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.

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 BuildConfig.groovy:

grails.compile.artefacts.closures.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"

Available Scopes

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

• servletContext - Also known as application scope, this scope lets you share state across the entire web application. The servletContext is an instance of ServletContext
• session - The session allows associating state with a given user and typically uses cookies to associate a session with a client. The session object is an instance of HttpSession
• request - The request object allows the storage of objects for the current request only. The request object is an instance of HttpServletRequest
• params - Mutable map of incoming request query string or POST parameters
• flash - See below

Accessing Scopes

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

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

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

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

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

Using Flash Scope

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

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

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

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

Scoped Controllers

By default, a new controller instance is created for each request. In fact, because the controller is prototype scoped, it is thread-safe since each request happens on its own thread.

You can change this behaviour by placing a controller in a particular scope. The supported 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 under in Config.groovy with the grails.controllers.defaultScope key, for example:

grails.controllers.defaultScope = "singleton"

Use scoped controllers wisely. For instance, we don't recommend having any properties in a singleton-scoped controller since they will be shared for all requests. Setting a default scope other than prototype may also lead to unexpected behaviors if you have controllers provided by installed plugins that expect that the scope is prototype.

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.

If no explicit model is returned the controller's properties will be used as the model, thus allowing you to write code like this:

class BookController {
    List books
    List authors
    def list() {
        books = Book.list()
        authors = Author.list()
    }
}

This is possible due to the fact that controllers are prototype scoped. In other words a new controller is created for each request. Otherwise code such as the above would not be thread-safe, and all users would share the same data.

In the above example the books and authors properties will be available in the view.

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 list 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.

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 {
        }
    }
    // …
}

Redirects

Actions can be redirected using the redirect controller method:

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

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

The redirect method expects one of:

• 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".

Chaining

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

class ExampleChainController {
    def first() {
        chain(action: second, model: [one: 1])
    }
    def second () {
        chain(action: third, model: [two: 2])
    }
    def third() {
        [three: 3])
    }
}

results in the model:

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

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

class ChainController {
    def nextInChain() {
        def model = chainModel.myModel
        …
    }
}

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

chain(action: "action1", model: [one: 1], params: [myparam: "param1"])

If your interceptor is likely to apply to more than one controller, you are almost certainly better off writing a Filter. Filters can be applied to multiple controllers or URIs without the need to change the logic of each controller

Before Interception

The beforeInterceptor intercepts processing before the action is executed. If it returns false then the intercepted action will not be executed. The interceptor can be defined for all actions in a controller as follows:

def beforeInterceptor = {
    println "Tracing action ${actionUri}"
}

The above is declared inside the body of the controller definition. It will be executed before all actions and does not interfere with processing. A common use case is very simplistic authentication:

def beforeInterceptor = [action: this.&auth, except: 'login']
// defined with private scope, so it's not considered an action
private auth() {
    if (!session.user) {
        redirect(action: 'login')
        return false
    }
}
def login() {
    // display login page
}

The above code defines a method called auth. A private method is used so that it is not exposed as an action to the outside world. The beforeInterceptor then defines an interceptor that is used on all actions except the login action and it executes the auth method. The auth method is referenced using Groovy's method pointer syntax. Within the method it detects whether there is a user in the session, and if not it redirects to the login action and returns false, causing the intercepted action to not be processed.

After Interception

Use the afterInterceptor property to define an interceptor that is executed after an action:

def afterInterceptor = { model ->
    println "Tracing action ${actionUri}"
}

The after interceptor takes the resulting model as an argument and can hence manipulate the model or response.

An after interceptor may also modify the Spring MVC ModelAndView object prior to rendering. In this case, the above example becomes:

def afterInterceptor = { model, modelAndView ->
    println "Current view is ${modelAndView.viewName}"
    if (model.someVar) modelAndView.viewName = "/mycontroller/someotherview"
    println "View is now ${modelAndView.viewName}"
}

This allows the view to be changed based on the model returned by the current action. Note that the modelAndView may be null if the action being intercepted called redirect or render.

Interception Conditions

Rails users will be familiar with the authentication example and how the 'except' condition was used when executing the interceptor (interceptors are called 'filters' in Rails; this terminology conflicts with Servlet filter terminology in Java):

def beforeInterceptor = [action: this.&auth, except: 'login']

This executes the interceptor for all actions except the specified action. A list of actions can also be defined as follows:

def beforeInterceptor = [action: this.&auth, except: ['login', 'register']]

The other supported condition is 'only', this executes the interceptor for only the specified action(s):

def beforeInterceptor = [action: this.&auth, only: ['secure']]

Grails uses Spring's underlying data binding capability to perform data binding.

Binding Request Data to the Model

There are two ways to bind request parameters onto the properties of a domain class. The first involves using a domain classes' Map constructor:

def save() {
    def b = new Book(params)
    b.save()
}

The data binding happens within the code new Book(params). By passing the params object to the domain class constructor Grails automatically recognizes that you are trying to bind from request parameters. So if we had an incoming request like:

/book/save?title=The%20Stand&author=Stephen%20King

Then the title and author request parameters would automatically be set on the domain class. You can use the properties property to perform data binding onto an existing instance:

def save() {
    def b = Book.get(params.id)
    b.properties = params
    b.save()
}

This has the same effect as using the implicit constructor.

Data binding and Single-ended Associations

If you have a one-to-one or many-to-one association you can use Grails' data binding capability to update these relationships too. For example if you have an incoming request such as:

/book/save?author.id=20

Grails will automatically detect the .id suffix on the request parameter and look up the Author instance for the given id when doing data binding such as:

def b = new Book(params)

An association property can be set to null by passing the literal String "null". For example:

/book/save?author.id=null

Data Binding and Many-ended Associations

If you have a one-to-many or many-to-many association there are different techniques for data binding depending of the association type.

If you have a Set based association (the default for a hasMany) then the simplest way to populate an association is to send a list of identifiers. For example consider the usage of <g:select> below:

<g:select name="books"
          from="${Book.list()}"
          size="5" multiple="yes" optionKey="id"
          value="${author?.books}" />

This produces a select box that lets you select multiple values. In this case if you submit the form Grails will automatically use the identifiers from the select box to populate the books association.

However, if you have a scenario where you want to update the properties of the associated objects the this technique won't work. Instead you use the subscript operator:

<g:textField name="books[0].title" value="the Stand" />
<g:textField name="books[1].title" value="the Shining" />

However, with Set based association it is critical that you render the mark-up in the same order that you plan to do the update in. This is because a Set has no concept of order, so although we're referring to 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. If you "skipped" a few elements in the middle:

<g:textField name="books[0].title" value="the Stand" />
<g:textField name="books[1].title" value="the Shining" />
<g:textField name="books[5].title" value="Red Madder" />

Then Grails will automatically create instances in between. For example in the above case Grails will create 4 additional instances if the association being bound had a size of 2.

You can bind existing instances of the associated type to a List using the same .id syntax as you would use with a single-ended association. For example:

<g:select name="books[0].id" from="${bookList}"
          value="${author?.books[0]?.id}" />
<g:select name="books[1].id" from="${bookList}"
          value="${author?.books[1]?.id}" />
<g:select name="books[2].id" from="${bookList}"
          value="${author?.books[2]?.id}" />

Would allow individual entries in the books List to be selected separately.

