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(Quick Reference) # 7 The Web Layer - Reference Documentation Authors: Graeme Rocher, Peter Ledbrook, Marc Palmer, Jeff Brown, Luke Daley, Burt Beckwith, Lari Hotari Version: 2.3.8 ## Table of Contents # 7 The Web Layer ## 7.1 Controllers A controller handles requests and creates or prepares the response. A controller can generate the response directly or delegate to a view. To create a controller, simply create a class whose name ends with `Controller` in the `grails-app/controllers` directory (in a subdirectory if it's in a package). The default URL Mapping configuration ensures that the first part of your controller name is mapped to a URI and each action defined within your controller maps to URIs within the controller name URI. ## 7.1.1 Understanding Controllers and Actions #### Creating a controller Controllers can be created with the create-controller or generate-controller command. For example try running the following command from the root of a Grails project:
    grails create-controller book
The command will create a controller at the location `grails-app/controllers/myapp/BookController.groovy`:

    package myappclass 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"
## 7.1.2 Controllers and Scopes #### Available Scopes Scopes are hash-like objects where you can store variables. The following scopes are available to controllers: servletContext - Also known as application scope, this scope lets you share state across the entire web application. The servletContext is an instance of ServletContext session - The session allows associating state with a given user and typically uses cookies to associate a session with a client. The session object is an instance of HttpSession request - The request object allows the storage of objects for the current request only. The request object is an instance of HttpServletRequest params - Mutable map of incoming request query string or POST parameters flash - See below #### Accessing Scopes Scopes can be accessed using the variable names above in combination with Groovy's array index operator, even on classes provided by the Servlet API such as the HttpServletRequest:
    class BookController {
        def find() {
            def findBy = params["findBy"]
            def appContext = request["foo"]
            def loggedUser = session["logged_user"]
        }
    }
You can also access values within scopes using the de-reference operator, making the syntax even more clear:
    class BookController {
        def find() {
            def findBy = params.findBy
            def appContext = request.foo
            def loggedUser = session.logged_user
        }
    }
This is one of the ways that Grails unifies access to the different scopes. #### Using Flash Scope Grails supports the concept of flash scope as a temporary store to make attributes available for this request and the next request only. Afterwards the attributes are cleared. This is useful for setting a message directly before redirecting, for example:
    def delete() {
        def b = Book.get(params.id)
        if (!b) {
            flash.message = "User not found for id ${params.id}"
            redirect(action:list)
        }
        … // remaining code
    }
When the `list` action is requested, the `message` value will be in scope and can be used to display an information message. It will be removed from the `flash` scope after this second request. Note that the attribute name can be anything you want, and the values are often strings used to display messages, but can be any object type. #### Scoped Controllers Supported controller scopes are: `prototype` (default) - A new controller will be created for each request (recommended for actions as Closure properties) `session` - One controller is created for the scope of a user session `singleton` - Only one instance of the controller ever exists (recommended for actions as methods) To enable one of the scopes, add a static `scope` property to your class with one of the valid scope values listed above, for example
    static scope = "singleton"
You can define the default strategy under in `Config.groovy` with the `grails.controllers.defaultScope` key, for example:
    grails.controllers.defaultScope = "singleton"
Newly created applications have the `grails.controllers.defaultScope` property set in `grails-app/conf/Config.groovy` with a value of "singleton". You may change this value to any of the supported scopes listed above. If the property is not assigned a value at all, controllers will default to "prototype" scope. 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. ## 7.1.3 Models and Views #### Returning the Model A model is a Map that the view uses when rendering. The keys within that Map correspond to variable names accessible by the view. There are a couple of ways to return a model. First, you can explicitly return a Map instance:
    def show() {
        [book: Book.get(params.id)]
    }
The above does not reflect what you should use with the scaffolding views - see the scaffolding section for more details. A more advanced approach is to return an instance of the Spring ModelAndView class:

    import org.springframework.web.servlet.ModelAndViewdef index() {
        // get some books just for the index page, perhaps your favorites
        def favoriteBooks = ...    // forward to the list view to show them
        return new ModelAndView("/book/list", [ bookList : favoriteBooks ])
    }
One thing to bear in mind is that certain variable names can not be used in your model: `attributes` `application` Currently, no error will be reported if you do use them, but this will hopefully change in a future version of Grails. #### Selecting the View In both of the previous two examples there was no code that specified which view to render. So how does Grails know which one to pick? The answer lies in the conventions. Grails will look for a view at the location `grails-app/views/book/show.gsp` for this `show` action:
    class BookController {
        def show() {
             [book: Book.get(params.id)]
        }
    }
To render a different view, use the render method:
    def show() {
        def map = [book: Book.get(params.id)]
        render(view: "display", model: map)
    }
In this case Grails will attempt to render a view at the location `grails-app/views/book/display.gsp`. Notice that Grails automatically qualifies the view location with the `book` directory of the `grails-app/views` directory. This is convenient, but to access shared views you need instead you can use an absolute path instead of a relative one:
    def show() {
        def map = [book: Book.get(params.id)]
        render(view: "/shared/display", model: map)
    }
In this case Grails will attempt to render a view at the location `grails-app/views/shared/display.gsp`. Grails also supports JSPs as views, so if a GSP isn't found in the expected location but a JSP is, it will be used instead. #### 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 {
            }
        }
        // …
    }
## 7.1.4 Redirects and Chaining #### Redirects Actions can be redirected using the redirect controller method:

    class OverviewController {    def login() {}    def find() {
            if (!session.user)
                redirect(action: 'login')
                return
            }
            …
        }
    }
Internally the redirect method uses the HttpServletResponse object's `sendRedirect` method. The `redirect` method expects one of: 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"])
## 7.1.5 Controller Interceptors Often it is useful to intercept processing based on either request, session or application state. This can be achieved with action interceptors. There are currently two types of interceptors: before and after. 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']]
## 7.1.6 Data Binding Data binding is the act of "binding" incoming request parameters onto the properties of an object or an entire graph of objects. Data binding should deal with all necessary type conversion since request parameters, which are typically delivered by a form submission, are always strings whilst the properties of a Groovy or Java object may well not be. #### Map Based Binding The data binder is capable of converting and assigning values in a Map to properties of an object. The binder will associate entries in the Map to properties of the object using the keys in the Map that have values which correspond to property names on the object. The following code demonstrates the basics:
    // grails-app/domain/Person.groovy
    class Person {
        String firstName
        String lastName
        Integer age
    }

    def bindingMap = [firstName: 'Peter', lastName: 'Gabriel', age: 63]def person = new Person(bindingMap)assert person.firstName == 'Peter'
    assert person.lastName == 'Gabriel'
    assert person.age == 63
To update properties of a domain object you may assign a Map to the `properties` property of the domain class:

    def bindingMap = [firstName: 'Peter', lastName: 'Gabriel', age: 63]def person = Person.get(someId)
    person.properties = bindingMapassert person.firstName == 'Peter'
    assert person.lastName == 'Gabriel'
    assert person.age == 63
The binder can populate a full graph of objects using Maps of Maps.

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

    def bindingMap = [firstName: 'Peter', lastName: 'Gabriel', age: 63, homeAddress: [county: 'Surrey', country: 'England'] ]def person = new Person(bindingMap)assert person.firstName == 'Peter'
    assert person.lastName == 'Gabriel'
    assert person.age == 63
    assert person.homeAddress.county == 'Surrey'
    assert person.homeAddress.country == 'England'
#### Binding To Collections And Maps The data binder can populate and update Collections and Maps. The following code shows a simple example of populating a `List` of objects in a domain class:

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

    def bindingMap = [name: 'Genesis',
                      'albums[0]': [title: 'Foxtrot', numberOfTracks: 6],
                      'albums[1]': [title: 'Nursery Cryme', numberOfTracks: 7]]def band = new Band(bindingMap)assert band.name == 'Genesis'
    assert band.albums.size() == 2
    assert band.albums[0].title == 'Foxtrot'
    assert band.albums[0].numberOfTracks == 6
    assert band.albums[1].title == 'Nursery Cryme'
    assert band.albums[1].numberOfTracks == 7
That code would work in the same way if `albums` were an array instead of a `List`. Note that when binding to a `Set` the structure of the `Map` being bound to the `Set` is the same as that of a `Map` being bound to a `List` but since a `Set` is unordered, the indexes don't necessarily correspond to the order of elements in the `Set`. In the code example above, if `albums` were a `Set` instead of a `List`, the `bindingMap` could look exactly the same but 'Foxtrot' might be the first album in the `Set` or it might be the second. When updating existing elements in a `Set` the `Map` being assigned to the `Set` must have `id` elements in it which represent the element in the `Set` being updated, as in the following example:

    /*
     * The value of the indexes 0 and 1 in albums[0] and albums[1] are arbitrary
     * values that can be anything as long as they are unique within the Map.
     * They do not correspond to the order of elements in albums because albums
     * is a Set.
     */
    def bindingMap = ['albums[0]': [id: 9, title: 'The Lamb Lies Down On Broadway']
                      'albums[1]': [id: 4, title: 'Selling England By The Pound']]def band = Band.get(someBandId)/*
     * This will find the Album in albums that has an id of 9 and will set its title
     * to 'The Lamb Lies Down On Broadway' and will find the Album in albums that has
     * an id of 4 and set its title to 'Selling England By The Pound'.  In both
     * cases if the Album cannot be found in albums then the album will be retrieved
     * from the database by id, the Album will be added to albums and will be updated
     * with the values described above.  If a Album with the specified id cannot be
     * found in the database, then a binding error will be created and associated
     * with the band object.  More on binding errors later.
     */
    band.properties = bindingMap
When binding to a `Map` the structure of the binding `Map` is the same as the structore of a `Map` used for binding to a `List` or a `Set` and the index inside of square brackets corresponds to the key in the `Map` being bound to. See the following code:

    class Album {
        String title
        static hasMany = [players: Player]
        Map players
    }class Player {
        String name
    }

    def bindingMap = [title: 'The Lamb Lies Down On Broadway',
                      'players[guitar]': [name: 'Steve Hackett'],
                      'players[vocals]': [name: 'Peter Gabriel'],
                      'players[keyboards]': [name: 'Tony Banks']]def album = new Album(bindingMap)assert album.title == 'The Lamb Lies Down On Broadway'
    assert album.players.size() == 3
    assert album.players.guitar.name == 'Steve Hackett'
    assert album.players.vocals.name == 'Peter Gabriel'
    assert album.players.keyboards.name == 'Tony Banks'
When updating an exisiting `Map`, if the key specified in the binding `Map` does not exist in the `Map` being bound to then a new value will be created and added to the `Map` with the specified key as in the following example:

    def bindingMap = [title: 'The Lamb Lies Down On Broadway',
                      'players[guitar]': [name: 'Steve Hackett'],
                      'players[vocals]': [name: 'Peter Gabriel']
                      'players[keyboards]': [name: 'Tony Banks']]def album = new Album(bindingMap)assert album.title == 'The Lamb Lies Down On Broadway'
    assert album.players.size() == 3
    assert album.players.guitar == 'Steve Hackett'
    assert album.players.vocals == 'Peter Gabriel'
    assert album.players.keyboards == 'Tony Banks'def updatedBindingMap = ['players[drums]': [name: 'Phil Collins'],
                             'players[keyboards]': [name: 'Anthony George Banks']]album.properties = updatedBindingMapassert album.title == 'The Lamb Lies Down On Broadway'
    assert album.players.size() == 4
    assert album.players.guitar == 'Steve Hackett'
    assert album.players.vocals == 'Peter Gabriel'
    assert album.players.keyboards == 'Anthony George Banks'
    assert album.players.drums == 'Phil Collins'
#### Binding Request Data to the Model The params object that is available in a controller has special behavior that helps convert dotted request parameter names into nested Maps that the data binder can work with. For example, if a request includes request parameters named `person.homeAddress.country` and `person.homeAddress.city` with values 'USA' and 'St. Louis' respectively, `params` would include entries like these:
    [person: [homeAddress: [country: 'USA', city: 'St. Louis']]]
There are two ways to bind request parameters onto the properties of a domain class. The first involves using a domain classes' Map constructor:
    def save() {
        def b = new Book(params)
        b.save()
    }
The data binding happens within the code `new Book(params)`. By passing the params object to the domain class constructor Grails automatically recognizes that you are trying to bind from request parameters. So if we had an incoming request like:
    /book/save?title=The%20Stand&author=Stephen%20King
Then the `title` and `author` request parameters would automatically be set on the domain class. You can use the properties property to perform data binding onto an existing instance:
    def save() {
        def b = Book.get(params.id)
        b.properties = params
        b.save()
    }
This has the same effect as using the implicit constructor. When binding an empty String (a String with no characters in it, not even spaces), the data binder will convert the empty String to null. This simplifies the most common case where the intent is to treat an empty form field as having the value null since there isn't a way to actually submit a null as a request parameter. When this behavior is not desireable the application may assign the value directly. The mass property binding mechanism will by default automatically trim all Strings at binding time. To disable this behavior set the `grails.databinding.trimStrings` property to false in `grails-app/conf/Config.groovy`.

    // the default value is true
    grails.databinding.trimStrings = false// ...
The mass property binding mechanism will by default automatically convert all empty Strings to null at binding time. To disable this behavior set the `grails.databinding.convertEmptyStringsToNull` property to false in `grials-app/conf/Config.groovy`.

