9 Web Services

9.1 REST

Each HTTP method maps to an action type. For example GET for retrieving data, POST for creating data, PUT for updating and so on.

Grails includes flexible features that make it easy to create RESTful APIs. Creating a RESTful resource can be as simple as one line of code, as demonstrated in the next section.

9.1.1 Domain classes as REST resources

import grails.rest.*
@Resource(uri='/books')
class Book {
    String title
    static constraints = {
        title blank:false
    }
}

Simply by adding the Resource transformation and specifying a URI, your domain class will automatically be available as a REST resource in either XML or JSON formats. The transformation will automatically register the necessary RESTful URL mapping and create a controller called BookController.

You can try it out by adding some test data to BootStrap.groovy:

def init = { servletContext ->
        new Book(title:"The Stand").save()
        new Book(title:"The Shining").save()
    }

And then hitting the URL http://localhost:8080/myapp/books/1, which will render the response like:

<?xml version="1.0" encoding="UTF-8"?>
<book id="1">
    <title>The Stand</title>
</book>

If you change the URL to http://localhost:8080/myapp/books/1.json you will get a JSON response such as:

{"id":1,"title":"The Stand"}

If you wish to change the default to return JSON instead of XML, you can do this by setting the formats attribute of the Resource transformation:

import grails.rest.*
@Resource(uri='/books', formats=['json', 'xml'])
class Book {
    …
}

With the above example JSON will be prioritized. The list that is passed should contain the names of the formats that the resource should expose. The names of formats are defined in the grails.mime.types setting of Config.groovy:

grails.mime.types = [
    …
    json:          ['application/json', 'text/json'],
    …
    xml:           ['text/xml', 'application/xml']
]

See the section on Configuring Mime Types in the user guide for more information.

Instead of using the file extension in the URI, you can also obtain a JSON response using the ACCEPT header. Here's an example using the Unix curl tool:

$ curl -i -H "Accept: application/json" localhost:8080/myapp/books/1
{"id":1,"title":"The Stand"}

This works thanks to Grails' Content Negotiation features.

You can create a new resource by issuing a POST request:

$ curl -i -X POST -H "Content-Type: application/json" -d '{"title":"Along Came A Spider"}' localhost:8080/myapp/books
HTTP/1.1 201 Created
Server: Apache-Coyote/1.1
...

Updating can be done with a PUT request:

$ curl -i -X PUT -H "Content-Type: application/json" -d '{"title":"Along Came A Spider"}' localhost:8080/myapp/books/1
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
...

Finally a resource can be deleted with DELETE request:

$ curl -i -X DELETE localhost:8080/myapp/books/1
HTTP/1.1 204 No Content
Server: Apache-Coyote/1.1
...

As you can see, the Resource transformation enables all of the HTTP method verbs on the resource. You can enable only read-only capabilities by setting the readOnly attribute to true:

import grails.rest.*
@Resource(uri='/books', readOnly=true)
class Book {
    …
}

In this case POST, PUT and DELETE requests will be forbidden.

9.1.2 Mapping to REST resources

"/books"(resources:"book")

Extending your API to include more end points then becomes trivial:

"/books"(resources:"book") {
    "/publisher"(controller:"publisher", method:"GET")
}

The above example will expose the URI /books/1/publisher.

A more detailed explanation on creating RESTful URL mappings can be found in the URL Mappings section of the user guide.

9.1.3 Linking to REST resources

<g:link resource="${book}">My Link</g:link>

However, currently you cannot use g:link to link to the DELETE action and most browsers do not support sending the DELETE method directly.

The best way to accomplish this is to use a form submit:

<form action="/book/2" method="post">
 	<input type="hidden" name="_method" value="DELETE"/>
 </form>

Grails supports overriding the request method via the hidden _method parameter. This is for browser compatibility purposes. This is useful when using restful resource mappings to create powerful web interfaces. To make a link fire this type of event, perhaps capture all click events for links with a `data-method` attribute and issue a form submit via javascript.

9.1.4 Versioning REST resources

Versioning using the URI

A common approach is to use the URI to version APIs (although this approach is discouraged in favour of Hypermedia). For example, you can define the following URL mappings:

"/books/v1"(resources:"book", namespace:'v1')
"/books/v2"(resources:"book", namespace:'v2')

That will match the following controllers:

package myapp.v1
class BookController {
    static namespace = 'v1'
}
package myapp.v2
class BookController {
    static namespace = 'v2'
}

This approach has the disadvantage of requiring two different URI namespaces for your API.