Entries at particular indexes can be removed in the same way too. For example:

<g:select name="books[0].id"
          from="${Book.list()}"
          value="${author?.books[0]?.id}"
          noSelection="['null': '']"/>

Will render a select box that will remove the association at 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"
          from="${Image.list()}"
          value="${book?.images[cover]?.id}"
          noSelection="['null': '']"/>

This would bind the selected image into the Map property images under a key of "cover".

Data binding with Multiple domain classes

It is possible to bind data to multiple domain objects from the params object.

For example so you have an incoming request to:

/book/save?book.title=The%20Stand&author.name=Stephen%20King

You'll notice the difference with the above request is that each parameter has a prefix such as author. or book. which is used to isolate which parameters belong to which type. Grails' params object is like a multi-dimensional hash and you can index into it to isolate only a subset of the parameters to bind.

def b = new Book(params.book)

Notice how we use the prefix before the first dot of the book.title parameter to isolate only parameters below this level to bind. We could do the same with an Author domain class:

def a = new Author(params.author)

Data Binding and Action Arguments

Controller action arguments are subject to request parameter data binding. There are 2 categories of controller action arguments. The first category is command objects. Complex types are treated as command objects. See the Command Objects section of the user guide for details. The other category is basic object types. Supported types are the 8 primitives, their corresponding type wrappers and java.lang.String. The default behavior is to map request parameters to action arguments by name:

class AccountingController {
   // accountNumber will be initialized with the value of params.accountNumber
   // accountType will be initialized with params.accountType
   def displayInvoice(String accountNumber, int accountType) {
       // …
   }
}

For primitive arguments and arguments which are instances of any of the primitive type wrapper classes a type conversion has to be carried out before the request parameter value can be bound to the action argument. The type conversion happens automatically. In a case like the example shown above, the params.accountType request parameter has to be converted to an int. If type conversion fails for any reason, the argument will have its default value per normal Java behavior (null for type wrapper references, false for booleans and zero for numbers) and a corresponding error will be added to the errors property of the defining controller.

/accounting/displayInvoice?accountNumber=B59786&accountType=bogusValue

Since "bogusValue" cannot be converted to type int, the value of accountType will be zero, controller.errors.hasErrors() will be true, controller.errors.errorCount will be equal to 1 and controller.errors.getFieldError('accountType') will contain the corresponding error.

If the argument name does not match the name of the request parameter then the @grails.web.RequestParameter annotation may be applied to an argument to express the name of the request parameter which should be bound to that argument:

import grails.web.RequestParameter
class AccountingController {
   // mainAccountNumber will be initialized with the value of params.accountNumber
   // accountType will be initialized with params.accountType
   def displayInvoice(@RequestParameter('accountNumber') String mainAccountNumber, int accountType) {
       // …
   }
}

Data binding and type conversion errors

Sometimes when performing data binding it is not possible to convert a particular String into a particular target type. This results in a type conversion error. Grails will retain type conversion errors inside the errors property of a Grails domain class. For example:

class Book {
    …
    URL publisherURL
}

Here we have a domain class Book that uses the java.net.URL class to represent URLs. Given an incoming request such as:

/book/save?publisherURL=a-bad-url

it is not possible to bind the string a-bad-url to the publisherURL property as a type mismatch error occurs. You can check for these like this:

def b = new Book(params)
if (b.hasErrors()) {
    println "The value ${b.errors.getFieldError('publisherURL').rejectedValue}" +
            " is not a valid URL!"
}

Although we have not yet covered error codes (for more information see the section on Validation), for type conversion errors you would want a message from the grails-app/i18n/messages.properties file to use for the error. You can use a generic error message handler such as:

typeMismatch.java.net.URL=The field {0} is not a valid URL

Or a more specific one:

typeMismatch.Book.publisherURL=The publisher URL you specified is not a valid URL

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.

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:

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

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: "text/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"?>
<list>
  <book id="1">
    <author>Stephen King</author>
    <title>The Stand</title>
  </book>
  <book id="2">
    <author>Stephen King</author>
    <title>The Shining</title>
  </book>
</list>

An alternative to using the converters is to use the codecs feature of Grails. The codecs feature provides encodeAsXML and encodeAsJSON methods:

def xml = Book.list().encodeAsXML()
render xml

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:

[
    {"id":1,
     "class":"Book",
     "author":"Stephen King",
     "title":"The Stand"},
    {"id":2,
     "class":"Book",
     "author":"Stephen King",
     "releaseDate":new Date(1194127343161),
     "title":"The Shining"}
 ]

Again as an alternative you can use the encodeAsJSON to achieve the same effect.

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 Config.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: "text/json") {
    hello = "world"
}

The above will produce the JSON:

{"hello":"world"}

Rendering JSON Arrays

To render a list of objects simple assign a list:

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

This will produce:

{"categories":["a","b","c"]}

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

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

The above code produces:

[1,2,3]

Rendering Complex Objects

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

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

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

def results = Book.list()
render(contentType: "text/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

Programmatic File Uploads

Grails supports file uploads using Spring's MultipartHttpServletRequest interface. The first step for file uploading is to create a multipart form like this:

Upload Form: <br />
    <g:uploadForm action="upload">
        <input type="file" name="myFile" />
        <input type="submit" />
    </g:uploadForm>

The uploadForm tag conveniently adds the enctype="multipart/form-data" attribute to the standard <g:form> tag.

There are then a number of ways to handle the file upload. One is to work with the Spring MultipartFile instance directly:

def upload() {
    def f = request.getFile('myFile')
    if (f.empty) {
        flash.message = 'file cannot be empty'
        render(view: 'uploadForm')
        return
    }
    f.transferTo(new File('/some/local/dir/myfile.txt'))
    response.sendError(200, 'Done')
}

This is convenient for doing transfers to other destinations and manipulating the file directly as you can obtain an InputStream and so on with the MultipartFile interface.

File Uploads through Data Binding

File uploads can also be performed using data binding. Consider this Image domain class:

class Image {
    byte[] myFile
    static constraints = {
        // Limit upload file size to 2MB
        myFile maxSize: 1024 * 1024 * 2
    }
}

If you create an image using the params object in the constructor as in the example below, Grails will automatically bind the file's contents as a byte to the myFile property:

def img = new Image(params)

It's important that you set the size or maxSize constraints, otherwise your database may be created with a small column size that can't handle reasonably sized files. For example, both H2 and MySQL default to a blob size of 255 bytes for byte properties.

It is also possible to set the contents of the file as a string by changing the type of the myFile property on the image to a String type:

class Image {
   String myFile
}

Declaring Command Objects

Command objects are typically declared in the same source file as a controller, directly below the controller class definition. For example:

class UserController {
    …
}
class LoginCommand {
    String username
    String password
    static constraints = {
        username(blank: false, minSize: 6)
        password(blank: false, minSize: 6)
    }
}

As this example shows, you can define constraints in command objects just like in domain classes.

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, populate and validate.