    // the default value is true
    grails.databinding.convertEmptyStringsToNull = false// ...
The order of events is that the String trimming happens and then null conversion happens so if `trimStrings` is `true` and `convertEmptyStringsToNull` is `true`, not only will empty Strings be converted to null but also blank Strings. A blank String is any String such that the `trim()` method returns an empty String. These forms of data binding in Grails are very convenient, but also indiscriminate. In other words, they will bind all non-transient, typed instance properties of the target object, including ones that you may not want bound. Just because the form in your UI doesn't submit all the properties, an attacker can still send malign data via a raw HTTP request. Fortunately, Grails also makes it easy to protect against such attacks - see the section titled "Data Binding and Security concerns" for more information. #### Data binding and Single-ended Associations If you have a `one-to-one` or `many-to-one` association you can use Grails' data binding capability to update these relationships too. For example if you have an incoming request such as:
    /book/save?author.id=20
Grails will automatically detect the `.id` suffix on the request parameter and look up the `Author` instance for the given id when doing data binding such as:
    def b = new Book(params)
An association property can be set to `null` by passing the literal `String` "null". For example:
    /book/save?author.id=null
#### Data Binding and Many-ended Associations If you have a one-to-many or many-to-many association there are different techniques for data binding depending of the association type. If you have a `Set` based association (the default for a `hasMany`) then the simplest way to populate an association is to send a list of identifiers. For example consider the usage of `<g:select>` below:
    <g:select name="books"
              from="${Book.list()}"
              size="5" multiple="yes" optionKey="id"
              value="${author?.books}" />
This produces a select box that lets you select multiple values. In this case if you submit the form Grails will automatically use the identifiers from the select box to populate the `books` association. However, if you have a scenario where you want to update the properties of the associated objects the this technique won't work. Instead you use the subscript operator:
    <g:textField name="books[0].title" value="the Stand" />
    <g:textField name="books[1].title" value="the Shining" />
However, with `Set` based association it is critical that you render the mark-up in the same order that you plan to do the update in. This is because a `Set` has no concept of order, so although we're referring to `books0` and `books1` it is not guaranteed that the order of the association will be correct on the server side unless you apply some explicit sorting yourself. This is not a problem if you use `List` based associations, since a `List` has a defined order and an index you can refer to. This is also true of `Map` based associations. Note also that if the association you are binding to has a size of two and you refer to an element that is outside the size of association:
    <g:textField name="books[0].title" value="the Stand" />
    <g:textField name="books[1].title" value="the Shining" />
    <g:textField name="books[2].title" value="Red Madder" />
Then Grails will automatically create a new instance for you at the defined position. You can bind existing instances of the associated type to a `List` using the same `.id` syntax as you would use with a single-ended association. For example:

    <g:select name="books[0].id" from="${bookList}"
              value="${author?.books[0]?.id}" /><g:select name="books[1].id" from="${bookList}"
              value="${author?.books[1]?.id}" /><g:select name="books[2].id" from="${bookList}"
              value="${author?.books[2]?.id}" />
Would allow individual entries in the `books List` to be selected separately. Entries at particular indexes can be removed in the same way too. For example:
    <g:select name="books[0].id"
              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"`. When binding to Maps, Arrays and Collections the data binder will automatically grow the size of the collections as necessary. The default limit to how large the binder will grow a collection is 256. If the data binder encounters an entry that requires the collection be grown beyond that limit, the entry is ignored. The limit may be configured by assigning a value to the `grails.databinding.autoGrowCollectionLimit` property in `Config.groovy`.

    // grails-app/conf/Config.groovy// the default value is 256
    grails.databinding.autoGrowCollectionLimit = 128// ...
#### Data binding with Multiple domain classes It is possible to bind data to multiple domain objects from the params object. For example so you have an incoming request to:
    /book/save?book.title=The%20Stand&author.name=Stephen%20King
You'll notice the difference with the above request is that each parameter has a prefix such as `author.` or `book.` which is used to isolate which parameters belong to which type. Grails' `params` object is like a multi-dimensional hash and you can index into it to isolate only a subset of the parameters to bind.
    def b = new Book(params.book)
Notice how we use the prefix before the first dot of the `book.title` parameter to isolate only parameters below this level to bind. We could do the same with an `Author` domain class:
    def a = new Author(params.author)
#### Data Binding and Action Arguments Controller action arguments are subject to request parameter data binding. There are 2 categories of controller action arguments. The first category is command objects. Complex types are treated as command objects. See the Command Objects section of the user guide for details. The other category is basic object types. Supported types are the 8 primitives, their corresponding type wrappers and java.lang.String. The default behavior is to map request parameters to action arguments by name:

    class AccountingController {   // accountNumber will be initialized with the value of params.accountNumber
       // accountType will be initialized with params.accountType
       def displayInvoice(String accountNumber, int accountType) {
           // …
       }
    }
For primitive arguments and arguments which are instances of any of the primitive type wrapper classes a type conversion has to be carried out before the request parameter value can be bound to the action argument. The type conversion happens automatically. In a case like the example shown above, the `params.accountType` request parameter has to be converted to an `int`. If type conversion fails for any reason, the argument will have its default value per normal Java behavior (null for type wrapper references, false for booleans and zero for numbers) and a corresponding error will be added to the `errors` property of the defining controller.
    /accounting/displayInvoice?accountNumber=B59786&accountType=bogusValue
Since "bogusValue" cannot be converted to type int, the value of accountType will be zero, the controller's `errors.hasErrors()` will be true, the controller's `errors.errorCount` will be equal to 1 and the controller's `errors.getFieldError('accountType')` will contain the corresponding error. If the argument name does not match the name of the request parameter then the `@grails.web.RequestParameter` annotation may be applied to an argument to express the name of the request parameter which should be bound to that argument:

    import grails.web.RequestParameterclass AccountingController {   // mainAccountNumber will be initialized with the value of params.accountNumber
       // accountType will be initialized with params.accountType
       def displayInvoice(@RequestParameter('accountNumber') String mainAccountNumber, int accountType) {
           // …
       }
    }
#### Data binding and type conversion errors Sometimes when performing data binding it is not possible to convert a particular String into a particular target type. This results in a type conversion error. Grails will retain type conversion errors inside the errors property of a Grails domain class. For example:
    class Book {
        …
        URL publisherURL
    }
Here we have a domain class `Book` that uses the `java.net.URL` class to represent URLs. Given an incoming request such as:
    /book/save?publisherURL=a-bad-url
it is not possible to bind the string `a-bad-url` to the `publisherURL` property as a type mismatch error occurs. You can check for these like this:

    def b = new Book(params)if (b.hasErrors()) {
        println "The value ${b.errors.getFieldError('publisherURL').rejectedValue}" +
                " is not a valid URL!"
    }
Although we have not yet covered error codes (for more information see the section on Validation), for type conversion errors you would want a message from the `grails-app/i18n/messages.properties` file to use for the error. You can use a generic error message handler such as:
    typeMismatch.java.net.URL=The field {0} is not a valid URL
Or a more specific one:
    typeMismatch.Book.publisherURL=The publisher URL you specified is not a valid URL
#### The BindUsing Annotation The BindUsing annotation may be used to define a custom binding mechanism for a particular field in a class. Any time data binding is being applied to the field the closure value of the annotation will be invoked with 2 arguments. The first argument is the object that data binding is being applied to and the second argument is DataBindingSource which is the data source for the data binding. The value returned from the closure will be bound to the property. The following example would result in the upper case version of the `name` value in the source being applied to the `name` field during data binding.

    import org.grails.databinding.BindUsingclass SomeClass {
        @BindUsing({obj, source ->        //source is DataSourceBinding which is similar to a Map
            //and defines getAt operation but source.name cannot be used here.
            //In order to get name from source use getAt instead as shown below.        source['name']?.toUpperCase()
        })
        String name
    }
Note that data binding is only possible when the name of the request parameter matches with the field name in the class. Here, `name` from request parameters matches with `name` from `SomeClass`. The BindUsing annotation may be used to define a custom binding mechanism for all of the fields on a particular class. When the annotation is applied to a class, the value assigned to the annotation should be a class which implements the BindingHelper interface. An instance of that class will be used any time a value is bound to a property in the class that this annotation has been applied to.
    @BindUsing(SomeClassWhichImplementsBindingHelper)
    class SomeClass {
        String someProperty
        Integer someOtherProperty
    }
#### Custom Data Converters The binder will do a lot of type conversion automatically. Some applications may want to define their own mechanism for converting values and a simple way to do this is to write a class which implements ValueConverter and register an instance of that class as a bean in the Spring application context.

    package com.myapp.convertersimport org.grails.databinding.converters.ValueConverter/**
     * A custom converter which will convert String of the
     * form 'city:state' into an Address object.
     */
    class AddressValueConverter implements ValueConverter {    boolean canConvert(value) {
            value instanceof String
        }    def convert(value) {
            def pieces = value.split(':')
            new com.myapp.Address(city: pieces[0], state: pieces[1])
        }    Class<?> getTargetType() {
            com.myapp.Address
        }
    }
An instance of that class needs to be registered as a bean in the Spring application context. The bean name is not important. All beans that implemented ValueConverter will be automatically plugged in to the data binding process.

    // grails-app/conf/spring/resources.groovybeans = {    addressConverter com.myapp.converters.AddressValueConverter    // ...}

    class Person {
        String firstName
        Address homeAddress
    }class Address {
        String city
        String state
    }def person = new Person()
    person.properties = [firstName: 'Jeff', homeAddress: "O'Fallon:Missouri"]
    assert person.firstName == 'Jeff'
    assert person.homeAddress.city = "O'Fallon"
    assert person.homeAddress.state = 'Missouri'
#### Date Formats For Data Binding A custom date format may be specified to be used when binding a String to a Date value by applying the BindingFormat annotation to a Date field.

    import org.grails.databinding.BindingFormatclass Person {
        @BindingFormat('MMddyyyy')
        Date birthDate
    }
A global setting may be configured in `Config.groovy` to define date formats which will be used application wide when binding to Date.

    // grails-app/conf/Config.groovygrails.databinding.dateFormats = ['MMddyyyy', 'yyyy-MM-dd HH:mm:ss.S', "yyyy-MM-dd'T'hh:mm:ss'Z'"]
The formats specified in `grails.databinding.dateFormats` will be attempted in the order in which they are included in the List. If a property is marked with @BindingFormat, the @BindingFormat will take precedence over the values specified in `grails.databinding.dateFormats`. The default formats that are used are "yyyy-MM-dd HH:mm:ss.S" and "yyyy-MM-dd'T'hh:mm:ss'Z'". #### Custom Formatted Converters You may supply your own handler for the BindingFormat annotation by writing a class which implements the FormattedValueConverter interface and regiserting an instance of that class as a bean in the Spring application context. Below is an example of a trivial custom String formatter that might convert the case of a String based on the value assigned to the BindingFormat annotation.

    package com.myapp.convertersimport org.grails.databinding.converters.FormattedValueConverterclass FormattedStringValueConverter implements FormattedValueConverter {
        def convert(value, String format) {
            if('UPPERCASE' == format) {
                value = value.toUpperCase()
            } else if('LOWERCASE' == format) {
                value = value.toLowerCase()
            }
            value
        }    Class getTargetType() {
            // specifies the type to which this converter may be applied
            String
        }
    }
An instance of that class needs to be registered as a bean in the Spring application context. The bean name is not important. All beans that implemented FormattedValueConverter will be automatically plugged in to the data binding process.

    // grails-app/conf/spring/resources.groovybeans = {    formattedStringConverter com.myapp.converters.FormattedStringValueConverter    // ...}
With that in place the `BindingFormat` annotation may be applied to String fields to inform the data binder to take advantage of the custom converter.

    import org.grails.databinding.BindingFormatclass Person {
        @BindingFormat('UPPERCASE')
        String someUpperCaseString    @BindingFormat('LOWERCASE')
        String someLowerCaseString    String someOtherString
    }
#### Localized Binding Formats The `BindingFormat` annotation supports localized format strings by using the optional `code` attribute. If a value is assigned to the code attribute that value will be used as the message code to retrieve the binding format string from the `messageSource` bean in the Spring application context and that lookup will be localized.

    import org.grails.databinding.BindingFormatclass Person {
        @BindingFormat(code='date.formats.birthdays')
        Date birthDate
    }
    # grails-app/conf/i18n/messages.properties
    date.formats.birthdays=MMddyyyy
    # grails-app/conf/i18n/messages_es.properties
    date.formats.birthdays=ddMMyyyy
#### Structured Data Binding Editors A structured data binding editor is a helper class which can bind structured request parameters to a property. The common use case for structured binding is binding to a `Date` object which might be constructed from several smaller pieces of information contained in several request parameters with names like `birthday_month`, `birthday_date` and `birthday_year`. The structured editor would retrieve all of those individual pieces of information and use them to construct a `Date`. The framework provides a structured editor for binding to `Date` objects. An application may register its own structured editors for whatever types are appropriate. Consider the following classes:

    // src/groovy/databinding/Gadget.groovy
    package databindingclass Gadget {
        Shape expandedShape
        Shape compressedShape
    }

    // src/groovy/databinding/Shape.groovy
    package databindingclass Shape {
        int area
    }
A `Gadget` has 2 `Shape` fields. A `Shape` has an `area` property. It may be that the application wants to accept request parameters like `width` and `height` and use those to calculate the `area` of a `Shape` at binding time. A structured binding editor is well suited for that. The way to register a structured editor with the data binding process is to add an instance of the org.grails.databinding.TypedStructuredBindingEditor interface to the Spring application context. The easiest way to implement the `TypedStructuredBindingEditor` interface is to extend the org.grails.databinding.converters.AbstractStructuredBindingEditor abstract class and override the `getPropertyValue` method as shown below:

    // src/groovy/databinding/converters/StructuredShapeEditor.groovy
    package databinding.convertersimport databinding.Shapeimport org.grails.databinding.converters.AbstractStructuredBindingEditorclass StructuredShapeEditor extends AbstractStructuredBindingEditor<Shape> {    public Shape getPropertyValue(Map values) {
            // retrieve the individual values from the Map
            def width = values.width as int
            def height = values.height as int        // use the values to calculate the area of the Shape
            def area = width * height        // create and return a Shape with the appropriate area
            new Shape(area: area)
        }
    }
An instance of that class needs to be registered with the Spring application context:

    // grails-app/conf/spring/resources.groovy
    beans = {
        shapeEditor databinding.converters.StructuredShapeEditor    // …
    }
When the data binder binds to an instance of the `Gadget` class it will check to see if there are request parameters with names `compressedShape` and `expandedShape` which have a value of "struct" and if they do exist, that will trigger the use of the `StructuredShapeEditor`. The individual components of the structure need to have parameter names of the form propertyName_structuredElementName. In the case of the `Gadget` class above that would mean that the `compressedShape` request parameter should have a value of "struct" and the `compressedShape_width` and `compressedShape_height` parameters should have values which represent the width and the height of the compressed `Shape`. Similarly, the `expandedShape` request parameter should have a value of "struct" and the `expandedShape_width` and `expandedShape_height` parameters should have values which represent the width and the hight of the expanded `Shape`.