Versioning with the Accept-Version header

As an alternative Grails supports the passing of an Accept-Version header from clients. For example you can define the following URL mappings:

"/books"(version:'1.0', resources:"book", namespace:'v1')
"/books"(version:'2.0', resources:"book", namespace:'v2')

Then in the client simply pass which version you need using the Accept-Version header:

$ curl -i -H "Accept-Version: 1.0" -X GET http://localhost:8080/myapp/books

Versioning using Hypermedia / Mime Types

Another approach to versioning is to use Mime Type definitions to declare the version of your custom media types (see the section on "Hypermedia as the Engine of Application State" for more information about Hypermedia concepts). For example, in Config.groovy you can declare a custom Mime Type for your resource that includes a version parameter (the 'v' parameter):

grails.mime.types = [
    all: '*/*',
    book: "application/vnd.books.org.book+json;v=1.0",
    bookv2: "application/vnd.books.org.book+json;v=2.0",
    …
}

It is critical that place your new mime types after the 'all' Mime Type because if the Content Type of the request cannot be established then the first entry in the map is used for the response. If you have your new Mime Type at the top then Grails will always try and send back your new Mime Type if the requested Mime Type cannot be established.

Then override the renderer (see the section on "Customizing Response Rendering" for more information on custom renderers) to send back the custom Mime Type in grails-app/conf/spring/resourses.groovy:

import grails.rest.render.json.*
import org.codehaus.groovy.grails.web.mime.*
beans = {
    bookRendererV1(JsonRenderer, myapp.v1.Book, new MimeType("application/vnd.books.org.book+json", [v:"1.0"])) 
    bookRendererV2(JsonRenderer, myapp.v2.Book, new MimeType("application/vnd.books.org.book+json", [v:"2.0"])) 
}

Then using the Accept header you can specify which version you need using the Mime Type:

$ curl -i -H "Accept: application/vnd.books.org.book+json;v=1.0" -X GET http://localhost:8080/myapp/books

9.1.5 Implementing REST controllers

9.1.5.1 Extending the RestfulController super class

class BookController extends RestfulController {
    static responseFormats = ['json', 'xml']
    BookController() {
        super(Book)
    }
}

To customize any logic you can just override the appropriate action. The following table provides the names of the action names and the URIs they map to:

Note that the create and edit actions are only needed if the controller exposes an HTML interface.

As an example, if you have a nested resource then you would typically want to query both the parent and the child identifiers. For example, given the following URL mapping:

"/authors"(resources:'author') {
    "/books"(resources:'book')
}

You could implement the nested controller as follows:

class BookController extends RestfulController {
    static responseFormats = ['json', 'xml']
    BookController() {
        super(Book)
    }
    @Override
    protected Book queryForResource(Serializable id) {
        Book.where {
            id == id && author.id = params.authorId
        }.find()
    }
}

The example above subclasses RestfulController and overrides the protected queryForResource method to customize the query for the resource to take into account the parent resource.

Customizing Data Binding In A RestfulController Subclass

The RestfulController class contains code which does data binding for actions like save and update. The class defines a getObjectToBind() method which returns a value which will be used as the source for data binding. For example, the update action does something like this...

class RestfulController<T> {
    def update() {
        T instance = // retrieve instance from the database...
        instance.properties = getObjectToBind()
        // …
    }
    // …
}

By default the getObjectToBind() method returns the request object. When the request object is used as the binding source, if the request has a body then the body will be parsed and its contents will be used to do the data binding, otherwise the request parameters will be used to do the data binding. Subclasses of RestfulController may override the getObjectToBind() method and return anything that is a valid binding source, including a Map or a DataBindingSource. For most use cases binding the request is appropriate but the getObjectToBind() method allows for changing that behavior where desired.

Using custom subclass of RestfulController with Resource annotation

You can also customize the behaviour of the controller that backs the Resource annotation.

The class must provide a constructor that takes a domain class as it's argument. The second constructor is required for supporting Resource annotation with readOnly=true.

This is a template that can be used for subclassed RestfulController classes used in Resource annotations:

class SubclassRestfulController<T> extends RestfulController<T> {
    SubclassRestfulController(Class<T> domainClass) {
        this(domainClass, false)
    }
    SubclassRestfulController(Class<T> domainClass, boolean readOnly) {
        super(domainClass, readOnly)
    }
}

You can specify the super class of the controller that backs the Resource annotation with the superClass attribute.

import grails.rest.*
@Resource(uri='/books', superClass=SubclassRestfulController)
class Book {
    String title
    static constraints = {
        title blank:false
    }
}

9.1.5.2 Implementing REST Controllers Step by Step

$ grails create-controller book

Then add some useful imports and enable readOnly by default:

import grails.transaction.*
import static org.springframework.http.HttpStatus.*
import static org.springframework.http.HttpMethod.*
@Transactional(readOnly = true)
class BookController {
    …
}

Recall that each HTTP verb matches a particular Grails action according to the following conventions:

The 'create' and 'edit' actions are already required if you plan to implement an HTML interface for the REST resource. They are there in order to render appropriate HTML forms to create and edit a resource. If this is not a requirement they can be discarded.

The key to implementing REST actions is the respond method introduced in Grails 2.3. The respond method tries to produce the most appropriate response for the requested content type (JSON, XML, HTML etc.)

Implementing the 'index' action

For example, to implement the index action, simply call the respond method passing the list of objects to respond with:

def index(Integer max) {
    params.max = Math.min(max ?: 10, 100)
    respond Book.list(params), model:[bookCount: Book.count()]
}

Note that in the above example we also use the model argument of the respond method to supply the total count. This is only required if you plan to support pagination via some user interface.