Before the controller action is executed Grails will automatically create an instance of the command object class, populate its properties with by binding the request parameters, and validate the command object. For example:

class LoginController {
    def login = { LoginCommand cmd ->
        if (cmd.hasErrors()) {
            redirect(action: 'loginForm')
            return
        }
        // work with the command object data
    }
}

When using methods instead of Closures for actions, you can specify command objects in arguments:

class LoginController {
    def login(LoginCommand cmd) {
        if (cmd.hasErrors()) {
            redirect(action: 'loginForm')
            return
        }
        // work with the command object data
    }
}

Command Objects and Dependency Injection

Command objects can participate in dependency injection. This is useful if your command object has some custom validation logic uses Grails services:

class LoginCommand {
    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.

<g:form useToken="true" ...>

Then in your controller code you can use the withForm method to handle valid and invalid requests:

withForm {
   // good request
}.invalidToken {
   // bad request
}

If you only provide the withForm method and not the chained invalidToken method then by default Grails will store the invalid token in a flash.invalidToken variable and redirect the request back to the original page. This can then be checked in the view:

<g:if test="${flash.invalidToken}">
  Don't click the button twice!
</g:if>

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

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
}

grails.servlet.version = "3.0"

With that done ensure you do a clean re-compile as some async features are enabled at compile time.

With a Servlet target version of 3.0 you can only deploy on Servlet 3.0 containers such as Tomcat 7 and above.

Asynchronous Rendering

You can render content (templates, binary data etc.) in an asynchronous manner by calling the startAsync method which returns an instance of the Servlet 3.0 AsyncContext. Once you have a reference to the AsyncContext you can use Grails' regular render method to render content:

def index() {
    def ctx = startAsync()
    ctx.start {
        new Book(title:"The Stand").save()
        render template:"books", model:[books:Book.list()]
        ctx.complete()
    }
}

Note that you must call the complete() method to terminate the connection.

Resuming an Async Request

You resume processing of an async request (for example to delegate to view rendering) by using the dispatch method of the AsyncContext class:

def index() {
    def ctx = startAsync()
    ctx.start {
        // do working
        …
        // render view
        ctx.dispatch()
    }
}

GSPs live in the grails-app/views directory and are typically rendered automatically (by convention) or with the render method such as:

render(view: "index")

A GSP is typically a mix of mark-up and GSP tags which aid in view rendering.

Although it is possible to have Groovy logic embedded in your GSP and doing this will be covered in this document, the practice is strongly discouraged. Mixing mark-up and code is a bad thing and most GSP pages contain no code and needn't do so.

A GSP typically has a "model" which is a set of variables that are used for view rendering. The model is passed to the GSP view from a controller. For example consider the following controller action:

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

This action will look up a Book instance and create a model that contains a key called book. This key can then be referenced within the GSP view using the name book:

${book.title}

GSP supports the usage of <% %> scriptlet blocks to embed Groovy code (again this is discouraged):

<html>
   <body>
     <% out << "Hello GSP!" %>
   </body>
</html>

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

<html>
   <body>
     <%="Hello GSP!" %>
   </body>
</html>

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

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

<% now = new Date() %>

and then access those variables later in the page:

<%=now%>

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

• application - The javax.servlet.ServletContext instance
• applicationContext The Spring ApplicationContext instance
• flash - The flash object
• grailsApplication - The GrailsApplication instance
• out - The response writer for writing to the output stream
• params - The params object for retrieving request parameters
• request - The HttpServletRequest instance
• response - The HttpServletResponse instance
• session - The HttpSession instance
• webRequest - The GrailsWebRequest instance

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

As well as logical branching:

<html>
   <body>
      <% if (params.hello == 'true')%>
      <%="Hello!"%>
      <% else %>
      <%="Goodbye!"%>
   </body>
</html>

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="text/json" %>

The contentType directive allows using GSP to render other formats.

<html>
  <body>
    Hello ${params.name}
  </body>
</html>

However, unlike JSP EL you can have any Groovy expression within the ${..} block. Variables within the ${..} block are not escaped by default, so any HTML in the variable's string is rendered directly to the page. To reduce the risk of Cross-site-scripting (XSS) attacks, you can enable automatic HTML escaping with the grails.views.default.codec setting in grails-app/conf/Config.groovy:

grails.views.default.codec='html'

Other possible values are 'none' (for no default encoding) and 'base64'.

The section on Tag Libraries covers how to add your own custom tag libraries.

All built-in GSP tags start with the prefix g:. Unlike JSP, you don't specify any tag library imports. If a tag starts with g: it is automatically assumed to be a GSP tag. An example GSP tag would look like:

<g:example />

GSP tags can also have a body such as:

<g:example>
   Hello world
</g:example>

Expressions can be passed into GSP tag attributes, if an expression is not used it will be assumed to be a String value:

<g:example attr="${new Date()}">
   Hello world
</g:example>

Maps can also be passed into GSP tag attributes, which are often used for a named parameter style syntax:

<g:example attr="${new Date()}" attr2="[one:1, two:2, three:3]">
   Hello world
</g:example>

Note that within the values of attributes you must use single quotes for Strings:

<g:example attr="${new Date()}" attr2="[one:'one', two:'two']">
   Hello world
</g:example>

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

<g:set var="now" value="${new Date()}" />

Here we assign a variable called now to the result of a GSP expression (which simply constructs a new java.util.Date instance). You can also use the body of the <g:set> tag to define a variable:

<g:set var="myHTML">
   Some re-usable code on: ${new Date()}
</g:set>

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" />

<g:if test="${session.role == 'admin'}">
   <%-- show administrative functions --%>
</g:if>
<g:else>
   <%-- show basic functions --%>
</g:else>

Use the each and while tags for iteration:

<g:each in="${[1,2,3]}" var="num">
   <p>Number ${num}</p>
</g:each>
<g:set var="num" value="${1}" />
<g:while test="${num < 5 }">
    <p>Number ${num++}</p>
</g:while>

Stephen King's Books:
<g:findAll in="${books}" expr="it.author == 'Stephen King'">
     <p>Title: ${it.title}</p>
</g:findAll>

The expr attribute contains a Groovy expression that can be used as a filter. The grep tag does a similar job, for example filtering by class:

<g:grep in="${books}" filter="NonFictionBooks.class">
     <p>Title: ${it.title}</p>
</g:grep>

Or using a regular expression:

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

The above example is also interesting due to its usage of GPath. GPath is an XPath-like language in Groovy. The books variable is a collection of Book instances. Since each Book has a title, you can obtain a list of Book titles using the expression books.title. Groovy will auto-magically iterate the collection, obtain each title, and return a new list!

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 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 WYSWIG 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")

Template Basics

Grails uses the convention of placing an underscore before the name of a view to identify it as a template. For example, you might have a template that renders Books located at grails-app/views/book/_bookTemplate.gsp:

<div class="book" id="${book?.id}">
   <div>Title: ${book?.title}</div>
   <div>Author: ${book?.author?.name}</div>
</div>

Use the render tag to render this template from one of the views in grails-app/views/book:

<g:render template="bookTemplate" model="[book: myBook]" />

Notice how we pass into a model to use using the model attribute of the render tag. If you have multiple Book instances you can also render the template for each Book using the render tag with a collection attribute:

<g:render template="bookTemplate" var="book" collection="${bookList}" />

Shared Templates

In the previous example we had a template that was specific to the BookController and its views at grails-app/views/book. However, you may want to share templates across your application.

