperty name="page.header" />
        <div id="nav">
            <g:applyLayout name="navLayout">
                <g:pageProperty name="page.navbar" />
        <div id="body">
            <g:applyLayout name="bodyLayout">
                <g:pageProperty name="page.body" />
        <div id="footer">
            <g:applyLayout name="footerLayout">
                <g:pageProperty name="page.footer" />

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:

grails.gsp.enable.reloadalternative system property for enabling the GSP reload mode without changing application.groovy 
grails.gsp.reload.intervalinterval between checking the lastmodified time of the gsp source file, unit is milliseconds5000
grails.gsp.reload.granularitythe 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 times1000

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 javax.servlet.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

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>

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)

Note 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>

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:


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.


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/UrlMappings.groovy.

The UrlMappings class contains a single property called mappings that has been assigned a block of code:

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", {

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:


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 MethodURIGrails Action

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'])


"/books"(resources:'book', includes:['index', 'show'])

Explicit REST Mappings

As of Grails 3.1, if you prefer not to rely on the 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:


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 oppose to resources):


This results in the following URL mappings:

HTTP MethodURIGrails Action

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') {

The above will result in the following URL mappings:

HTTP MethodURLGrails Action

You can also nest regular URL mappings within a resource mapping:

"/books"(resources: "book") {

This will result in the following URL being available:

HTTP MethodURLGrails Action

To map a URI directly below a resource then use a collection:

"/books"(resources: "book") {
    collection {

This will result in the following URL being available (without the ID):

HTTP MethodURLGrails Action

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') {

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:


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 = {

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:


Optional File Extensions

If you wish to capture the extension of a particular path, then a special case mapping exists:


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") 

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 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:


However, it would also allow:


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 {

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

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

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/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 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.)

8.5.1 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'

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.

8.5.2 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() {

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.

8.5.3 Ordering Interceptor Execution

Interceptors can be ordered by defining an order property that defines a priority.

For example:

class AuthInterceptor {


… }

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:

    order: 50

Or in grails-app/conf/application.groovy:

beans {
  authInterceptor {
    order = 50

Thus giving you complete control over interceptor execution order.

8.6 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:

            all: '*/*'
            atom: application/atom+xml
            css: text/css
            csv: text/csv
            form: application/x-www-form-urlencoded
              - text/html
              - application/xhtml+xml
            js: text/javascript
              - application/json
              - text/json
            multipartForm: multipart/form-data
            rss: application/rss+xml
            text: text/plain
              - application/hal+json
              - application/hal+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:


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


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:


This works as a result of the default URL Mapping definition which is:


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:


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 }