    // grails-app/controllers/demo/DemoController.groovy
    class DemoController {    def createGadget(Gadget gadget) {
            /*        /demo/createGadget?expandedShape=struct&expandedShape_width=80&expandedShape_height=30
                              &compressedShape=struct&compressedShape_width=10&compressedShape_height=3        */        // with the request parameters shown above gadget.expandedShape.area would be 2400
            // and gadget.compressedShape.area would be 30        // ...    }
    }
Typically the request parameters with "struct" as their value would be represented by hidden form fields. #### Data Binding Event Listeners The DataBindingListener interface provides a mechanism for listeners to be notified of data binding events. The interface looks like this:

    package org.grails.databinding.events;import org.grails.databinding.errors.BindingError;public interface DataBindingListener {    /**
         * @return true if the listener is interested in events for the specified type.
         */
        boolean supports(Class<?> clazz);    /**
         * Called when data binding is about to start.
         *
         * @param target The object data binding is being imposed upon
         * @param errors the Spring Errors instance (a org.springframework.validation.BindingResult)
         * @return true if data binding should continue
         */
        Boolean beforeBinding(Object target, Object errors);    /**
         * Called when data binding is about to imposed on a property
         *
         * @param target The object data binding is being imposed upon
         * @param propertyName The name of the property being bound to
         * @param value The value of the property being bound
         * @param errors the Spring Errors instance (a org.springframework.validation.BindingResult)
         * @return true if data binding should continue, otherwise return false
         */
        Boolean beforeBinding(Object target, String propertyName, Object value, Object errors);    /**
         * Called after data binding has been imposed on a property
         *
         * @param target The object data binding is being imposed upon
         * @param propertyName The name of the property that was bound to
         * @param errors the Spring Errors instance (a org.springframework.validation.BindingResult)
         */
        void afterBinding(Object target, String propertyName, Object errors);    /**
         * Called after data binding has finished.
         *
         * @param target The object data binding is being imposed upon
         * @param errors the Spring Errors instance (a org.springframework.validation.BindingResult)
         */
        void afterBinding(Object target, Object errors);    /**
         * Called when an error occurs binding to a property
         * @param error encapsulates information about the binding error
         * @param errors the Spring Errors instance (a org.springframework.validation.BindingResult)
         * @see BindingError
         */
        void bindingError(BindingError error, Object errors);
    }
Any bean in the Spring application context which implements that interface will automatically be registered with the data binder. The DataBindingListenerAdapter class implements the `DataBindingListener` interface and provides default implementations for all of the methods in the interface so this class is well suited for subclassing so your listener class only needs to provide implementations for the methods your listener is interested in. The Grails data binder has limited support for the older BindEventListener style listeners. `BindEventListener` looks like this:

    package org.codehaus.groovy.grails.web.binding;import org.springframework.beans.MutablePropertyValues;
    import org.springframework.beans.TypeConverter;public interface BindEventListener {    /**
         * @param target The target to bind to
         * @param source The source of the binding, typically a Map
         * @param typeConverter The type converter to be used
         */
        void doBind(Object target, MutablePropertyValues source, TypeConverter typeConverter);
    }
Support for `BindEventListener` is disabled by default. To enable support assign a value of `true` to the `grails.databinding.enableSpringEventAdapter` property in `grails-app/conf/Config.groovy`.

    // grails-app/conf/Config.groovy
    grails.databinding.enableSpringEventAdapter=true...
With `enableSpringEventAdapter` set to `true` instances of `BindEventListener` which are in the Spring application context will automatically be registered with the data binder. Notice that the `MutablePropertyValues` and `TypeConverter` arguments to the `doBind` method in `BindEventListener` are Spring specific classes and are not relevant to the current data binder. The event adapter will pass `null` values for those arguments. The only real value passed into the `doBind` method will be the object being bound to. This limited support is provided for backward compatibility and will be useful for a subset of scenarios. Developers are encouraged to migrate their `BindEventListener` beans to the newer `DataBindingListener` model. #### Using The Data Binder Directly There are situations where an application may want to use the data binder directly. For example, to do binding in a Service on some arbitrary object which is not a domain class. The following will not work because the `properties` property is read only.

    // src/groovy/bindingdemo/Widget.groovy
    package bindingdemoclass Widget {
        String name
        Integer size
    }

    // grails-app/services/bindingdemo/WidgetService.groovy
    package bindingdemoclass WidgetService {    def updateWidget(Widget widget, Map data) {
            // this will throw an exception because
            // properties is read-only
            widget.properties = data
        }
    }
An instance of the data binder is in the Spring application context with a bean name of `grailsWebDataBinder`. That bean implements the DataBinder interface. The following code demonstrates using the data binder directly.

    // grails-app/services/bindingdmeo/WidgetService
    package bindingdemoimport org.grails.databinding.SimpleMapDataBindingSourceclass WidgetService {    // this bean will be autowired into the service
        def grailsWebDataBinder    def updateWidget(Widget widget, Map data) {
            grailsWebDataBinder.bind widget, data as SimpleMapDataBindingSource
        }}
See the DataBinder documentation for more information about overloaded versions of the `bind` method. #### Data Binding and Security Concerns When batch updating properties from request parameters you need to be careful not to allow clients to bind malicious data to domain classes and be persisted in the database. You can limit what properties are bound to a given domain class using the subscript operator:

    def p = Person.get(1)p.properties['firstName','lastName'] = params
In this case only the `firstName` and `lastName` properties will be bound. Another way to do this is is to use Command Objects as the target of data binding instead of domain classes. Alternatively there is also the flexible bindData method. The `bindData` method allows the same data binding capability, but to arbitrary objects:
    def p = new Person()
    bindData(p, params)
The `bindData` method also lets you exclude certain parameters that you don't want updated:
    def p = new Person()
    bindData(p, params, [exclude: 'dateOfBirth'])
Or include only certain properties:
    def p = new Person()
    bindData(p, params, [include: ['firstName', 'lastName']])
Note that if an empty List is provided as a value for the `include` parameter then all fields will be subject to binding if they are not explicitly excluded. ## 7.1.7 XML and JSON Responses #### Using the render method to output XML Grails supports a few different ways to produce XML and JSON responses. The first is the render method. The `render` method can be passed a block of code to do mark-up building in XML:

    def list() {    def results = Book.list()    render(contentType: "text/xml") {
            books {
                for (b in results) {
                    book(title: b.title)
                }
            }
        }
    }
The result of this code would be something like:
    <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: "application/json") {
            books = array {
                for (b in results) {
                    book title: b.title
                }
            }
        }
    }
In this case the result would be something along the lines of:
    [
        {"title":"The Stand"},
        {"title":"The Shining"}
    ]
The same dangers with naming conflicts described above for XML also apply to JSON building. #### Automatic XML Marshalling Grails also supports automatic marshalling of domain classes to XML using special converters. To start off with, import the `grails.converters` package into your controller:
    import grails.converters.*
Now you can use the following highly readable syntax to automatically convert domain classes to XML:
    render Book.list() as XML
The resulting output would look something like the following::
    <?xml version="1.0" encoding="ISO-8859-1"?>
    <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>
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"}
     ]
## 7.1.8 More on JSONBuilder The previous section on on XML and JSON responses covered simplistic examples of rendering XML and JSON responses. Whilst the XML builder used by Grails is the standard XmlSlurper found in Groovy, the JSON builder is a custom implementation specific to Grails. #### JSONBuilder and Grails versions JSONBuilder behaves different depending on the version of Grails you use. For version below 1.2 the deprecated grails.web.JSONBuilder class is used. This section covers the usage of the Grails 1.2 JSONBuilder For backwards compatibility the old `JSONBuilder` class is used with the `render` method for older applications; to use the newer/better `JSONBuilder` class set the following in `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: "application/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: "application/json") {
        categories = ['a', 'b', 'c']
    }
This will produce:
    {"categories":["a","b","c"]}
You can also render lists of complex objects, for example:
    render(contentType: "application/json") {
        categories = [ { a = "A" }, { b = "B" } ]
    }
This will produce:
    {"categories":[ {"a":"A"} , {"b":"B"}] }
Use the special `element` method to return a list as the root:
    render(contentType: "application/json") {
        element 1
        element 2
        element 3
    }
The above code produces:
    [1,2,3]
#### Rendering Complex Objects Rendering complex objects can be done with Closures. For example:
    render(contentType: "application/json") {
        categories = ['a', 'b', 'c']
        title = "Hello JSON"
        information = {
            pages = 10
        }
    }
The above will produce the JSON:
    {"categories":["a","b","c"],"title":"Hello JSON","information":{"pages":10}}
#### Arrays of Complex Objects As mentioned previously you can nest complex objects within arrays using Closures:
    render(contentType: "application/json") {
        categories = [ { a = "A" }, { b = "B" } ]
    }
You can use the `array` method to build them up dynamically:
    def results = Book.list()
    render(contentType: "application/json") {
        books = array {
            for (b in results) {
                book title: b.title
            }
        }
    }
#### Direct JSONBuilder API Access If you don't have access to the `render` method, but still want to produce JSON you can use the API directly:

    def builder = new JSONBuilder()def result = builder.build {
        categories = ['a', 'b', 'c']
        title = "Hello JSON"
        information = {
            pages = 10
        }
    }// prints the JSON text
    println result.toString()def sw = new StringWriter()
    result.render sw
## 7.1.9 Uploading Files #### Programmatic File Uploads Grails supports file uploads using Spring's MultipartHttpServletRequest interface. The first step for file uploading is to create a multipart form like this:
    Upload Form: <br />
        <g:uploadForm action="upload">
            <input type="file" name="myFile" />
            <input type="submit" />
        </g:uploadForm>
The `uploadForm` tag conveniently adds the `enctype="multipart/form-data"` attribute to the standard `<g:form>` tag. There are then a number of ways to handle the file upload. One is to work with the Spring MultipartFile instance directly:

    def upload() {
        def f = request.getFile('myFile')
        if (f.empty) {
            flash.message = 'file cannot be empty'
            render(view: 'uploadForm')
            return
        }    f.transferTo(new File('/some/local/dir/myfile.txt'))
        response.sendError(200, 'Done')
    }
This is convenient for doing transfers to other destinations and manipulating the file directly as you can obtain an `InputStream` and so on with the MultipartFile interface. #### File Uploads through Data Binding File uploads can also be performed using data binding. Consider this `Image` domain class:

    class Image {
        byte[] myFile    static constraints = {
            // Limit upload file size to 2MB
            myFile maxSize: 1024 * 1024 * 2
        }
    }
If you create an image using the `params` object in the constructor as in the example below, Grails will automatically bind the file's contents as a `byte` to the `myFile` property:
    def img = new Image(params)
It's important that you set the size or maxSize constraints, otherwise your database may be created with a small column size that can't handle reasonably sized files. For example, both H2 and MySQL default to a blob size of 255 bytes for `byte` properties. It is also possible to set the contents of the file as a string by changing the type of the `myFile` property on the image to a String type:
    class Image {
       String myFile
    }
## 7.1.10 Command Objects Grails controllers support the concept of command objects. A command object is a class that is used in conjunction with data binding, usually to allow validation of data that may not fit into an existing domain class. Note: A class is only considered to be a command object when it is used as a parameter of an action. #### Declaring Command Objects Command object classes are defined just like any other class.

    @grails.validation.Validateable
    class LoginCommand {
        String username
        String password    static constraints = {
            username(blank: false, minSize: 6)
            password(blank: false, minSize: 6)
        }
    }
In this example, the command object is marked with the `Validateable` annotation. The `Validateable` annotation allows the definition of constraints just like in domain classes. If the command object is defined in the same source file as the controller that is using it, Grails will automatically mark it as `Validateable`. It is not required that command object classes be validateable. #### Using Command Objects To use command objects, controller actions may optionally specify any number of command object parameters. The parameter types must be supplied so that Grails knows what objects to create and initialize. Before the controller action is executed Grails will automatically create an instance of the command object class and populate its properties by binding the request parameters. If the command object class is marked with `Validateable` then the command object will be validated. For example:

    class LoginController {    def login(LoginCommand cmd) {
            if (cmd.hasErrors()) {
                redirect(action: 'loginForm')
                return
            }        // work with the command object data
        }
    }
If the command object's type is that of a domain class and there is an `id` request parameter then instead of invoking the domain class constructor to create a new instance a call will be made to the static `get` method on the domain class and the value of the `id` parameter will be passed as an argument. Whatever is returned from that call to `get` is what will be passed into the controller action. This means that if there is an `id` request parameter and no corresponding record is found in the database then the value of the command object will be `null`. #### Command Objects And Request Parameter Names Normally request parameter names will be mapped directly to property names in the command object. Nested parameter names may be used to bind down the object graph in an intuitive way. In the example below a request parameter named `name` will be bound to the `name` property of the `PersonCommand` instance and a request parameter named `address.city` will be bound to the `city` property of the `address` property in the `PersonCommand`.

    class StoreController {
        def buy(PersonCommand buyer) {
            // …
        }
    }class PersonCommand {
        String name
        Address address
    }class Address {
        String city
    }
A problem may arise if a controller action accepts multiple command objects which happen to contain the same property name. Consider the following example.

    class StoreController {
        def buy(PersonCommand buyer, ProductCommand product) {
            // …
        }
    }class PersonCommand {
        String name
        Address address
    }class Address {
        String city
    }class ProductCommand {
        String name
    }
If there is a request parameter named `name` it isn't clear if that should represent the name of the `ProductCommand` or the name of the `PersonCommand`. To help deal with this the framework imposes special rules for mapping parameter names to command object types. If a command object class name ends with "Command" and there are request parameters with a name which begins with the name of the command object class name minus the "Command" suffix then the data binding will treat all parameters that begin with that name as belonging to the corresponding command object. For example, the `product.name` request parameter will be bound to the `name` property in the `ProductCommand` instance, the `person.name` request parameter will be bound to the `name` property in the `PersonCommand` instance and the `person.address.city` request parameter will be bound to the `city` property of the `address` property in the `PersonCommand` instance. A potentially problematic scenario is one where the command object class name ends with "Command" and the class contains a property with a name that matches the command object class name minus the "Command" suffix as shown below.

    class StoreController {
        def buy(PersonCommand buyer) {
            // …
        }
    }class PersonCommand {
        Person person
    }class Person {
        String name
    }
In the example above one might expect a request parameter named `person.name` to be bound to the `name` property of the `person` property in the `PersonCommand` instance but because of the special handling described above that would not work. The problem is since the `person.name` request parameter begins with "person" and that happens to match the beginning of the command object class name, "PersonCommand", everything after the "person." prefix represents what is bound to the command object. Since there is no `name` property in the `PersonCommand` class that request parameter is ignored during data binding. The application could deal with this by renaming the `PersonCommand` class, renaming the `person` property name in the `PersonCommand` class or by fully qualifying the request parameter name to be `person.person.name`. The first "person" there is the qualifier which tells the framework the type of command object to bind this request paramter to. The second "person" there represents the property name in `PersonCommand` that `name` should be bound to. #### Command Objects and Dependency Injection Command objects can participate in dependency injection. This is useful if your command object has some custom validation logic which uses a Grails service:

    @grails.validation.Validateable
    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`. #### Binding The Request Body To Command Objects When a request is made to a controller action which accepts a command object and the request contains a body, Grails will attempt to parse the body of the request based on the request content type and use the body to do data binding on the command object. See the following example.