In this case you can place them in the root views directory at grails-app/views or any subdirectory below that location, and then with the template attribute use an absolute location starting with / instead of a relative location. For example if you had a template called grails-app/views/shared/_mySharedTemplate.gsp, you would reference it as:

<g:render template="/shared/mySharedTemplate" />

You can also use this technique to reference templates in any directory from any view or controller:

<g:render template="/book/bookTemplate" model="[book: myBook]" />

The Template Namespace

Since templates are used so frequently there is template namespace, called tmpl, available that makes using templates easier. Consider for example the following usage pattern:

<g:render template="bookTemplate" model="[book:myBook]" />

This can be expressed with the tmpl namespace as follows:

<tmpl:bookTemplate book="${myBook}" />

Templates in Controllers and Tag Libraries

You can also render templates from controllers using the render controller method. This is useful for Ajax 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.

Creating Layouts

Grails leverages Sitemesh, a decorator engine, to support view layouts. Layouts are located in the grails-app/views/layouts directory. A typical layout can be seen below:

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

The key elements are the layoutHead, layoutTitle and layoutBody tag invocations:

• layoutTitle - outputs the target page's title
• layoutHead - outputs the target page's head tag contents
• layoutBody - outputs the target page's body tag contents

The previous example also demonstrates the pageProperty tag which can be used to inspect and return aspects of the target page.

Triggering Layouts

There are a few ways to trigger a layout. The simplest is to add a meta tag to the view:

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

In this case a layout called grails-app/views/layouts/main.gsp will be used to layout the page. If we were to use the layout from the previous section the output would resemble this:

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

Specifying A Layout In A Controller

Another way to specify a layout is to specify the name of the layout by assigning a value to the "layout" property in a controller. For example, if you have a controller such as:

class BookController {
    static layout = 'customer'
    def list() { … }
}

You can create a layout called grails-app/views/layouts/customer.gsp which will be applied to all views that the BookController delegates to. The value of the "layout" property may contain a directory structure relative to the grails-app/views/layouts/ directory. For example:

class BookController {
    static layout = 'custom/customer'
    def list() { … }
}

Views rendered from that controller would be decorated with the grails-app/views/layouts/custom/customer.gsp template.

Layout by Convention

Another way to associate layouts is to use "layout by convention". For example, if you have this controller:

class BookController {
    def list() { … }
}

You can create a layout called grails-app/views/layouts/book.gsp, which will be applied to all views that the BookController delegates to.

Alternatively, you can create a layout called grails-app/views/layouts/book/list.gsp which will only be applied to the list action within the BookController.

If you have both the above mentioned layouts in place the layout specific to the action will take precedence when the list action is executed.

If a layout may not be located using any of those conventions, the convention of last resort is to look for the application default layout which is grails-app/views/layouts/application.gsp. The name of the application default layout may be changed by defining a property in grails-app/conf/Config.groovy as follows:

grails.sitemesh.default.layout = 'myLayoutName'

With that property in place, the application default layout will be grails-app/views/layouts/myLayoutName.gsp.

Inline Layouts

Grails' also supports Sitemesh's concept of inline layouts with the applyLayout tag. This can be used to apply a layout to a template, URL or arbitrary section of content. This lets you even further modularize your view structure by "decorating" your template includes.

Some examples of usage can be seen below:

<g:applyLayout name="myLayout" template="bookTemplate" collection="${books}" />
<g:applyLayout name="myLayout" url="http://www.google.com" />
<g:applyLayout name="myLayout">
The content to apply a layout to
</g:applyLayout>

Server-Side Includes

While the applyLayout tag is useful for applying layouts to external content, if you simply want to include external content in the current page you use the include tag:

<g:include controller="book" action="list" />

You can even combine the include tag and the applyLayout tag for added flexibility:

<g:applyLayout name="myLayout">
   <g:include controller="book" action="list" />
</g:applyLayout>

Finally, you can also call the include tag from a controller or tag library as a method:

def content = include(controller:"book", action:"list")

The resulting content will be provided via the return value of the include tag.

The basic way to include a link to a static resource 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 Resources framework tackles are:

• Web application performance tuning is difficult
• Correct ordering of resources, and deferred inclusion of JavaScript
• Resources that depend on others that must be loaded first
• The need for a standard way to expose static resources in plugins and applications
• The need for an extensible processing chain to optimize resources
• Preventing multiple inclusion of the same resource

The plugin achieves this by introducing new artefacts and processing the resources using the server's local file system.

It adds artefacts for declaring resources, for declaring "mappers" that can process resources, and a servlet filter to serve processed resources.

What you get is an incredibly advanced resource system that enables you to easily create highly optimized web applications that run the same in development and in production.

The Resources plugin documentation provides a more detailed overview of the concepts which will be beneficial when reading the following guide.

Pulling in resources with r:require

To use resources, your GSP page must indicate which resource modules it requires. For example with the jQuery plugin, which exposes a "jquery" resource module, to use jQuery in any page on your site you simply add:

<html>
   <head>
      <r:require module="jquery"/>
      <r:layoutResources/>
   </head>
   <body>
      …
      <r:layoutResources/>
   </body>
</html>

This will automatically include all resources needed for jQuery, including them at the correct locations in the page. By default the plugin sets the disposition to be "head", so they load early in the page.

You can call r:require multiple times in a GSP page, and you use the "modules" attribute to provide a list of modules:

<html>
   <head>
      <r:require modules="jquery, main, blueprint, charting"/>
      <r:layoutResources/>
   </head>
   <body>
      …
      <r:layoutResources/>
   </body>
</html>

The above may result in many JavaScript and CSS files being included, in the correct order, with some JavaScript files loading at the end of the body to improve the apparent page load time.

However you cannot use r:require in isolation - as per the examples you must have the <r:layoutResources/> tag to actually perform the render.

Rendering the links to resources with r:layoutResources

When you have declared the resource modules that your GSP page requires, the framework needs to render the links to those resources at the correct time.

To achieve this correctly, you must include the r:layoutResources tag twice in your page, or more commonly, in your GSP layout:

<html>
   <head>
      <g:layoutTitle/>
      <r:layoutResources/>
   </head>
   <body>
      <g:layoutBody/>
      <r:layoutResources/>
   </body>
</html>

This represents the simplest Sitemesh layout you can have that supports Resources.

The Resources framework has the concept of a "disposition" for every resource. This is an indication of where in the page the resource should be included.

The default disposition applied depends on the type of resource. All CSS must be rendered in <head> in HTML, so "head" is the default for all CSS, and will be rendered by the first r:layoutResources. Page load times are improved when JavaScript is loaded after the page content, so the default for JavaScript files is "defer", which means it is rendered when the second r:layoutResources is invoked.

Note that both your GSP page and your Sitemesh layout (as well as any GSP template fragments) can call r:require to depend on resources. The only limitation is that you must call r:require before the r:layoutResources that should render it.

Adding page-specific JavaScript code with r:script

Grails has the javascript tag which is adapted to defer to Resources plugin if installed, but it is recommended that you call r:script directly when you need to include fragments of JavaScript code.

This lets you write some "inline" JavaScript which is actually not rendered inline, but either in the <head> or at the end of the body, based on the disposition.

Given a Sitemesh layout like this:

<html>
   <head>
      <g:layoutTitle/>
      <r:layoutResources/>
   </head>
   <body>
      <g:layoutBody/>
      <r:layoutResources/>
   </body>
</html>

...in your GSP you can inject some JavaScript code into the head or deferred regions of the page like this:

<html>
   <head>
      <title>Testing r:script magic!</title>
   </head>
   <body>
      <r:script disposition="head">
         window.alert('This is at the end of <head>');
      </r:script>
      <r:script disposition="defer">
         window.alert('This is at the end of the body, and the page has loaded.');
      </r:script>
   </body>
</html>

The default disposition is "defer", so the disposition in the latter r:script is purely included for demonstration.

Note that such r:script code fragments always load after any modules that you have used, to ensure that any required libraries have loaded.

Linking to images with r:img

This tag is used to render <img> markup, using the Resources framework to process the resource on the fly (if configured to do so - e.g. make it eternally cacheable).

This includes any extra attributes on the <img> tag if the resource has been previously declared in a module.

With this mechanism you can specify the width, height and any other attributes in the resource declaration in the module, and they will be pulled in as necessary.

Example:

<html>
   <head>
      <title>Testing r:img</title>
   </head>
   <body>
      <r:img uri="/images/logo.png"/>
   </body>
</html>

Note that Grails has a built-in g:img tag as a shortcut for rendering <img> tags that refer to a static resource. The Grails img tag is Resources-aware and will delegate to r:img if found. However it is recommended that you use r:img directly if using the Resources plugin.

Alongside the regular Grails resource tag attributes, this also supports the "uri" attribute for increased brevity.

See r:resource documentation for full details.

r:resource

This is equivalent to the Grails resource tag, returning a link to the processed static resource. Grails' own g:resource tag delegates to this implementation if found, but if your code requires the Resources plugin, you should use r:resource directly.

Alongside the regular Grails resource tag attributes, this also supports the "uri" attribute for increased brevity.

See r:resource documentation for full details.

r:external

This is a resource-aware version of Grails external tag which renders the HTML markup necessary to include an external file resource such as CSS, JS or a favicon.

See r:resource documentation for full details.

Note that you do not need to declare all your static resources, especially images. However you must to establish dependencies or other resources-specific attributes. Any resource that is not declared is called "ad-hoc" and will still be processed using defaults for that resource type.

Consider this example resource configuration file, grails-app/conf/MyAppResources.groovy:

modules = {
    core {
        dependsOn 'jquery, utils'
        resource url: '/js/core.js', disposition: 'head'
        resource url: '/js/ui.js'
        resource url: '/css/main.css',
        resource url: '/css/branding.css'
        resource url: '/css/print.css', attrs: [media: 'print']
    }
    utils {
        dependsOn 'jquery'
        resource url: '/js/utils.js'
    }
    forms {
        dependsOn 'core,utils'
        resource url: '/css/forms.css'
        resource url: '/js/forms.js'
    }
}

This defines three resource modules; 'core', 'utils' and 'forms'. The resources in these modules will be automatically bundled out of the box according to the module name, resulting in fewer files. You can override this with bundle:'someOtherName' on each resource, or call defaultBundle on the module (see resources plugin documentation).

It declares dependencies between them using dependsOn, which controls the load order of the resources.

When you include an <r:require module="forms"/> in your GSP, it will pull in all the resources from 'core' and 'utils' as well as 'jquery', all in the correct order.

You'll also notice the disposition:'head' on the core.js file. This tells Resources that while it can defer all the other JS files to the end of the body, this one must go into the <head>.

The CSS file for print styling adds custom attributes using the attrs map option, and these are passed through to the r:external tag when the engine renders the link to the resource, so you can customize the HTML attributes of the generated link.

There is no limit to the number of modules or xxxResources.groovy artefacts you can provide, and plugins can supply them to expose modules to applications, which is exactly how the jQuery plugin works.

To define modules like this in your application's Config.groovy, you simply assign the DSL closure to the grails.resources.modules Config variable.

For full details of the resource DSL please see the resources plugin documentation.

For example, you may wish to bundle jQuery and some other libraries all together in one file. There is a load-time and caching trade-off here, but often it is the case that you'd like to override some of these settings.

To do this, the DSL supports an "overrides" clause, within which you can change the defaultBundle setting for a module, or attributes of individual resources that have been declared with a unique id:

modules = {
    core {
        dependsOn 'jquery, utils'
        defaultBundle 'monolith'
        resource url: '/js/core.js', disposition: 'head'
        resource url: '/js/ui.js'
        resource url: '/css/main.css',
        resource url: '/css/branding.css'
        resource url: '/css/print.css', attrs: [media: 'print']
    }
    utils {
        dependsOn 'jquery'
        defaultBundle 'monolith'
        resource url: '/js/utils.js'
    }
    forms {
        dependsOn 'core,utils'
        defaultBundle 'monolith'
        resource url: '/css/forms.css'
        resource url: '/js/forms.js'
    }
    overrides {
        jquery {
            defaultBundle 'monolith'
        }
    }
}

This will put all code into a single bundle named 'monolith'. Note that this can still result in multiple files, as separate bundles are required for head and defer dispositions, and JavaScript and CSS files are bundled separately.

Note that overriding individual resources requires the original declaration to have included a unique id for the resource.

For full details of the resource DSL please see the resources plugin documentation.

The resource mappers are applied to each static resource once, in a specific order. You can create your own resource mappers, and several plugins provide some already for zipping, caching and minifying.

Out of the box, the Resources plugin provides bundling of resources into fewer files, which is achieved with a few mappers that also perform CSS re-writing to handle when your CSS files are moved into a bundle.

Bundling multiple resources into fewer files

The 'bundle' mapper operates by default on any resource with a "bundle" defined - or inherited from a defaultBundle clause on the module. Modules have an implicit default bundle name the same as the name of the module.

Files of the same kind will be aggregated into this bundle file. Bundles operate across module boundaries:

modules = {
    core {
        dependsOn 'jquery, utils'
        defaultBundle 'common'
        resource url: '/js/core.js', disposition: 'head'
        resource url: '/js/ui.js', bundle: 'ui'
        resource url: '/css/main.css', bundle: 'theme'
        resource url: '/css/branding.css'
        resource url: '/css/print.css', attrs: [media: 'print']
    }
    utils {
        dependsOn 'jquery'
        resource url: '/js/utils.js', bundle: 'common'
    }
    forms {
        dependsOn 'core,utils'
        resource url: '/css/forms.css', bundle: 'ui'
        resource url: '/js/forms.js', bundle: 'ui'
    }
}

Here you see that resources are grouped into bundles; 'common', 'ui' and 'theme' - across module boundaries.

Note that auto-bundling by module does not occur if there is only one resource in the module.

Making resources cache "eternally" in the client browser

Caching resources "eternally" in the client is only viable if the resource has a unique name that changes whenever the contents change, and requires caching headers to be set on the response.

The cached-resources plugin provides a mapper that achieves this by hashing your files and renaming them based on this hash. It also sets the caching headers on every response for those resources. To use, simply install the cached-resources plugin.