    // grails-app/controllers/bindingdemo/DemoController.groovy
    package bindingdemoclass DemoController {    def createWidget(Widget w) {
            render "Name: ${w?.name}, Size: ${w?.size}"
        }
    }class Widget {
        String name
        Integer size
    }
    $ curl -H "Content-Type: application/json" -d '{"name":"Some Widget","size":"42"}' localhost:8080/myapp/demo/createWidget
     Name: Some Widget, Size: 42
    ~ $
    $ curl -H "Content-Type: application/xml" -d '<widget><name>Some Other Widget</name><size>2112</size></widget>' localhost:8080/bodybind/demo/createWidget
     Name: Some Other Widget, Size: 2112
    ~ $
Note that the body of the request is being parsed to make that work. Any attempt to read the body of the request after that will fail since the corresponding input stream will be empty. The controller action can either use a command object or it can parse the body of the request on its own (either directly, or by referring to something like request.JSON), but cannot do both.

    // grails-app/controllers/bindingdemo/DemoController.groovy
    package bindingdemoclass DemoController {    def createWidget(Widget w) {
            // this will fail because it requires reading the body,
            // which has already been read.
            def json = request.JSON        // ...    }
    }
## 7.1.11 Handling Duplicate Form Submissions Grails has built-in support for handling duplicate form submissions using the "Synchronizer Token Pattern". To get started you define a token on the form tag:
    <g:form useToken="true" ...>
Then in your controller code you can use the withForm method to handle valid and invalid requests:
    withForm {
       // good request
    }.invalidToken {
       // bad request
    }
If you only provide the withForm method and not the chained `invalidToken` method then by default Grails will store the invalid token in a `flash.invalidToken` variable and redirect the request back to the original page. This can then be checked in the view:
    <g:if test="${flash.invalidToken}">
      Don't click the button twice!
    </g:if>
The withForm tag makes use of the session and hence requires session affinity or clustered sessions if used in a cluster. ## 7.1.12 Simple Type Converters ### Type Conversion Methods If you prefer to avoid the overhead of Data Binding and simply want to convert incoming parameters (typically Strings) into another more appropriate type the params object has a number of convenience methods for each type:
    def total = params.int('total')
The above example uses the `int` method, and there are also methods for `boolean`, `long`, `char`, `short` and so on. Each of these methods is null-safe and safe from any parsing errors, so you don't have to perform any additional checks on the parameters. Each of the conversion methods allows a default value to be passed as an optional second argument. The default value will be returned if a corresponding entry cannot be found in the map or if an error occurs during the conversion. Example:
    def total = params.int('total', 42)
These same type conversion methods are also available on the `attrs` parameter of GSP tags. ### Handling Multi Parameters A common use case is dealing with multiple request parameters of the same name. For example you could get a query string such as `?name=Bob&name=Judy`. In this case dealing with one parameter and dealing with many has different semantics since Groovy's iteration mechanics for `String` iterate over each character. To avoid this problem the params object provides a `list` method that always returns a list:
    for (name in params.list('name')) {
        println name
    }
## 7.1.13 Declarative Controller Exception Handling Grails controllers support a simple mechanism for declarative exception handling. If a controller declares a method that accepts a single argument and the argument type is `java.lang.Exception` or some subclass of `java.lang.Exception`, that method will be invoked any time an action in that controller throws an exception of that type. See the following example.

    // grails-app/controllers/demo/DemoController.groovy
    package democlass DemoController {    def someAction() {
            // do some work
        }    def handleSQLException(SQLException e) {
            render 'A SQLException Was Handled'
        }    def handleBatchUpdateException(BatchUpdateException e) {
            redirect controller: 'logging', action: 'batchProblem'
        }    def handleNumberFormatException(NumberFormatException nfe) {
            [problemDescription: 'A Number Was Invalid']
        }
    }
That controller will behave as if it were written something like this...

    // grails-app/controllers/demo/DemoController.groovy
    package democlass DemoController {    def someAction() {
            try {
                // do some work
            } catch (BatchUpdateException e) {
                return handleBatchUpdateException(e)
            } catch (SQLException e) {
                return handleSQLException(e)
            } catch (NumberFormatException e) {
                return handleNumberFormatException(e)
            }
        }    def handleSQLException(SQLException e) {
            render 'A SQLException Was Handled'
        }    def handleBatchUpdateException(BatchUpdateException e) {
            redirect controller: 'logging', action: 'batchProblem'
        }    def handleNumberFormatException(NumberFormatException nfe) {
            [problemDescription: 'A Number Was Invalid']
        }
    }
The exception handler method names can be any valid method name. The name is not what makes the method an exception handler, the `Exception` argument type is the important part. The exception handler methods can do anything that a controller action can do including invoking `render`, `redirect`, returning a model, etc. The exception handler methods are inherited into sublcasses so an application could define the exception handlers in an abstract class that multiple controllers extend from. Exception handler methods must be present at compile time. Specifically, exception handler methods which are runtime metaprogrammed onto a controller class are not supported. ## 7.2 Groovy Server Pages Groovy Servers Pages (or GSP for short) is Grails' view technology. It is designed to be familiar for users of technologies such as ASP and JSP, but to be far more flexible and intuitive. GSPs live in the `grails-app/views` directory and are typically rendered automatically (by convention) or with the render method such as:
    render(view: "index")
A GSP is typically a mix of mark-up and GSP tags which aid in view rendering. Although it is possible to have Groovy logic embedded in your GSP and doing this will be covered in this document, the practice is strongly discouraged. Mixing mark-up and code is a bad thing and most GSP pages contain no code and needn't do so. A GSP typically has a "model" which is a set of variables that are used for view rendering. The model is passed to the GSP view from a controller. For example consider the following controller action:
    def show() {
        [book: Book.get(params.id)]
    }
This action will look up a `Book` instance and create a model that contains a key called `book`. This key can then be referenced within the GSP view using the name `book`:
    ${book.title}
Embedding data received from user input has the risk of making your application vulnerable to an Cross Site Scripting (XSS) attack. Please read the documentation on XSS prevention for information on how to prevent XSS attacks. ## 7.2.1 GSP Basics In the next view sections we'll go through the basics of GSP and what is available to you. First off let's cover some basic syntax that users of JSP and ASP should be familiar with. GSP supports the usage of `<% %>` scriptlet blocks to embed Groovy code (again this is discouraged):
    <html>
       <body>
         <% out << "Hello GSP!" %>
       </body>
    </html>
You can also use the `<%= %>` syntax to output values:
    <html>
       <body>
         <%="Hello GSP!" %>
       </body>
    </html>
GSP also supports JSP-style server-side comments (which are not rendered in the HTML response) as the following example demonstrates:
    <html>
       <body>
         <%-- This is my comment --%>
         <%="Hello GSP!" %>
       </body>
    </html>
Embedding data received from user input has the risk of making your application vulnerable to an Cross Site Scripting (XSS) attack. Please read the documentation on XSS prevention for information on how to prevent XSS attacks. ## 7.2.1.1 Variables and Scopes Within the `<% %>` brackets you can declare variables:
    <% now = new Date() %>
and then access those variables later in the page:
    <%=now%>
Within the scope of a GSP there are a number of pre-defined variables, including: `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 ## 7.2.1.2 Logic and Iteration Using the `<% %>` syntax you can embed loops and so on using this syntax:
    <html>
       <body>
          <% [1,2,3,4].each { num -> %>
             <p><%="Hello ${num}!" %></p>
          <%}%>
       </body>
    </html>
As well as logical branching:
    <html>
       <body>
          <% if (params.hello == 'true')%>
          <%="Hello!"%>
          <% else %>
          <%="Goodbye!"%>
       </body>
    </html>
## 7.2.1.3 Page Directives GSP also supports a few JSP-style page directives. The import directive lets you import classes into the page. However, it is rarely needed due to Groovy's default imports and GSP Tags:
    <%@ page import="java.awt.*" %>
GSP also supports the contentType directive:
    <%@ page contentType="application/json" %>
The contentType directive allows using GSP to render other formats. ## 7.2.1.4 Expressions In GSP the `<%= %>` syntax introduced earlier is rarely used due to the support for GSP expressions. A GSP expression is similar to a JSP EL expression or a Groovy GString and takes the form `${expr}`:
    <html>
      <body>
        Hello ${params.name}
      </body>
    </html>
However, unlike JSP EL you can have any Groovy expression within the `${..}` block. Embedding data received from user input has the risk of making your application vulnerable to an Cross Site Scripting (XSS) attack. Please read the documentation on XSS prevention for information on how to prevent XSS attacks. ## 7.2.2 GSP Tags Now that the less attractive JSP heritage has been set aside, the following sections cover GSP's built-in tags, which are the preferred way to define GSP pages. The section on Tag Libraries covers how to add your own custom tag libraries. All built-in GSP tags start with the prefix `g:`. Unlike JSP, you don't specify any tag library imports. If a tag starts with `g:` it is automatically assumed to be a GSP tag. An example GSP tag would look like:
    <g:example />
GSP tags can also have a body such as:
    <g:example>
       Hello world
    </g:example>
Expressions can be passed into GSP tag attributes, if an expression is not used it will be assumed to be a String value:
    <g:example attr="${new Date()}">
       Hello world
    </g:example>
Maps can also be passed into GSP tag attributes, which are often used for a named parameter style syntax:
    <g:example attr="${new Date()}" attr2="[one:1, two:2, three:3]">
       Hello world
    </g:example>
Note that within the values of attributes you must use single quotes for Strings:
    <g:example attr="${new Date()}" attr2="[one:'one', two:'two']">
       Hello world
    </g:example>
With the basic syntax out the way, the next sections look at the tags that are built into Grails by default. ## 7.2.2.1 Variables and Scopes Variables can be defined within a GSP using the set tag:
    <g:set var="now" value="${new Date()}" />
Here we assign a variable called `now` to the result of a GSP expression (which simply constructs a new `java.util.Date` instance). You can also use the body of the `<g:set>` tag to define a variable:
    <g:set var="myHTML">
       Some re-usable code on: ${new Date()}
    </g:set>
The assigned value can also be a bean from the applicationContext:
    <g:set var="bookService" bean="bookService" />
Variables can also be placed in one of the following scopes: `page` - Scoped to the current page (default) `request` - Scoped to the current request `flash` - Placed within flash scope and hence available for the next request `session` - Scoped for the user session `application` - Application-wide scope. To specify the scope, use the `scope` attribute:
    <g:set var="now" value="${new Date()}" scope="request" />
## 7.2.2.2 Logic and Iteration GSP also supports logical and iterative tags out of the box. For logic there are if, else and elseif tags for use with branching:
    <g:if test="${session.role == 'admin'}">
       <%-- show administrative functions --%>
    </g:if>
    <g:else>
       <%-- show basic functions --%>
    </g:else>
Use the each and while tags for iteration:

    <g:each in="${[1,2,3]}" var="num">
       <p>Number ${num}</p>
    </g:each><g:set var="num" value="${1}" />
    <g:while test="${num < 5 }">
        <p>Number ${num++}</p>
    </g:while>
## 7.2.2.3 Search and Filtering If you have collections of objects you often need to sort and filter them. Use the findAll and grep tags for these tasks:
    Stephen King's Books:
    <g:findAll in="${books}" expr="it.author == 'Stephen King'">
         <p>Title: ${it.title}</p>
    </g:findAll>
The `expr` attribute contains a Groovy expression that can be used as a filter. The grep tag does a similar job, for example filtering by class:
    <g:grep in="${books}" filter="NonFictionBooks.class">
         <p>Title: ${it.title}</p>
    </g:grep>
Or using a regular expression:
    <g:grep in="${books.title}" filter="~/.*?Groovy.*?/">
         <p>Title: ${it}</p>
    </g:grep>
The above example is also interesting due to its usage of GPath. GPath is an XPath-like language in Groovy. The `books` variable is a collection of `Book` instances. Since each `Book` has a `title`, you can obtain a list of Book titles using the expression `books.title`. Groovy will auto-magically iterate the collection, obtain each title, and return a new list! ## 7.2.2.4 Links and Resources GSP also features tags to help you manage linking to controllers and actions. The link tag lets you specify controller and action name pairing and it will automatically work out the link based on the URL Mappings, even if you change them! For example:

    <g:link action="show" id="1">Book 1</g:link><g:link action="show" id="${currentBook.id}">${currentBook.name}</g:link><g:link controller="book">Book Home</g:link><g:link controller="book" action="list">Book List</g:link><g:link url="[action: 'list', controller: 'book']">Book List</g:link><g:link params="[sort: 'title', order: 'asc', author: currentBook.author]"
            action="list">Book List</g:link>
## 7.2.2.5 Forms and Fields #### Form Basics GSP supports many different tags for working with HTML forms and fields, the most basic of which is the form tag. This is a controller/action aware version of the regular HTML form tag. The `url` attribute lets you specify which controller and action to map to:
    <g:form name="myForm" url="[controller:'book',action:'list']">...</g:form>
In this case we create a form called `myForm` that submits to the `BookController`'s `list` action. Beyond that all of the usual HTML attributes apply. #### Form Fields In addition to easy construction of forms, GSP supports custom tags for dealing with different types of fields, including: textField - For input fields of type 'text' passwordField - For input fields of type 'password' checkBox - For input fields of type 'checkbox' radio - For input fields of type 'radio' hiddenField - For input fields of type 'hidden' select - For dealing with HTML select boxes Each of these allows GSP expressions for the value:
    <g:textField name="myField" value="${myValue}" />
GSP also contains extended helper versions of the above tags such as radioGroup (for creating groups of radio tags), localeSelect, currencySelect and timeZoneSelect (for selecting locales, currencies and time zones respectively). #### Multiple Submit Buttons The age old problem of dealing with multiple submit buttons is also handled elegantly with Grails using the actionSubmit tag. It is just like a regular submit, but lets you specify an alternative action to submit to:
    <g:actionSubmit value="Some update label" action="update" />
## 7.2.2.6 Tags as Method Calls One major different between GSP tags and other tagging technologies is that GSP tags can be called as either regular tags or as method calls from controllers, tag libraries or GSP views. #### Tags as method calls from GSPs Tags return their results as a String-like object (a `StreamCharBuffer` which has all of the same methods as String) instead of writing directly to the response when called as methods. For example:
    Static Resource: ${createLinkTo(dir: "images", file: "logo.jpg")}
This is particularly useful for using a tag within an attribute:
    <img src="${createLinkTo(dir: 'images', file: 'logo.jpg')}" />
In view technologies that don't support this feature you have to nest tags within tags, which becomes messy quickly and often has an adverse effect of 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")
## 7.2.3 Views and Templates Grails also has the concept of templates. These are useful for partitioning your views into maintainable chunks, and combined with Layouts provide a highly re-usable mechanism for structured views. #### Template Basics Grails uses the convention of placing an underscore before the name of a view to identify it as a template. For example, you might have a template that renders Books located at `grails-app/views/book/_bookTemplate.gsp`:
    <div class="book" id="${book?.id}">
       <div>Title: ${book?.title}</div>
       <div>Author: ${book?.author?.name}</div>
    </div>
Use the render tag to render this template from one of the views in `grails-app/views/book`:
    <g:render template="bookTemplate" model="[book: myBook]" />
Notice how we pass into a model to use using the `model` attribute of the `render` tag. If you have multiple `Book` instances you can also render the template for each `Book` using the render tag with a `collection` attribute:
    <g:render template="bookTemplate" var="book" collection="${bookList}" />
#### Shared Templates In the previous example we had a template that was specific to the `BookController` and its views at `grails-app/views/book`. However, you may want to share templates across your application. In this case you can place them in the root views directory at grails-app/views or any subdirectory below that location, and then with the template attribute use an absolute location starting with `/` instead of a relative location. For example if you had a template called `grails-app/views/shared/_mySharedTemplate.gsp`, you would reference it as:
    <g:render template="/shared/mySharedTemplate" />
You can also use this technique to reference templates in any directory from any view or controller:
    <g:render template="/book/bookTemplate" model="[book: myBook]" />
#### The Template Namespace Since templates are used so frequently there is template namespace, called `tmpl`, available that makes using templates easier. Consider for example the following usage pattern:
    <g:render template="bookTemplate" model="[book:myBook]" />
This can be expressed with the `tmpl` namespace as follows:
    <tmpl:bookTemplate book="${myBook}" />
#### Templates in Controllers and Tag Libraries You can also render templates from controllers using the render controller method. This is useful for Ajax applications where you generate small HTML or data responses to partially update the current page instead of performing new request:
    def bookData() {
        def b = Book.get(params.id)
        render(template:"bookTemplate", model:[book:b])
    }
The render controller method writes directly to the response, which is the most common behaviour. To instead obtain the result of template as a String you can use the render tag:
    def bookData() {
        def b = Book.get(params.id)
        String content = g.render(template:"bookTemplate", model:[book:b])
        render content
    }
Notice the usage of the `g` namespace which tells Grails we want to use the tag as method call instead of the render method. ## 7.2.4 Layouts with Sitemesh #### Creating Layouts Grails leverages Sitemesh, a decorator engine, to support view layouts. Layouts are located in the `grails-app/views/layouts` directory. A typical layout can be seen below:
    <html>
        <head>
            <title><g:layoutTitle default="An example decorator" /></title>
            <g:layoutHead />
        </head>
        <body onload="${pageProperty(name:'body.onload')}">
            <div class="menu"><!--my common menu goes here--></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. ## 7.2.5 Static Resources Grails 2.0 integrates with the Resources plugin to provide sophisticated static resource management. This plugin is installed by default in new Grails applications. 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. ## 7.2.5.1 Including resources using the resource tags #### 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. ## 7.2.5.2 Other resource tags #### 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. ## 7.2.5.3 Declaring resources A DSL is provided for declaring resources and modules. This can go either in your `Config.groovy` in the case of application-specific resources, or more commonly in a resources artefact in `grails-app/conf`. 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. ## 7.2.5.4 Overriding plugin resources Because a resource module can define the bundle groupings and other attributes of resources, you may find that the settings provided are not correct for your application. 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. ## 7.2.5.5 Optimizing your resources The Resources framework uses "mappers" to mutate the resources into the final format served to the user. 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. ## 7.2.5.6 Debugging When your resources are being moved around, renamed and otherwise mutated, it can be hard to debug client-side issues. Modern browsers, especially Safari, Chrome and Firefox have excellent tools that let you view all the resources requested by a page, including the headers and other information about them. 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. ## 7.2.5.7 Preventing processing of resources Sometimes you do not want a resource to be processed in a particular way, or even at all. Occasionally you may also want to disable all resource mapping. #### 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']
## 7.2.5.8 Other Resources-aware plugins At the time of writing, the following plugins include support for the Resources framework: jquery jquery-ui blueprint lesscss-resources zipped-resources cached-resources ## 7.2.6 Sitemesh Content Blocks Although it is useful to decorate an entire page sometimes you may find the need to decorate independent sections of your site. To do this you can use content blocks. To get started, partition the page to be decorated using the `<content>` tag:

    <content tag="navbar">
    … draw the navbar here…
    </content><content tag="header">
    … draw the header here…
    </content><content tag="footer">
    … draw the footer here…
    </content><content tag="body">
    … draw the body here…
    </content>
Then within the layout you can reference these components and apply individual layouts to each:
    <html>
        <body>
            <div id="header">
                <g:applyLayout name="headerLayout">
                    <g:pageProperty name="page.header" />
                </g:applyLayout>
            </div>
            <div id="nav">
                <g:applyLayout name="navLayout">
                    <g:pageProperty name="page.navbar" />
                </g:applyLayout>
            </div>
            <div id="body">
                <g:applyLayout name="bodyLayout">
                    <g:pageProperty name="page.body" />
                </g:applyLayout>
            </div>
            <div id="footer">
                <g:applyLayout name="footerLayout">
                    <g:pageProperty name="page.footer" />
                </g:applyLayout>
            </div>
        </body>
    </html>
## 7.2.7 Making Changes to a Deployed Application One of the main issues with deploying a Grails application (or typically any servlet-based one) is that any change to the views requires that you redeploy your whole application. If all you want to do is fix a typo on a page, or change an image link, it can seem like a lot of unnecessary work. For such simple requirements, Grails does have a solution: the `grails.gsp.view.dir` configuration setting. How does this work? The first step is to decide where the GSP files should go. Let's say we want to keep them unpacked in a `/var/www/grails/my-app` directory. We add these two lines to `grails-app/conf/Config.groovy` :
    grails.gsp.enable.reload = true
    grails.gsp.view.dir = "/var/www/grails/my-app/"
The first line tells Grails that modified GSP files should be reloaded at runtime. If you don't have this setting, you can make as many changes as you like but they won't be reflected in the running application until you restart. The second line tells Grails where to load the views and layouts from. The trailing slash on the `grails.gsp.view.dir` value is important! Without it, Grails will look for views in the parent directory. Setting "grails.gsp.view.dir" is optional. If it's not specified, you can update files directly to the application server's deployed war directory. Depending on the application server, these files might get overwritten when the server is restarted. Most application servers support "exploded war deployment" which is recommended in this case. With those settings in place, all you need to do is copy the views from your web application to the external directory. On a Unix-like system, this would look something like this:
    mkdir -p /var/www/grails/my-app/grails-app/views
    cp -R grails-app/views/* /var/www/grails/my-app/grails-app/views
The key point here is that you must retain the view directory structure, including the `grails-app/views` bit. So you end up with the path `/var/www/grails/my-app/grails-app/views/...` . One thing to bear in mind with this technique is that every time you modify a GSP, it uses up permgen space. So at some point you will eventually hit "out of permgen space" errors unless you restart the server. So this technique is not recommended for frequent or large changes to the views. There are also some System properties to control GSP reloading:
Name | Description | Default
--- | --- | ---
grails.gsp.enable.reload | altervative system property for enabling the GSP reload mode without changing Config.groovy |
grails.gsp.reload.interval | interval between checking the lastmodified time of the gsp source file, unit is milliseconds | 5000
grails.gsp.reload.granularity | the number of milliseconds leeway to give before deciding a file is out of date. this is needed because different roundings usually cause a 1000ms difference in lastmodified times | 1000
GSP reloading is supported for precompiled GSPs since Grails 1.3.5 . ## 7.2.8 GSP Debugging #### Viewing the generated source code Adding "?showSource=true" or "&showSource=true" to the url shows the generated Groovy source code for the view instead of rendering it. It won't show the source code of included templates. This only works in development mode The saving of all generated source code can be activated by setting the property "grails.views.gsp.keepgenerateddir" (in 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. ## 7.3 Tag Libraries Like Java Server Pages (JSP), GSP supports the concept of custom tag libraries. Unlike JSP, Grails' tag library mechanism is simple, elegant and completely reloadable at runtime. Quite simply, to create a tag library create a Groovy class that ends with the convention `TagLib` and place it within the `grails-app/taglib` directory:

    class SimpleTagLib {}
Now to create a tag create a Closure property that takes two arguments: the tag attributes and the body content:

    class SimpleTagLib {
        def simple = { attrs, body ->    }
    }
The `attrs` argument is a Map of the attributes of the tag, whilst the `body` argument is a Closure that returns the body content when invoked:
    class SimpleTagLib {
        def emoticon = { attrs, body ->
           out << body() << (attrs.happy == 'true' ? " :-)" : " :-(")
        }
    }
As demonstrated above there is an implicit `out` variable that refers to the output `Writer` which you can use to append content to the response. Then you can reference the tag inside your GSP; no imports are necessary:
    <g:emoticon happy="true">Hi John</g:emoticon>
> To help IDEs like Spring Tool Suite (STS) and others autocomplete tag attributes, you should add Javadoc comments to your tag closures with `@attr` descriptions. Since taglibs use Groovy code it can be difficult to reliably detect all usable attributes. For example:

>     class SimpleTagLib {    /**
>          * Renders the body with an emoticon.
>          *
>          * @attr happy whether to show a happy emoticon ('true') or
>          * a sad emoticon ('false')
>          */
>         def emoticon = { attrs, body ->
>            out << body() << (attrs.happy == 'true' ? " :-)" : " :-(")
>         }
>     }
> and any mandatory attributes should include the REQUIRED keyword, e.g.

>     class SimpleTagLib {    /**
>          * Creates a new password field.
>          *
>          * @attr name REQUIRED the field name
>          * @attr value the field value
>          */
>         def passwordField = { attrs ->
>             attrs.type = "password"
>             attrs.tagName = "passwordField"
>             fieldImpl(out, attrs)
>         }
>     }
## 7.3.1 Variables and Scopes Within the scope of a tag library there are a number of pre-defined variables including: `actionName` - The currently executing action name `controllerName` - The currently executing controller name `flash` - The flash object `grailsApplication` - The GrailsApplication instance `out` - The response writer for writing to the output stream `pageScope` - A reference to the pageScope object used for GSP rendering (i.e. the binding) `params` - The params object for retrieving request parameters `pluginContextPath` - The context path to the plugin that contains the tag library `request` - The HttpServletRequest instance `response` - The HttpServletResponse instance `servletContext` - The javax.servlet.ServletContext instance `session` - The HttpSession instance ## 7.3.2 Simple Tags As demonstrated in the previous example it is easy to write simple tags that have no body and just output content. Another example is a `dateFormat` style tag:
    def dateFormat = { attrs, body ->
        out << new java.text.SimpleDateFormat(attrs.format).format(attrs.date)
    }
The above uses Java's `SimpleDateFormat` class to format a date and then write it to the response. The tag can then be used within a GSP as follows:
    <g:dateFormat format="dd-MM-yyyy" date="${new Date()}" />
With simple tags sometimes you need to write HTML mark-up to the response. One approach would be to embed the content directly:
    def formatBook = { attrs, body ->
        out << "<div id="${attrs.book.id}">"
        out << "Title : ${attrs.book.title}"
        out << "</div>"
    }
Although this approach may be tempting it is not very clean. A better approach would be to reuse the render tag:
    def formatBook = { attrs, body ->
        out << render(template: "bookTemplate", model: [book: attrs.book])
    }
And then have a separate GSP template that does the actual rendering. ## 7.3.3 Logical Tags You can also create logical tags where the body of the tag is only output once a set of conditions have been met. An example of this may be a set of security tags:
    def isAdmin = { attrs, body ->
        def user = attrs.user
        if (user && checkUserPrivs(user)) {
            out << body()
        }
    }
The tag above checks if the user is an administrator and only outputs the body content if he/she has the correct set of access privileges:
    <g:isAdmin user="${myUser}">
        // some restricted content
    </g:isAdmin>
## 7.3.4 Iterative Tags Iterative tags are easy too, since you can invoke the body multiple times:
    def repeat = { attrs, body ->
        attrs.times?.toInteger()?.times { num ->
            out << body(num)
        }
    }
In this example we check for a `times` attribute and if it exists convert it to a number, then use Groovy's `times` method to iterate the specified number of times:
    <g:repeat times="3">
    <p>Repeat this 3 times! Current repeat = ${it}</p>
    </g:repeat>
Notice how in this example we use the implicit `it` variable to refer to the current number. This works because when we invoked the body we passed in the current value inside the iteration:
    out << body(num)
That value is then passed as the default variable `it` to the tag. However, if you have nested tags this can lead to conflicts, so you should instead name the variables that the body uses:
    def repeat = { attrs, body ->
        def var = attrs.var ?: "num"
        attrs.times?.toInteger()?.times { num ->
            out << body((var):num)
        }
    }
Here we check if there is a `var` attribute and if there is use that as the name to pass into the body invocation on this line:
    out << body((var):num)
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. ## 7.3.5 Tag Namespaces By default, tags are added to the default Grails namespace and are used with the `g:` prefix in GSP pages. However, you can specify a different namespace by adding a static property to your `TagLib` class:

    class SimpleTagLib {
        static namespace = "my"    def example = { attrs ->
            …
        }
    }
Here we have specified a `namespace` of `my` and hence the tags in this tag lib must then be referenced from GSP pages like this:
    <my:example name="..." />
where the prefix is the same as the value of the static `namespace` property. Namespaces are particularly useful for plugins. Tags within namespaces can be invoked as methods using the namespace as a prefix to the method call:
    out << my.example(name:"foo")
This works from GSP, controllers or tag libraries ## 7.3.6 Using JSP Tag Libraries In addition to the simplified tag library mechanism provided by GSP, you can also use JSP tags from GSP. To do so simply declare the JSP to use with the `taglib` directive:
    <%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
Then you can use it like any other tag:
    <fmt:formatNumber value="${10}" pattern=".00"/>
With the added bonus that you can invoke JSP tags like methods:
    ${fmt.formatNumber(value:10, pattern:".00")}
## 7.3.7 Tag return value Since Grails 1.2, a tag library call returns an instance of `org.codehaus.groovy.grails.web.util.StreamCharBuffer` class by default. This change improves performance by reducing object creation and optimizing buffering during request processing. In earlier Grails versions, a `java.lang.String` instance was returned. Tag libraries can also return direct object values to the caller since Grails 1.2.. Object returning tag names are listed in a static `returnObjectForTags` property in the tag library class. Example:

    class ObjectReturningTagLib {
        static namespace = "cms"
        static returnObjectForTags = ['content']    def content = { attrs, body ->
            CmsContent.findByCode(attrs.code)?.content
        }
    }
## 7.4 URL Mappings Throughout the documentation so far the convention used for URLs has been the default of `/controller/action/id`. However, this convention is not hard wired into Grails and is in fact controlled by a URL Mappings class located at `grails-app/conf/UrlMappings.groovy`. The `UrlMappings` class contains a single property called `mappings` that has been assigned a block of code:
    class UrlMappings {
        static mappings = {
        }
    }
## 7.4.1 Mapping to Controllers and Actions To create a simple mapping simply use a relative URL as the method name and specify named parameters for the controller and action to map to:
    "/product"(controller: "product", action: "list")
In this case we've mapped the URL `/product` to the `list` action of the `ProductController`. Omit the action definition to map to the default action of the controller:
    "/product"(controller: "product")
An alternative syntax is to assign the controller and action to use within a block passed to the method:
    "/product" {
        controller = "product"
        action = "list"
    }
Which syntax you use is largely dependent on personal preference. If you have mappings that all fall under a particular path you can group mappings with the `group` method:
    group "/product", {
        "/apple"(controller:"product", id:"apple")
        "/htc"(controller:"product", id:"htc")
    }
To rewrite one URI onto another explicit URI (rather than a controller/action pair) do something like this:
    "/hello"(uri: "/hello.dispatch")
Rewriting specific URIs is often useful when integrating with other frameworks. ## 7.4.2 Mapping to REST resources Since Grails 2.3, it possible to create RESTful URL mappings that map onto controllers by convention. The syntax to do so is as follows:
    "/books"(resources:'book')
You define a base URI and the name of the controller to map to using the `resources` parameter. The above mapping will result in the following URLs:
HTTP Method | URI | Grails Action
--- | --- | ---
GET | /books | index
GET | /books/create | create
POST | /books | save
GET | /books/${id} | show
GET | /books/${id}/edit | edit
PUT | /books/${id} | update
DELETE | /books/${id} | delete
If you wish to include or exclude any of the generated URL mappings you can do so with the `includes` or `excludes` parameter, which accepts the name of the Grails action to include or exclude:

    "/books"(resources:'book', excludes:['delete', 'update'])or"/books"(resources:'book', includes:['index', 'show'])
#### Single resources A single resource is a resource for which there is only one (possibly per user) in the system. You can create a single resource using the `resource` parameter (as oppose to `resources`):
    "/book"(resource:'book')
This results in the following URL mappings:
HTTP Method | URI | Grails Action
--- | --- | ---
GET | /book/create | create
POST | /book | save
GET | /book | show
GET | /book/edit | edit
PUT | /book | update
DELETE | /book | delete
The main difference is that the id is not included in the URL mapping. #### Nested Resources You can nest resource mappings to generate child resources. For example:
    "/books"(resources:'book') {
      "/authors"(resources:"author")
    }
The above will result in the following URL mappings:
HTTP Method | URL | Grails Action
--- | --- | ---
GET | /books/${bookId}/authors | index
GET | /books/${bookId}/authors/create | create
POST | /books/${bookId}/authors | save
GET | /books/${bookId}/authors/${id} | show
GET | /books/${bookId}/authors/edit/${id} | edit
PUT | /books/${bookId}/authors/${id} | update
DELETE | /books/${bookId}/authors/${id} | delete
You can also nest regular URL mappings within a resource mapping:
    "/books"(resources: "book") {
        "/publisher"(controller:"publisher")
    }
This will result in the following URL being available:
HTTP Method | URL | Grails Action
--- | --- | ---
GET | /books/1/publisher | index
#### Linking to RESTful Mappings You can link to any URL mapping created with the `g:link` tag provided by Grails simply by referencing the controller and action to link to:
    <g:link controller="book" action="index">My Link</g:link>
As a convenience you can also pass a domain instance to the `resource` attribute of the `link` tag:
    <g:link resource="${book}">My Link</g:link>
This will automatically produce the correct link (in this case "/books/1" for an id of "1"). The case of nested resources is a little different as they typically required two identifiers (the id of the resource and the one it is nested within). For example given the nested resources:
    "/books"(resources:'book') {
      "/authors"(resources:"author")
    }
If you wished to link to the `show` action of the `author` controller, you would write:
    // Results in /books/1/authors/2
    <g:link controller="author" action="show" method="GET" params="[bookId:1]" id="2">The Author</g:link>
However, to make this more concise there is a `resource` attribute to the link tag which can be used instead:
    // Results in /books/1/authors/2
    <g:link resource="book/author" action="show" bookId="1" id="2">My Link</g:link>
The resource attribute accepts a path to the resource separated by a slash (in this case "book/author"). The attributes of the tag can be used to specify the necessary `bookId` parameter. ## 7.4.3 Redirects In URL Mappings Since Grails 2.3, it is possible to define URL mappings which specify a redirect. When a URL mapping specifies a redirect, any time that mapping matches an incoming request, a redirect is initiated with information provided by the mapping. When a URL mapping specifies a redirect the mapping must either supply a String representing a URI to redirect to or must provide a Map representing the target of the redirect. That Map is structured just like the Map that may be passed as an argument to the `redirect` method in a controller.
    "/viewBooks"(redirect: '/books/list')
    "/viewAuthors"(redirect: [controller: 'author', action: 'list'])
    "/viewPublishers"(redirect: [controller: 'publisher', action: 'list', permanent: true])
Request parameters that were part of the original request will be included in the redirect. ## 7.4.4 Embedded Variables #### Simple Variables The previous section demonstrated how to map simple URLs with concrete "tokens". In URL mapping speak tokens are the sequence of characters between each slash, '/'. A concrete token is one which is well defined such as as `/product`. However, in many circumstances you don't know what the value of a particular token will be until runtime. In this case you can use variable placeholders within the URL for example:
    static mappings = {
      "/product/$id"(controller: "product")
    }
In this case by embedding a $id variable as the second token Grails will automatically map the second token into a parameter (available via the params object) called `id`. For example given the URL `/product/MacBook`, the following code will render "MacBook" to the response:
    class ProductController {
         def index() { render params.id }
    }
You can of course construct more complex examples of mappings. For example the traditional blog URL format could be mapped as follows:
    static mappings = {
       "/$blog/$year/$month/$day/$id"(controller: "blog", action: "show")
    }
The above mapping would let you do things like:
    /graemerocher/2007/01/10/my_funky_blog_entry
The individual tokens in the URL would again be mapped into the params object with values available for `year`, `month`, `day`, `id` and so on. #### Dynamic Controller and Action Names Variables can also be used to dynamically construct the controller and action name. In fact the default Grails URL mappings use this technique:
    static mappings = {
        "/$controller/$action?/$id?"()
    }
Here the name of the controller, action and id are implicitly obtained from the variables `controller`, `action` and `id` embedded within the URL. You can also resolve the controller name and action name to execute dynamically using a closure:
    static mappings = {
        "/$controller" {
            action = { params.goHere }
        }
    }
#### Optional Variables Another characteristic of the default mapping is the ability to append a ? at the end of a variable to make it an optional token. In a further example this technique could be applied to the blog URL mapping to have more flexible linking:
    static mappings = {
        "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show")
    }
With this mapping all of these URLs would match with only the relevant parameters being populated in the params object:
    /graemerocher/2007/01/10/my_funky_blog_entry
    /graemerocher/2007/01/10
    /graemerocher/2007/01
    /graemerocher/2007
    /graemerocher
#### Optional File Extensions If you wish to capture the extension of a particular path, then a special case mapping exists:
    "/$controller/$action?/$id?(.$format)?"()
By adding the `(.$format)?` mapping you can access the file extension using the `response.format` property in a controller:
    def index() {
        render "extension is ${response.format}"
    }
#### Arbitrary Variables You can also pass arbitrary parameters from the URL mapping into the controller by just setting them in the block passed to the mapping:
    "/holiday/win" {
         id = "Marrakech"
         year = 2007
    }
This variables will be available within the params object passed to the controller. #### Dynamically Resolved Variables The hard coded arbitrary variables are useful, but sometimes you need to calculate the name of the variable based on runtime factors. This is also possible by assigning a block to the variable name:
    "/holiday/win" {
         id = { params.id }
         isEligible = { session.user != null } // must be logged in
    }
In the above case the code within the blocks is resolved when the URL is actually matched and hence can be used in combination with all sorts of logic. ## 7.4.5 Mapping to Views You can resolve a URL to a view without a controller or action involved. For example to map the root URL `/` to a GSP at the location `grails-app/views/index.gsp` you could use:
    static mappings = {
        "/"(view: "/index")  // map the root URL
    }
Alternatively if you need a view that is specific to a given controller you could use:
    static mappings = {
       "/help"(controller: "site", view: "help") // to a view for a controller
    }
## 7.4.6 Mapping to Response Codes Grails also lets you map HTTP response codes to controllers, actions or views. Just use a method name that matches the response code you are interested in:
    static mappings = {
       "403"(controller: "errors", action: "forbidden")
       "404"(controller: "errors", action: "notFound")
       "500"(controller: "errors", action: "serverError")
    }
Or you can specify custom error pages:
    static mappings = {
       "403"(view: "/errors/forbidden")
       "404"(view: "/errors/notFound")
       "500"(view: "/errors/serverError")
    }
#### Declarative Error Handling In addition you can configure handlers for individual exceptions:
    static mappings = {
       "403"(view: "/errors/forbidden")
       "404"(view: "/errors/notFound")
       "500"(controller: "errors", action: "illegalArgument",
             exception: IllegalArgumentException)
       "500"(controller: "errors", action: "nullPointer",
             exception: NullPointerException)
       "500"(controller: "errors", action: "customException",
             exception: MyException)
       "500"(view: "/errors/serverError")
    }
With this configuration, an `IllegalArgumentException` will be handled by the `illegalArgument` action in `ErrorsController`, a `NullPointerException` will be handled by the `nullPointer` action, and a `MyException` will be handled by the `customException` action. Other exceptions will be handled by the catch-all rule and use the `/errors/serverError` view. You can access the exception from your custom error handing view or controller action using the request's `exception` attribute like so:
    class ErrorController {
        def handleError() {
            def exception = request.exception
            // perform desired processing to handle the exception
        }
    }
If your error-handling controller action throws an exception as well, you'll end up with a `StackOverflowException`. ## 7.4.7 Mapping to HTTP methods URL mappings can also be configured to map based on the HTTP method (GET, POST, PUT or DELETE). This is very useful for RESTful APIs and for restricting mappings based on HTTP method. As an example the following mappings provide a RESTful API URL mappings for the `ProductController`:
    static mappings = {
       "/product/$id"(controller:"product", action: "update", method: "PUT")
    }
## 7.4.8 Mapping Wildcards Grails' URL mappings mechanism also supports wildcard mappings. For example consider the following mapping:
    static mappings = {
        "/images/*.jpg"(controller: "image")
    }
This mapping will match all paths to images such as `/image/logo.jpg`. Of course you can achieve the same effect with a variable:
    static mappings = {
        "/images/$name.jpg"(controller: "image")
    }
However, you can also use double wildcards to match more than one level below:
    static mappings = {
        "/images/**.jpg"(controller: "image")
    }
In this cases the mapping will match `/image/logo.jpg` as well as `/image/other/logo.jpg`. Even better you can use a double wildcard variable:
    static mappings = {
        // will match /image/logo.jpg and /image/other/logo.jpg
        "/images/$name**.jpg"(controller: "image")
    }
In this case it will store the path matched by the wildcard inside a `name` parameter obtainable from the params object:
    def name = params.name
    println name // prints "logo" or "other/logo"
If you use wildcard URL mappings then you may want to exclude certain URIs from Grails' URL mapping process. To do this you can provide an `excludes` setting inside the `UrlMappings.groovy` class:
    class UrlMappings {
        static excludes = ["/images/*", "/css/*"]
        static mappings = {
            …
        }
    }
In this case Grails won't attempt to match any URIs that start with `/images` or `/css`. ## 7.4.9 Automatic Link Re-Writing Another great feature of URL mappings is that they automatically customize the behaviour of the link tag so that changing the mappings don't require you to go and change all of your links. This is done through a URL re-writing technique that reverse engineers the links from the URL mappings. So given a mapping such as the blog one from an earlier section:
    static mappings = {
       "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show")
    }
If you use the link tag as follows:

    <g:link controller="blog" action="show"
            params="[blog:'fred', year:2007]">
        My Blog
    </g:link><g:link controller="blog" action="show"
            params="[blog:'fred', year:2007, month:10]">
        My Blog - October 2007 Posts
    </g:link>
Grails will automatically re-write the URL in the correct format:
    <a href="/fred/2007">My Blog</a>
    <a href="/fred/2007/10">My Blog - October 2007 Posts</a>
## 7.4.10 Applying Constraints URL Mappings also support Grails' unified validation constraints mechanism, which lets you further "constrain" how a URL is matched. For example, if we revisit the blog sample code from earlier, the mapping currently looks like this:
    static mappings = {
       "/$blog/$year?/$month?/$day?/$id?"(controller:"blog", action:"show")
    }
This allows URLs such as:
    /graemerocher/2007/01/10/my_funky_blog_entry
However, it would also allow:
    /graemerocher/not_a_year/not_a_month/not_a_day/my_funky_blog_entry
This is problematic as it forces you to do some clever parsing in the controller code. Luckily, URL Mappings can be constrained to further validate the URL tokens:
    "/$blog/$year?/$month?/$day?/$id?" {
         controller = "blog"
         action = "show"
         constraints {
              year(matches:/\d{4}/)
              month(matches:/\d{2}/)
              day(matches:/\d{2}/)
         }
    }
In this case the constraints ensure that the `year`, `month` and `day` parameters match a particular valid pattern thus relieving you of that burden later on. ## 7.4.11 Named URL Mappings URL Mappings also support named mappings, that is mappings which have a name associated with them. The name may be used to refer to a specific mapping when links are generated. The syntax for defining a named mapping is as follows:
    static mappings = {
       name <mapping name>: <url pattern> {
          // …
       }
    }
For example:
    static mappings = {
        name personList: "/showPeople" {
            controller = 'person'
            action = 'list'
        }
        name accountDetails: "/details/$acctNumber" {
            controller = 'product'
            action = 'accountDetails'
        }
    }
The mapping may be referenced in a link tag in a GSP.
    <g:link mapping="personList">List People</g:link>
That would result in:
    <a href="/showPeople">List People</a>
Parameters may be specified using the params attribute.
    <g:link mapping="accountDetails" params="[acctNumber:'8675309']">
        Show Account
    </g:link>
That would result in:
    <a href="/details/8675309">Show Account</a>
Alternatively you may reference a named mapping using the link namespace.
    <link:personList>List People</link:personList>
That would result in:
    <a href="/showPeople">List People</a>
The link namespace approach allows parameters to be specified as attributes.
    <link:accountDetails acctNumber="8675309">Show Account</link:accountDetails>
That would result in:
    <a href="/details/8675309">Show Account</a>
To specify attributes that should be applied to the generated `href`, specify a `Map` value to the `attrs` attribute. These attributes will be applied directly to the href, not passed through to be used as request parameters.
    <link:accountDetails attrs="[class: 'fancy']" acctNumber="8675309">
        Show Account
    </link:accountDetails>
That would result in:
    <a href="/details/8675309" class="fancy">Show Account</a>
## 7.4.12 Customizing URL Formats The default URL Mapping mechanism supports camel case names in the URLs. The default URL for accessing an action named `addNumbers` in a controller named `MathHelperController` would be something like `/mathHelper/addNumbers`. Grails allows for the customization of this pattern and provides an implementation which replaces the camel case convention with a hyphenated convention that would support URLs like `/math-helper/add-numbers`. To enable hyphenated URLs assign a value of "hyphenated" to the `grails.web.url.converter` property in `grails-app/conf/Config.groovy`.