Note that the caching headers can only be set if your resources are being served by your application. If you have another server serving the static content from your app (e.g. Apache HTTPD), configure it to send caching headers. Alternatively you can configure it to request and proxy the resources from your container.

Zipping resources

Returning gzipped resources is another way to reduce page load times and reduce bandwidth.

The zipped-resources plugin provides a mapper that automatically compresses your content, excluding by default already compressed formats such as gif, jpeg and png.

Simply install the zipped-resources plugin and it works.

Minifying

There are a number of CSS and JavaScript minifiers available to obfuscate and reduce the size of your code. At the time of writing none are publicly released but releases are imminent.

There are several debugging features built in to the Resources framework.

X-Grails-Resources-Original-Src Header

Every resource served in development mode will have the X-Grails-Resources-Original-Src: header added, indicating the original source file(s) that make up the response.

Adding the debug flag

If you add a query parameter _debugResources=y to your URL and request the page, Resources will bypass any processing so that you can see your original source files.

This also adds a unique timestamp to all your resource URLs, to defeat any caching that browsers may use. This means that you should always see your very latest code when you reload the page.

Turning on debug all the time

You can turn on the aforementioned debug mechanism without requiring a query parameter, but turning it on in Config.groovy:

grails.resources.debug = true

You can of course set this per-environment.

Preventing the application of a specific mapper to an individual resource

All resource declarations support a convention of noXXXX:true where XXXX is a mapper name.

So for example to prevent the "hashandcache" mapper from being applied to a resource (which renames and moves it, potentially breaking relative links written in JavaScript code), you would do this:

modules = {
    forms {
        resource url: '/css/forms.css', nohashandcache: true
        resource url: '/js/forms.js', nohashandcache: true
    }
}

Excluding/including paths and file types from specific mappers

Mappers have includes/excludes Ant patterns to control whether they apply to a given resource. Mappers set sensible defaults for these based on their activity, for example the zipped-resources plugin's "zip" mapper is set to exclude images by default.

You can configure this in your Config.groovy using the mapper name e.g:

// We wouldn't link to .exe files using Resources but for the sake of example:
grails.resources.zip.excludes = ['**/*.zip', '**/*.exe']
// Perhaps for some reason we want to prevent bundling on "less" CSS files:
grails.resources.bundle.excludes = ['**/*.less']

There is also an "includes" inverse. Note that settings these replaces the default includes/excludes for that mapper - it is not additive.

Controlling what is treated as an "ad-hoc" (legacy) resource

Ad-hoc resources are those undeclared, but linked to directly in your application without using the Grails or Resources linking tags (resource, img or external).

These may occur with some legacy plugins or code with hardcoded paths in.

There is a Config.groovy setting grails.resources.adhoc.patterns which defines a list of Servlet API compliant filter URI mappings, which the Resources filter will use to detect such "ad-hoc resource" requests.

By default this is set to:

grails.resources.adhoc.patterns = ['images/*', '*.js', '*.css']

• jquery
• jquery-ui
• blueprint
• lesscss-resources
• zipped-resources
• cached-resources

<content tag="navbar">
… draw the navbar here…
</content>
<content tag="header">
… draw the header here…
</content>
<content tag="footer">
… draw the footer here…
</content>
<content tag="body">
… draw the body here…
</content>

Then within the layout you can reference these components and apply individual layouts to each:

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

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/Config.groovy :

grails.gsp.enable.reload = true
grails.gsp.view.dir = "/var/www/grails/my-app/"

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

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:

GSP reloading is supported for precompiled GSPs since Grails 1.3.5 .

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 Config.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

• See Debugging GSP in STS

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.

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 SpringSource 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)
    }
}

• 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

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.

def isAdmin = { attrs, body ->
    def user = attrs.user
    if (user && checkUserPrivs(user)) {
        out << body()
    }
}

The tag above checks if the user is an administrator and only outputs the body content if he/she has the correct set of access privileges:

<g:isAdmin user="${myUser}">
    // some restricted content
</g:isAdmin>

def repeat = { attrs, body ->
    attrs.times?.toInteger()?.times { num ->
        out << body(num)
    }
}

In this example we check for a times attribute and if it exists convert it to a number, then use Groovy's times method to iterate the specified number of times:

<g:repeat times="3">
<p>Repeat this 3 times! Current repeat = ${it}</p>
</g:repeat>

Notice how in this example we use the implicit it variable to refer to the current number. This works because when we invoked the body we passed in the current value inside the iteration:

out << body(num)

That value is then passed as the default variable it to the tag. However, if you have nested tags this can lead to conflicts, so you should 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>
</g:repeat>

Notice how we use the var attribute to define the name of the variable j and then we are able to reference that variable within the body of the tag.

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

class ExampleFilters {
   def filters = {
        // your filters here
   }
}

Each filter you define within the filters block has a name and a scope. The name is the method name and the scope is defined using named arguments. For example to define a filter that applies to all controllers and all actions you can use wildcards:

sampleFilter(controller:'*', action:'*') {
  // interceptor definitions
}

The scope of the filter can be one of the following things:

• A controller and/or action name pairing with optional wildcards
• A URI, with Ant path matching syntax

Filter rule attributes:

• controller - controller matching pattern, by default * is replaced with .* and a regex is compiled
• controllerExclude - controller exclusion pattern, by default * is replaced with .* and a regex is compiled
• action - action matching pattern, by default * is replaced with .* and a regex is compiled
• actionExclude - action exclusion pattern, by default * is replaced with .* and a regex is compiled
• regex (true/false) - use regex syntax (don't replace '*' with '.*')
• uri - a uri to match, expressed with as Ant style path (e.g. /book/**)
• uriExclude - a uri pattern to exclude, expressed with as Ant style path (e.g. /book/**)
• find (true/false) - rule matches with partial match (see java.util.regex.Matcher.find())
• invert (true/false) - invert the rule (NOT rule)

Some examples of filters include:

• All controllers and actions

all(controller: '*', action: '*') {
}

• Only for the BookController

justBook(controller: 'book', action: '*') {
}

• All controllers except the BookController

notBook(controller: 'book', invert: true) {
}

• All actions containing 'save' in the action name

saveInActionName(action: '*save*', find: true) {
}

• All actions starting with the letter 'b' except for actions beginning with the phrase 'bad*'

actionBeginningWithBButNotBad(action: 'b*', actionExclude: 'bad*', find: true) {
}

• Applied to a URI space

someURIs(uri: '/book/**') {
}

• Applied to all URIs

allURIs(uri: '/**') {
}

In addition, the order in which you define the filters within the filters code block dictates the order in which they are executed. To control the order of execution between Filters classes, you can use the dependsOn property discussed in filter dependencies section.

Note: When exclude patterns are used they take precedence over the matching patterns. For example, if action is 'b*' and actionExclude is 'bad*' then actions like 'best' and 'bien' will have that filter applied but actions like 'bad' and 'badlands' will not.

• before - Executed before the action. Return false to indicate that the response has been handled that that all future filters and the action should not execute
• after - Executed after an action. Takes a first argument as the view model to allow modification of the model before rendering the view
• afterView - Executed after view rendering. Takes an Exception as an argument which will be non-null if an exception occurs during processing. Note: this Closure is called before the layout is applied.

For example to fulfill the common simplistic authentication use case you could define a filter as follows:

class SecurityFilters {
   def filters = {
       loginCheck(controller: '*', action: '*') {
           before = {
              if (!session.user && !actionName.equals('login')) {
                  redirect(action: 'login')
                  return false
               }
           }
       }
   }
}

Here the loginCheck filter uses a before interceptor to execute a block of code that checks if a user is in the session and if not redirects to the login action. Note how returning false ensure that the action itself is not executed.

Here's a more involved example that demonstrates all three filter types:

import java.util.concurrent.atomic.AtomicLong
class LoggingFilters {
   private static final AtomicLong REQUEST_NUMBER_COUNTER = new AtomicLong()
   private static final String START_TIME_ATTRIBUTE = 'Controller__START_TIME__'
   private static final String REQUEST_NUMBER_ATTRIBUTE = 'Controller__REQUEST_NUMBER__'
   def filters = {
      logFilter(controller: '*', action: '*') {
         before = {
            if (!log.debugEnabled) return true
            long start = System.currentTimeMillis()
            long currentRequestNumber = REQUEST_NUMBER_COUNTER.incrementAndGet()
            request[START_TIME_ATTRIBUTE] = start
            request[REQUEST_NUMBER_ATTRIBUTE] = currentRequestNumber
            log.debug "preHandle request #$currentRequestNumber : " +
               "'$request.servletPath'/'$request.forwardURI', " +
               "from $request.remoteHost ($request.remoteAddr) " +
               " at ${new Date()}, Ajax: $request.xhr, controller: $controllerName, " +
               "action: $actionName, params: ${new TreeMap(params)}"
            return true
         }
         after = { Map model ->
            if (!log.debugEnabled) return true
            long start = request[START_TIME_ATTRIBUTE]
            long end = System.currentTimeMillis()
            long requestNumber = request[REQUEST_NUMBER_ATTRIBUTE]
            def msg = "postHandle request #$requestNumber: end ${new Date()}, " +
                      "controller total time ${end - start}ms"
            if (log.traceEnabled) {
            	log.trace msg + "; model: $model"
            }
            else {
            	log.debug msg
            }
         }
         afterView = { Exception e ->
            if (!log.debugEnabled) return true
            long start = request[START_TIME_ATTRIBUTE]
            long end = System.currentTimeMillis()
            long requestNumber = request[REQUEST_NUMBER_ATTRIBUTE]
            def msg = "afterCompletion request #$requestNumber: " +
                      "end ${new Date()}, total time ${end - start}ms"
            if (e) {
               log.debug "$msg \n\texception: $e.message", e
            }
            else {
               log.debug msg
            }
         }
      }
   }
}

In this logging example we just log various request information, but note that the model map in the after filter is mutable. If you need to add or remove items from the model map you can do that in the after filter.

• request - The HttpServletRequest object
• response - The HttpServletResponse object
• session - The HttpSession object
• servletContext - The ServletContext object
• flash - The flash object
• params - The request parameters object
• actionName - The action name that is being dispatched to
• controllerName - The controller name that is being dispatched to
• grailsApplication - The Grails application currently running
• applicationContext - The ApplicationContext object

However, filters only support a subset of the methods available to controllers and tag libraries. These include:

• redirect - For redirects to other controllers and actions
• render - For rendering custom responses

Take the following example Filters classes:

class MyFilters {
    def dependsOn = [MyOtherFilters]
    def filters = {
        checkAwesome(uri: "/*") {
            before = {
                if (request.isAwesome) { // do something awesome }
            }
        }
        checkAwesome2(uri: "/*") {
            before = {
                if (request.isAwesome) { // do something else awesome }
            }
        }
    }
}

class MyOtherFilters {
    def filters = {
        makeAwesome(uri: "/*") {
            before = {
                request.isAwesome = true
            }
        }
        doNothing(uri: "/*") {
            before = {
                // do nothing
            }
        }
    }
}

MyFilters specifically dependsOn MyOtherFilters. This will cause all the filters in MyOtherFilters whose scope matches the current request to be executed before those in MyFilters. For a request of "/test", which will match the scope of every filter in the example, the execution order would be as follows:

• MyOtherFilters - makeAwesome
• MyOtherFilters - doNothing
• MyFilters - checkAwesome
• MyFilters - checkAwesome2

The filters within the MyOtherFilters class are processed in order first, followed by the filters in the MyFilters class. Execution order between Filters classes are enabled and the execution order of filters within each Filters class are preserved.

If any cyclical dependencies are detected, the filters with cyclical dependencies will be added to the end of the filter chain and processing will continue. Information about any cyclical dependencies that are detected will be written to the logs. Ensure that your root logging level is set to at least WARN or configure an appender for the Grails Filters Plugin (org.codehaus.groovy.grails.plugins.web.filters.FiltersGrailsPlugin) when debugging filter dependency issues.

This section covers Grails' support for Ajax in general. To get started, add this line to the <head> tag of your page:

<g:javascript library="jquery" />

You can replace jQuery with any other library supplied by a plugin you have installed. This works because of Grails' support for adaptive tag libraries. Thanks to Grails' plugin system there is support for a number of different Ajax libraries including (but not limited to):

• jQuery
• Prototype
• Dojo
• YUI
• MooTools

<g:remoteLink action="delete" id="1">Delete Book</g:remoteLink>

The above link sends an asynchronous request to the delete action of the current controller with an id of 1.

def delete() {
    def b = Book.get(params.id)
    b.delete()
    render "Book ${b.id} was deleted"
}

GSP code:

<div id="message"></div>
<g:remoteLink action="delete" id="1" update="message">
Delete Book
</g:remoteLink>

The above example will call the action and set the contents of the message div to the response in this case "Book 1 was deleted". This is done by the update attribute on the tag, which can also take a Map to indicate what should be updated on failure:

<div id="message"></div>
<div id="error"></div>
<g:remoteLink update="[success: 'message', failure: 'error']"
              action="delete" id="1">
Delete Book
</g:remoteLink>

Here the error div will be updated if the request failed.

<g:formRemote url="[controller: 'book', action: 'delete']"
              update="[success: 'message', failure: 'error']">
    <input type="hidden" name="id" value="1" />
    <input type="submit" value="Delete Book!" />
</g:formRemote >

Or alternatively you can use the submitToRemote tag to create a submit button. This allows some buttons to submit remotely and some not depending on the action:

<form action="delete">
    <input type="hidden" name="id" value="1" />
    <g:submitToRemote action="delete"
                      update="[success: 'message', failure: 'error']" />
</form>

<g:remoteLink action="show"
              id="1"
              update="success"
              onLoading="showProgress()"
              onComplete="hideProgress()">Show Book 1</g:remoteLink>

The above code will execute the "showProgress()" function which may show a progress bar or whatever is appropriate. Other events include:

• onSuccess - The JavaScript function to call if successful
• onFailure - The JavaScript function to call if the call failed
• on_ERROR_CODE - The JavaScript function to call to handle specified error codes (eg on404="alert('not found!')")
• onUninitialized - The JavaScript function to call the a Ajax engine failed to initialise
• onLoading - The JavaScript function to call when the remote function is loading the response
• onLoaded - The JavaScript function to call when the remote function is completed loading the response
• onComplete - The JavaScript function to call when the remote function is complete, including any updates