    // grails-app/conf/Config.groovygrails.web.url.converter = 'hyphenated'
Arbitrary strategies may be plugged in by providing a class which implements the UrlConverter interface and adding an instance of that class to the Spring application context with the bean name of `grails.web.UrlConverter.BEAN_NAME`. If Grails finds a bean in the context with that name, it will be used as the default converter and there is no need to assign a value to the `grails.web.url.converter` config property.

    // src/groovy/com/myapplication/MyUrlConverterImpl.groovypackage com.myapplicationclass MyUrlConverterImpl implements grails.web.UrlConverter {    String toUrlElement(String propertyOrClassName) {
            // return some representation of a property or class name that should be used in URLs…
        }
    }

    // grails-app/conf/spring/resources.groovybeans = {
        "${grails.web.UrlConverter.BEAN_NAME}"(com.myapplication.MyUrlConverterImpl)
    }
## 7.4.13 Namespaced Controllers If an application defines multiple controllers with the same name in different packages, the controllers must be defined in a namespace. The way to define a namespace for a controller is to define a static property named `namespace` in the controller and assign a String to the property that represents the namespace.

    // grails-app/controllers/com/app/reporting/AdminController.groovy
    package com.app.reportingclass AdminController {    static namespace = 'reports'    // …
    }

    // grails-app/controllers/com/app/security/AdminController.groovy
    package com.app.securityclass AdminController {    static namespace = 'users'    // …
    }
When defining url mappings which should be associated with a namespaced controller, the `namespace` variable needs to be part of the URL mapping.

    // grails-app/conf/UrlMappings.groovy
    class UrlMappings {    static mappings = {
            '/userAdmin' {
                controller = 'admin'
                namespace = 'users'
            }        '/reportAdmin' {
                controller = 'admin'
                namespace = 'reports'
            }        "/$namespace/$controller/$action?"()
        }
    }
Reverse URL mappings also require that the `namespace` be specified.
    <g:link controller="admin" namespace="reports">Click For Report Admin</g:link>
    <g:link controller="admin" namespace="users">Click For User Admin</g:link>
When resolving a URL mapping (forward or reverse) to a namespaced controller, a mapping will only match if the `namespace` has been provided. If the application provides several controllers with the same name in different packages, at most 1 of them may be defined without a `namespace` property. If there are multiple controllers with the same name that do not define a `namespace` property, the framework will not know how to distinguish between them for forward or reverse mapping resolutions. It is allowed for an application to use a plugin which provides a controller with the same name as a controller provided by the application and for neither of the controllers to define a `namespace` property as long as the controllers are in separate packages. For example, an application may include a controller named `com.accounting.ReportingController` and the application may use a plugin which provides a controller named `com.humanresources.ReportingController`. The only issue with that is the URL mapping for the controller provided by the plugin needs to be explicit in specifying that the mapping applies to the `ReportingController` which is provided by the plugin. See the following example.
    static mappings = {
        "/accountingReports" {
            controller = "reporting"
        }
        "/humanResourceReports" {
            controller = "reporting"
            plugin = "humanResources"
        }
    }
With that mapping in place, a request to `/accountingReports` will be handled by the `ReportingController` which is defined in the application. A request to `/humanResourceReports` will be handled by the `ReportingController` which is provided by the `humanResources` plugin. There could be any number of `ReportingController` controllers provided by any number of plugins but no plugin may provide more than one `ReportingController` even if they are defined in separate packages. Assigning a value to the `plugin` variable in the mapping is only required if there are multiple controllers with the same name available at runtime provided by the application and/or plugins. If the `humanResources` plugin provides a `ReportingController` and there is no other `ReportingController` available at runtime, the following mapping would work.
    static mappings = {
        "/humanResourceReports" {
            controller = "reporting"
        }
    }
It is best practice to be explicit about the fact that the controller is being provided by a plugin. ## 7.5 Web Flow #### Overview Grails supports the creation of web flows built on the Spring Web Flow project. A web flow is a conversation that spans multiple requests and retains state for the scope of the flow. A web flow also has a defined start and end state. Web flows don't require an HTTP session, but instead store their state in a serialized form, which is then restored using a flow execution key that Grails passes around as a request parameter. This makes flows far more scalable than other forms of stateful application that use the HttpSession and its inherit memory and clustering concerns. Web flow is essentially an advanced state machine that manages the "flow" of execution from one state to the next. Since the state is managed for you, you don't have to be concerned with ensuring that users enter an action in the middle of some multi step flow, as web flow manages that for you. This makes web flow perfect for use cases such as shopping carts, hotel booking and any application that has multi page work flows. From Grails 1.2 onwards Webflow is no longer in Grails core, so you must install the Webflow plugin to use this feature. #### Creating a Flow To create a flow create a regular Grails controller and add an action that ends with the convention `Flow`. For example:

    class BookController {   def index() {
          redirect(action: "shoppingCart")
       }   def shoppingCartFlow = {
            …
       }
    }
Notice when redirecting or referring to the flow as an action we omit the `Flow` suffix. In other words the name of the action of the above flow is `shoppingCart`. ## 7.5.1 Start and End States As mentioned before a flow has a defined start and end state. A start state is the state which is entered when a user first initiates a conversation (or flow). The start state of a Grails flow is the first method call that takes a block. For example:
    class BookController {
       …
       def shoppingCartFlow ={
           showCart {
               on("checkout").to "enterPersonalDetails"
               on("continueShopping").to "displayCatalogue"
           }
           …
           displayCatalogue {
               redirect(controller: "catalogue", action: "show")
           }
           displayInvoice()
       }
    }
Here the `showCart` node is the start state of the flow. Since the showCart state doesn't define an action or redirect it is assumed be a view state that, by convention, refers to the view `grails-app/views/book/shoppingCart/showCart.gsp`. Notice that unlike regular controller actions, the views are stored within a directory that matches the name of the flow: `grails-app/views/book/shoppingCart`. The `shoppingCart` flow also has two possible end states. The first is `displayCatalogue` which performs an external redirect to another controller and action, thus exiting the flow. The second is `displayInvoice` which is an end state as it has no events at all and will simply render a view called `grails-app/views/book/shoppingCart/displayInvoice.gsp` whilst ending the flow at the same time. Once a flow has ended it can only be resumed from the start state, in this case `showCart`, and not from any other state. ## 7.5.2 Action States and View States #### View states A view state is a one that doesn't define an `action` or a `redirect`. So for example this is a view state:
    enterPersonalDetails {
       on("submit").to "enterShipping"
       on("return").to "showCart"
    }
It will look for a view called `grails-app/views/book/shoppingCart/enterPersonalDetails.gsp` by default. Note that the `enterPersonalDetails` state defines two events: `submit` and `return`. The view is responsible for triggering these events. Use the `render` method to change the view to be rendered:
    enterPersonalDetails {
       render(view: "enterDetailsView")
       on("submit").to "enterShipping"
       on("return").to "showCart"
    }
Now it will look for `grails-app/views/book/shoppingCart/enterDetailsView.gsp`. Start the `view` parameter with a / to use a shared view:
    enterPersonalDetails {
       render(view: "/shared/enterDetailsView")
       on("submit").to "enterShipping"
       on("return").to "showCart"
    }
Now it will look for `grails-app/views/shared/enterDetailsView.gsp` #### Action States An action state is a state that executes code but does not render a view. The result of the action is used to dictate flow transition. To create an action state you define an action to to be executed. This is done by calling the `action` method and passing it a block of code to be executed:
    listBooks {
       action {
          [bookList: Book.list()]
       }
       on("success").to "showCatalogue"
       on(Exception).to "handleError"
    }
As you can see an action looks very similar to a controller action and in fact you can reuse controller actions if you want. If the action successfully returns with no errors the `success` event will be triggered. In this case since we return a Map, which is regarded as the "model" and is automatically placed in flow scope. In addition, in the above example we also use an exception handler to deal with errors on the line:
    on(Exception).to "handleError"
This makes the flow transition to a state called `handleError` in the case of an exception. You can write more complex actions that interact with the flow request context:

    processPurchaseOrder {
        action {
            def a =  flow.address
            def p = flow.person
            def pd = flow.paymentDetails
            def cartItems = flow.cartItems
            flow.clear()        def o = new Order(person: p, shippingAddress: a, paymentDetails: pd)
            o.invoiceNumber = new Random().nextInt(9999999)
            for (item in cartItems) { o.addToItems item }
            o.save()
            [order: o]
        }
        on("error").to "confirmPurchase"
        on(Exception).to "confirmPurchase"
        on("success").to "displayInvoice"
    }
Here is a more complex action that gathers all the information accumulated from the flow scope and creates an `Order` object. It then returns the order as the model. The important thing to note here is the interaction with the request context and "flow scope". #### Transition Actions Another form of action is what is known as a transition action. A transition action is executed directly prior to state transition once an event has been triggered. A simple example of a transition action can be seen below:
    enterPersonalDetails {
       on("submit") {
           log.trace "Going to enter shipping"
       }.to "enterShipping"
       on("return").to "showCart"
    }
Notice how we pass a block of the code to `submit` event that simply logs the transition. Transition states are very useful for data binding and validation, which is covered in a later section. ## 7.5.3 Flow Execution Events In order to transition execution of a flow from one state to the next you need some way of trigger an event that indicates what the flow should do next. Events can be triggered from either view states or action states. #### Triggering Events from a View State As discussed previously the start state of the flow in a previous code listing deals with two possible events. A `checkout` event and a `continueShopping` event:
    def shoppingCartFlow = {
        showCart {
            on("checkout").to "enterPersonalDetails"
            on("continueShopping").to "displayCatalogue"
        }
        …
    }
Since the `showCart` event is a view state it will render the view `grails-app/book/shoppingCart/showCart.gsp`. Within this view you need to have components that trigger flow execution. On a form this can be done use the submitButton tag:
    <g:form>
        <g:submitButton name="continueShopping" value="Continue Shopping" />
        <g:submitButton name="checkout" value="Checkout" />
    </g:form>
The form automatically submits back to the `shoppingCart` flow. The name attribute of each submitButton tag signals which event will be triggered. If you don't have a form you can also trigger an event with the link tag as follows:
    <g:link event="checkout" />
Prior to 2.0.0, it was required to specify the controller and/or action in forms and links, which caused the url to change when entering a subflow state. When the controller and action are not specified, all url's are relative to the main flow execution url, which makes your flows reusable as subflows and prevents issues with the browser's back button. #### Triggering Events from an Action To trigger an event from an `action` you invoke a method. For example there is the built in `error()` and `success()` methods. The example below triggers the `error()` event on validation failure in a transition action:
    enterPersonalDetails {
       on("submit") {
             def p = new Person(params)
             flow.person = p
             if (!p.validate()) return error()
       }.to "enterShipping"
       on("return").to "showCart"
    }
In this case because of the error the transition action will make the flow go back to the `enterPersonalDetails` state. With an action state you can also trigger events to redirect flow:
    shippingNeeded {
       action {
           if (params.shippingRequired) yes()
           else no()
       }
       on("yes").to "enterShipping"
       on("no").to "enterPayment"
    }
## 7.5.4 Flow Scopes #### Scope Basics You'll notice from previous examples that we used a special object called `flow` to store objects within "flow scope". Grails flows have five different scopes you can utilize: `request` - Stores an object for the scope of the current request `flash` - Stores the object for the current and next request only `flow` - Stores objects for the scope of the flow, removing them when the flow reaches an end state `conversation` - Stores objects for the scope of the conversation including the root flow and nested subflows `session` - Stores objects in the user's session Grails service classes can be automatically scoped to a web flow scope. See the documentation on Services for more information. Returning a model Map from an action will automatically result in the model being placed in flow scope. For example, using a transition action, you can place objects within `flow` scope as follows:
    enterPersonalDetails {
        on("submit") {
            [person: new Person(params)]
        }.to "enterShipping"
       on("return").to "showCart"
    }
Be aware that a new request is always created for each state, so an object placed in request scope in an action state (for example) will not be available in a subsequent view state. Use one of the other scopes to pass objects from one state to another. Also note that Web Flow: Moves objects from flash scope to request scope upon transition between states; Merges objects from the flow and conversation scopes into the view model before rendering (so you shouldn't include a scope prefix when referencing these objects within a view, e.g. GSP pages). #### Flow Scopes and Serialization When placing objects in `flash`, `flow` or `conversation` scope they must implement `java.io.Serializable` or an exception will be thrown. This has an impact on domain classes in that domain classes are typically placed within a scope so that they can be rendered in a view. For example consider the following domain class:
    class Book {
        String title
    }
To place an instance of the `Book` class in a flow scope you will need to modify it as follows:
    class Book implements Serializable {
        String title
    }
This also impacts associations and closures you declare within a domain class. For example consider this:
    class Book implements Serializable {
        String title
        Author author
    }
Here if the `Author` association is not `Serializable` you will also get an error. This also impacts closures used in GORM events such as `onLoad`, `onSave` and so on. The following domain class will cause an error if an instance is placed in a flow scope:

    class Book implements Serializable {    String title    def onLoad = {
            println "I'm loading"
        }
    }
The reason is that the assigned block on the `onLoad` event cannot be serialized. To get around this you should declare all events as `transient`:

    class Book implements Serializable {    String title    transient onLoad = {
            println "I'm loading"
        }
    }
or as methods:

    class Book implements Serializable {    String title    def onLoad() {
            println "I'm loading"
        }
    }
> The flow scope contains a reference to the Hibernate session. As a result, any object loaded into the session through a GORM query will also be in the flow and will need to implement Serializable. If you don't want your domain class to be Serializable or stored in the flow, then you will need to evict the entity manually before the end of the state:
>     flow.persistenceContext.evict(it)
## 7.5.5 Data Binding and Validation In the section on start and end states, the start state in the first example triggered a transition to the `enterPersonalDetails` state. This state renders a view and waits for the user to enter the required information:
    enterPersonalDetails {
       on("submit").to "enterShipping"
       on("return").to "showCart"
    }
The view contains a form with two submit buttons that either trigger the submit event or the return event:
    <g:form>
        <!-- Other fields -->
        <g:submitButton name="submit" value="Continue"></g:submitButton>
        <g:submitButton name="return" value="Back"></g:submitButton>
    </g:form>
However, what about the capturing the information submitted by the form? To capture the form info we can use a flow transition action:
    enterPersonalDetails {
       on("submit") {
          flow.person = new Person(params)
          !flow.person.validate() ? error() : success()
       }.to "enterShipping"
       on("return").to "showCart"
    }
Notice how we perform data binding from request parameters and place the `Person` instance within `flow` scope. Also interesting is that we perform validation and invoke the `error()` method if validation fails. This signals to the flow that the transition should halt and return to the `enterPersonalDetails` view so valid entries can be entered by the user, otherwise the transition should continue and go to the `enterShipping` state. Like regular actions, flow actions also support the notion of Command Objects by defining the first argument of the closure:
    enterPersonalDetails {
       on("submit") { PersonDetailsCommand cmd ->
           flow.personDetails = cmd
          !flow.personDetails.validate() ? error() : success()
       }.to "enterShipping"
       on("return").to "showCart"
    }
## 7.5.6 Subflows and Conversations #### Calling subflows Grails' Web Flow integration also supports subflows. A subflow is like a flow within a flow. For example take this search flow:
    def searchFlow = {
        displaySearchForm {
            on("submit").to "executeSearch"
        }
        executeSearch {
            action {
                [results:searchService.executeSearch(params.q)]
            }
            on("success").to "displayResults"
            on("error").to "displaySearchForm"
        }
        displayResults {
            on("searchDeeper").to "extendedSearch"
            on("searchAgain").to "displaySearchForm"
        }
        extendedSearch {
            // Extended search subflow
            subflow(controller: "searchExtensions", action: "extendedSearch")
            on("moreResults").to "displayMoreResults"
            on("noResults").to "displayNoMoreResults"
        }
        displayMoreResults()
        displayNoMoreResults()
    }
It references a subflow in the `extendedSearch` state. The controller parameter is optional if the subflow is defined in the same controller as the calling flow. Prior to 1.3.5, the previous subflow call would look like `subflow(new SearchExtensionsController().extendedSearchFlow)`, with the requirement that the name of the subflow state be the same as the called subflow (minus `Flow`). This way of calling a subflow is deprecated and only supported for backward compatibility. The subflow is another flow entirely:
    def extendedSearchFlow = {
        startExtendedSearch {
            on("findMore").to "searchMore"
            on("searchAgain").to "noResults"
        }
        searchMore {
            action {
               def results = searchService.deepSearch(ctx.conversation.query)
               if (!results) return error()
               conversation.extendedResults = results
            }
            on("success").to "moreResults"
            on("error").to "noResults"
        }
        moreResults()
        noResults()
    }
Notice how it places the `extendedResults` in conversation scope. This scope differs to flow scope as it lets you share state that spans the whole conversation, i.e. a flow execution including all subflows, not just the flow itself. Also notice that the end state (either `moreResults` or `noResults` of the subflow triggers the events in the main flow:
    extendedSearch {
        // Extended search subflow
        subflow(controller: "searchExtensions", action: "extendedSearch")
        on("moreResults").to "displayMoreResults"
        on("noResults").to "displayNoMoreResults"
    }
#### Subflow input and output Using conversation scope for passing input and output between flows can be compared with using global variables to pass information between methods. While this is OK in certain situations, it is usually better to use method arguments and return values. In webflow speak, this means defining input and output arguments for flows. Consider following flow for searching a person with a certain expertise:

    def searchFlow = {
            input {
                expertise(required: true)
                title("Search person")
            }        search {
                onEntry {
                    [personInstanceList: Person.findAllByExpertise(flow.expertise)]
                }
                on("select") {
                    flow.person = Person.get(params.id)
                }.to("selected")
                on("cancel").to("cancel")
            }        selected {
                output {
                    person {flow.person}
                }
            }
            cancel()
        }}
This flow accepts two input parameters: a required expertise argument an optional title argument with a default value All input arguments are stored in flow scope and are, just like local variables, only visible within this flow. A flow that contains required input will throw an exception when an execution is started without providing the input. The consequence is that these flows can only be started as subflows. Notice how an end state can define one or more named output values. If the value is a closure, this closure will be evaluated at the end of each flow execution. If the value is not a closure, the value will be a constant that is only calculated once at flow definition time. When a subflow is called, we can provide it a map with input values:

    def newProjectWizardFlow = {
        ...    managerSearch {
            subflow(controller: "person", action: "search",
                    input: [expertise : "management", title: "Search project manager"])
            on("selected") {
                flow.projectInstance.manager = currentEvent.attributes.person
            }.to "techleadSearch"
        }    techleadSearch {
            subflow(controller: "person", action: "search",
                    input: [expertise : { flow.technology }, title: "Search technical lead"])
            on("selected") {
                flow.projectInstance.techlead = currentEvent.attributes.person
            }.to "projectDetails"
        }    ...}
Notice again the difference between constant values like `expertise : "management"` and dynamic values like `expertise : { flow.technology }` The subflow output is available via `currentEvent.attributes` ## 7.6 Filters Although Grails controllers support fine grained interceptors, these are only really useful when applied to a few controllers and become difficult to manage with larger applications. Filters on the other hand can be applied across a whole group of controllers, a URI space or to a specific action. Filters are far easier to plugin and maintain completely separately to your main controller logic and are useful for all sorts of cross cutting concerns such as security, logging, and so on. ## 7.6.1 Applying Filters To create a filter create a class that ends with the convention `Filters` in the `grails-app/conf` directory. Within this class define a code block called `filters` that contains the filter definitions:
    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. ## 7.6.2 Filter Types Within the body of the filter you can then define one or several of the following interceptor types for the filter: `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.AtomicLongclass 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. ## 7.6.3 Variables and Scopes Filters support all the common properties available to controllers and tag libraries, plus the application context: 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 ## 7.6.4 Filter Dependencies In a `Filters` class, you can specify any other `Filters` classes that should first be executed using the `dependsOn` property. This is used when a `Filters` class depends on the behavior of another `Filters` class (e.g. setting up the environment, modifying the request/session, etc.) and is defined as an array of `Filters` classes. 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. ## 7.7 Ajax Ajax is the driving force behind the shift to richer web applications. These types of applications in general are better suited to agile, dynamic frameworks written in languages like Groovy and Ruby Grails provides support for building Ajax applications through its Ajax tag library. For a full list of these see the Tag Library Reference. Note: JavaScript examples use the jQuery library. ## 7.7.1 Ajax Support By default Grails ships with the jQuery library, but through the Plugin system provides support for other frameworks such as Prototype, Dojo:http://dojotoolkit.org/, Yahoo UI:http://developer.yahoo.com/yui/ and the Google Web Toolkit. 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 ## 7.7.1.1 Remoting Linking Remote content can be loaded in a number of ways, the most commons way is through the remoteLink tag. This tag allows the creation of HTML anchor tags that perform an asynchronous request and optionally set the response in an element. The simplest way to create a remote link is as follows:
    <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`. ## 7.7.1.2 Updating Content This is great, but usually you provide feedback to the user about what happened:
    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. ## 7.7.1.3 Remote Form Submission An HTML form can also be submitted asynchronously in one of two ways. Firstly using the formRemote tag which expects similar attributes to those for the remoteLink tag:
    <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>
## 7.7.1.4 Ajax Events Specific JavaScript can be called if certain events occur, all the events start with the "on" prefix and let you give feedback to the user where appropriate, or take other action:
    <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 You can simply refer to the `XMLHttpRequest` variable to obtain the request:
    <g:javascript>
        function fireMe(event) {
            alert("XmlHttpRequest = " + event)
        }
    }
    </g:javascript>
    <g:remoteLink action="example"
                  update="success"
                  onFailure="fireMe(XMLHttpRequest)">Ajax Link</g:remoteLink>
## 7.7.2 Ajax with Prototype Grails features an external plugin to add Prototype support to Grails. To install the plugin, list it in BuildConfig.groovy:
    runtime ":prototype:latest.release"
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. ## 7.7.3 Ajax with Dojo Grails features an external plugin to add Dojo support to Grails. To install the plugin, list it in BuildConfig.groovy:
    compile ":dojo:latest.release"
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. ## 7.7.4 Ajax with GWT Grails also features support for the Google Web Toolkit through a plugin. There is comprehensive documentation available on the Grails wiki. ## 7.7.5 Ajax on the Server There are a number of different ways to implement Ajax which are typically broken down into: 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.JSONdef 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(data) {
        $("#book" + data.id + "_title").html( data.title );
    }
    </g:javascript>
    <g:remoteLink action="showBook" id="${book.id}" onSuccess="updateBook(data)">
        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.XMLdef 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(data) {
        var id = $(data).find("book").attr("id");
        $("#book" + id + "_title").html( $(data).find("title").text() );
    }
    </g:javascript>
    <g:remoteLink action="showBook" id="${book.id}" onSuccess="updateBook(data)">
        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').html('${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]
        }
    }
## 7.8 Content Negotiation Grails has built in support for Content negotiation using either the HTTP `Accept` header, an explicit format request parameter or the extension of a mapped URI. #### Configuring Mime Types Before you can start dealing with content negotiation you need to tell Grails what content types you wish to support. By default Grails comes configured with a number of different content types within `grails-app/conf/Config.groovy` using the `grails.mime.types` setting:
    grails.mime.types = [ // the first one is the default format
        all:           '*/*', // 'all' maps to '*' or the first available format in withFormat
        atom:          'application/atom+xml',
        css:           'text/css',
        csv:           'text/csv',
        form:          'application/x-www-form-urlencoded',
        html:          ['text/html','application/xhtml+xml'],
        js:            'text/javascript',
        json:          ['application/json', 'text/json'],
        multipartForm: 'multipart/form-data',
        rss:           'application/rss+xml',
        text:          'text/plain',
        hal:           ['application/hal+json','application/hal+xml'],
        xml:           ['text/xml', 'application/xml']
    ]
The above bit of configuration allows Grails to detect to format of a request containing either the 'text/xml' or 'application/xml' media types as simply 'xml'. You can add your own types by simply adding new entries into the map. The first one is the default format. #### Content Negotiation using the format parameter Let's say a controller action can return a resource in a variety of formats: HTML, XML, and JSON. What format will the client get? The easiest and most reliable way for the client to control this is through a `format` URL parameter. So if you, as a browser or some other client, want a resource as XML, you can use a URL like this:
    http://my.domain.org/books?format=xml
The result of this on the server side is a `format` property on the `response` object with the value `xml` . You could code your controller action to return XML based on this property, but you can also make use of the controller-specific `withFormat()` method:

    import grails.converters.JSON
    import grails.converters.XMLclass BookController {    def list() {
            def books = Book.list()        withFormat {
                html bookList: books
                json { render books as JSON }
                xml { render books as XML }
                * { render books as JSON }
            }
        }
    }
In this example, Grails will only execute the block inside `withFormat()` that matches the requested content type. So if the preferred format is `html` then Grails will execute the `html()` call only. Each 'block' can either be a map model for the corresponding view (as we are doing for 'html' in the above example) or a closure. The closure can contain any standard action code, for example it can return a model or render content directly. When no format matches explicitly, a `` (wildcard) block can be used to handle all other formats. There is a special format, "all", that is handled differently from the explicit formats. If "all" is specified (normally this happens through the Accept header - see below), then the first block of `withFormat()` is executed when there isn't a `` (wildcard) block available. You should not add an explicit "all" block. In this example, a format of "all" will trigger the `html` handler (`html` is the first block and there is no `*` block).
    withFormat {
                html bookList: books
                json { render books as JSON }
                xml { render books as XML }
            }
When using withFormat make sure it is the last call in your controller action as the return value of the `withFormat` method is used by the action to dictate what happens next. #### Using the Accept header Every incoming HTTP request has a special Accept header that defines what media types (or mime types) a client can "accept". In older browsers this is typically:
    */*
which simply means anything. However, newer browsers send more interesting values such as this one sent by Firefox:
    text/xml, application/xml, application/xhtml+xml, text/html;q=0.9,
        text/plain;q=0.8, image/png, */*;q=0.5
This particular accept header is unhelpful because it indicates that XML is the preferred response format whereas the user is really expecting HTML. That's why Grails ignores the accept header by default for browsers. However, non-browser clients are typically more specific in their requirements and can send accept headers such as
    application/json
As mentioned the default configuration in Grails is to ignore the accept header for browsers. This is done by the configuration setting `grails.mime.disable.accept.header.userAgents`, which is configured to detect the major rendering engines and ignore their ACCEPT headers. This allows Grails' content negotiation to continue to work for non-browser clients:
    grails.mime.disable.accept.header.userAgents = ['Gecko', 'WebKit', 'Presto', 'Trident']
For example, if it sees the accept header above ('application/json') it will set `format` to `json` as you'd expect. And of course this works with the `withFormat()` method in just the same way as when the `format` URL parameter is set (although the URL parameter takes precedence). An accept header of '*/*' results in a value of `all` for the `format` property. #### 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
This works as a result of the default URL Mapping definition which is:
    "/$controller/$action?/$id?(.$format)?"{
Note the inclusion of the `format` variable in the path. If you do not wish to use content negotiation via the file extension then simply remove this part of the URL mapping:
    "/$controller/$action?/$id?"{
#### Testing Content Negotiation To test content negotiation in a unit or integration test (see the section on Testing) you can either manipulate the incoming request headers:

    void testJavascriptOutput() {
        def controller = new TestController()
        controller.request.addHeader "Accept",
                  "text/javascript, text/html, application/xml, text/xml, */*"    controller.testAction()
        assertEquals "alert('hello')", controller.response.contentAsString
    }
Or you can set the format parameter to achieve a similar effect:

    void testJavascriptOutput() {
        def controller = new TestController()
        controller.params.format = 'js'    controller.testAction()
        assertEquals "alert('hello')", controller.response.contentAsString
    }
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