If you need a reference to the XmlHttpRequest object you can use the implicit event parameter e to obtain it:

<g:javascript>
    function fireMe(e) {
        alert("XmlHttpRequest = " + e)
    }
}
</g:javascript>
<g:remoteLink action="example"
              update="success"
              onSuccess="fireMe(e)">Ajax Link</g:remoteLink>

grails install-plugin prototype

This will download the current supported version of the Prototype plugin and install it into your Grails project. With that done you can add the following reference to the top of your page:

<g:javascript library="prototype" />

If you require Scriptaculous too you can do the following instead:

<g:javascript library="scriptaculous" />

Now all of Grails tags such as remoteLink, formRemote and submitToRemote work with Prototype remoting.

grails install-plugin dojo

This will download the current supported version of Dojo and install it into your Grails project. With that done you can add the following reference to the top of your page:

<g:javascript library="dojo" />

Now all of Grails tags such as remoteLink, formRemote and submitToRemote work with Dojo remoting.

• Content Centric Ajax - Where you just use the HTML result of a remote call to update the page
• Data Centric Ajax - Where you actually send an XML or JSON response from the server and programmatically update the page
• Script Centric Ajax - Where the server sends down a stream of JavaScript to be evaluated on the fly

Most of the examples in the Ajax section cover Content Centric Ajax where you are updating the page, but you may also want to use Data Centric or Script Centric. This guide covers the different styles of Ajax.

Content Centric Ajax

Just to re-cap, content centric Ajax involves sending some HTML back from the server and is typically done by rendering a template with the render method:

def showBook() {
    def b = Book.get(params.id)
    render(template: "bookTemplate", model: [book: b])
}

Calling this on the client involves using the remoteLink tag:

<g:remoteLink action="showBook" id="${book.id}"
              update="book${book.id}">Update Book</g:remoteLink>
<div id="book${book.id}">
   <!--existing book mark-up -->
</div>

Data Centric Ajax with JSON

Data Centric Ajax typically involves evaluating the response on the client and updating programmatically. For a JSON response with Grails you would typically use Grails' JSON marshalling capability:

import grails.converters.JSON
def showBook() {
    def b = Book.get(params.id)
    render b as JSON
}

And then on the client parse the incoming JSON request using an Ajax event handler:

<g:javascript>
function updateBook(e) {
    var book = eval("("+e.responseText+")") // evaluate the JSON
    $("book" + book.id + "_title").innerHTML = book.title
}
<g:javascript>
<g:remoteLink action="test" update="foo" onSuccess="updateBook(e)">
    Update Book
</g:remoteLink>
<g:set var="bookId">book${book.id}</g:set>
<div id="${bookId}">
    <div id="${bookId}_title">The Stand</div>
</div>

Data Centric Ajax with XML

On the server side using XML is equally simple:

import grails.converters.XML
def showBook() {
    def b = Book.get(params.id)
    render b as XML
}

However, since DOM is involved the client gets more complicated:

<g:javascript>
function updateBook(e) {
    var xml = e.responseXML
    var id = xml.getElementsByTagName("book").getAttribute("id")
    $("book" + id + "_title") = xml.getElementsByTagName("title")[0].textContent
}
<g:javascript>
<g:remoteLink action="test" update="foo" onSuccess="updateBook(e)">
    Update Book
</g:remoteLink>
<g:set var="bookId">book${book.id}</g:set>
<div id="${bookId}">
    <div id="${bookId}_title">The Stand</div>
</div>

Script Centric Ajax with JavaScript

Script centric Ajax involves actually sending JavaScript back that gets evaluated on the client. An example of this can be seen below:

def showBook() {
    def b = Book.get(params.id)
    response.contentType = "text/javascript"
    String title = b.title.encodeAsJavascript()
    render "$('book${b.id}_title')='${title}'"
}

The important thing to remember is to set the contentType to text/javascript. If you use Prototype on the client the returned JavaScript will automatically be evaluated due to this contentType setting.

Obviously in this case it is critical that you have an agreed client-side API as you don't want changes on the client breaking the server. This is one of the reasons Rails has something like RJS. Although Grails does not currently have a feature such as RJS there is a Dynamic JavaScript Plugin that offers similar capabilities.

Responding to both Ajax and non-Ajax requests

It's straightforward to have the same Grails controller action handle both Ajax and non-Ajax requests. Grails adds the isXhr() method to HttpServletRequest which can be used to identify Ajax requests. For example you could render a page fragment using a template for Ajax requests or the full page for regular HTTP requests:

def listBooks() {
    def books = Book.list(params)
    if (request.xhr) {
        render template: "bookTable", model: [books: books]
    } else {
        render view: "list", model: [books: books]
    }
}

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/Config.groovy using the grails.mime.types setting:

grails.mime.types = [ xml: ['text/xml', 'application/xml'],
                      text: 'text-plain',
                      js: 'text/javascript',
                      rss: 'application/rss+xml',
                      atom: 'application/atom+xml',
                      css: 'text/css',
                      csv: 'text/csv',
                      all: '*/*',
                      json: 'text/json',
                      html: ['text/html','application/xhtml+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.

Content Negotiation 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, on newer browser something all together more useful is sent such as (an example of a Firefox Accept header):

text/xml, application/xml, application/xhtml+xml, text/html;q=0.9,
text/plain;q=0.8, image/png, */*;q=0.5

Grails parses this incoming format and adds a property to the response object that outlines the preferred response format. For the above example the following assertion would pass:

assert 'html' == response.format

Why? The text/html media type has the highest "quality" rating of 0.9, therefore is the highest priority. If you have an older browser as mentioned previously the result is slightly different:

assert 'all' == response.format

In this case 'all' possible formats are accepted by the client. To deal with different kinds of requests from Controllers you can use the withFormat method that acts as kind of a switch statement:

import grails.converters.XML
class BookController {
    def list() {
        def books = Book.list()
        withFormat {
            html bookList: books
            js { render "alert('hello')" }
            xml { render books as XML }
        }
    }
}

If the preferred format is html then Grails will execute the html() call only. This causes Grails to look for a view called either grails-app/views/books/list.html.gsp or grails-app/views/books/list.gsp. If the format is xml then the closure will be invoked and an XML response rendered.

How do we handle the "all" format? Simply order the content-types within your withFormat block so that whichever one you want executed comes first. So in the above example, "all" will trigger the html handler.

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.

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 the format Request Parameter

If fiddling with request headers if not your favorite activity you can override the format used by specifying a format request parameter:

/book/list?format=xml

You can also define this parameter in the URL Mappings definition:

"/book/list"(controller:"book", action:"list") {
    format = "xml"
}

Content Negotiation with URI Extensions

Grails also supports content negotiation using URI extensions. For example given the following URI:

/book/list.xml

Grails will remove the extension and map it to /book/list instead whilst simultaneously setting the content format to xml based on this extension. This behaviour is enabled by default, so if you wish to turn it off, you must set the grails.mime.file.extensions property in grails-app/conf/Config.groovy to false:

grails.mime.file.extensions = false

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
}