The respond method will, using Content Negotiation, attempt to reply with the most appropriate response given the content type requested by the client (via the ACCEPT header or file extension).

If the content type is established to be HTML then a model will be produced such that the action above would be the equivalent of writing:

def index(Integer max) {
    params.max = Math.min(max ?: 10, 100)
    [bookList: Book.list(params), bookCount: Book.count()]
}

By providing an index.gsp file you can render an appropriate view for the given model. If the content type is something other than HTML then the respond method will attempt to lookup an appropriate grails.rest.render.Renderer instance that is capable of rendering the passed object. This is done by inspecting the grails.rest.render.RendererRegistry.

By default there are already renderers configured for JSON and XML, to find out how to register a custom renderer see the section on "Customizing Response Rendering".

Implementing the 'show' action

The show action, which is used to display and individual resource by id, can be implemented in one line of Groovy code (excluding the method signature):

def show(Book book) {
    respond book
}

By specifying the domain instance as a parameter to the action Grails will automatically attempt to lookup the domain instance using the id parameter of the request. If the domain instance doesn't exist, then null will be passed into the action. The respond method will return a 404 error if null is passed otherwise once again it will attempt to render an appropriate response. If the format is HTML then an appropriate model will produced. The following action is functionally equivalent to the above action:

def show(Book book) {
    if(book == null) {
        render status:404
    }
    else {
        return [book: book]
    }
}

Implementing the 'save' action

The save action creates new resource representations. To start off, simply define an action that accepts a resource as the first argument and mark it as Transactional with the grails.transaction.Transactional transform:

@Transactional
def save(Book book) {
    …
}

Then the first thing to do is check whether the resource has any validation errors and if so respond with the errors:

if(book.hasErrors()) {
    respond book.errors, view:'create' 
}
else {
    …
}

In the case of HTML the 'create' view will be rendered again so the user can correct the invalid input. In the case of other formats (JSON, XML etc.), the errors object itself will be rendered in the appropriate format and a status code of 422 (UNPROCESSABLE_ENTITY) returned.

If there are no errors then the resource can be saved and an appropriate response sent:

book.save flush:true
    withFormat {
        html { 
            flash.message = message(code: 'default.created.message', args: [message(code: 'book.label', default: 'Book'), book.id])
            redirect book 
        }
        '*' { render status: CREATED }
    }

In the case of HTML a redirect is issued to the originating resource and for other formats a status code of 201 (CREATED) is returned.

Implementing the 'update' action

The update action updates an existing resource representations and is largely similar to the save action. First define the method signature:

@Transactional
def update(Book book) {
    …
}

If the resource exists then Grails will load the resource, otherwise null we passed. In the case of null, you should return a 404:

if(book == null) {
        render status: NOT_FOUND
    }
    else {
        …
    }

Then once again check for errors validation errors and if so respond with the errors:

if(book.hasErrors()) {
    respond book.errors, view:'edit' 
}
else {
    …
}

In the case of HTML the 'edit' view will be rendered again so the user can correct the invalid input. In the case of other formats (JSON, XML etc.) the errors object itself will be rendered in the appropriate format and a status code of 422 (UNPROCESSABLE_ENTITY) returned.

If there are no errors then the resource can be saved and an appropriate response sent:

book.save flush:true
withFormat {
    html { 
        flash.message = message(code: 'default.updated.message', args: [message(code: 'book.label', default: 'Book'), book.id])
        redirect book 
    }
    '*' { render status: OK }
}

In the case of HTML a redirect is issued to the originating resource and for other formats a status code of 200 (OK) is returned.

Implementing the 'delete' action

The delete action deletes an existing resource. The implementation is largely similar to the update action, expect the delete() method is called instead:

book.delete flush:true
withFormat {
    html { 
        flash.message = message(code: 'default.deleted.message', args: [message(code: 'Book.label', default: 'Book'), book.id])
        redirect action:"index", method:"GET" 
    }
    '*'{ render status: NO_CONTENT } 
}

Notice that for an HTML response a redirect is issued back to the index action, whilst for other content types a response code 204 (NO_CONTENT) is returned.

9.1.5.3 Generating a REST controller using scaffolding

$ grails generate-controller [Domain Class Name]

9.1.6 Customizing Response Rendering

9.1.6.1 Customizing the Default Renderers

You can easily customize response rendering using these default renderers. A common change you may want to make is to include or exclude certain properties from rendering.

Including or Excluding Properties from Rendering

As mentioned previously, Grails maintains a registry of grails.rest.render.Renderer instances. There are some default configured renderers and the ability to register or override renderers for a given domain class or even for a collection of domain classes. To include a particular property from rendering you need to register a custom renderer by defining a bean in grails-app/conf/spring/resources.groovy:

import grails.rest.render.xml.*
beans = {
    bookRenderer(XmlRenderer, Book) {
        includes = ['title']
    }
}

The bean name is not important (Grails will scan the application context for all registered renderer beans), but for organizational and readability purposes it is recommended you name it something meaningful.

To exclude a property, the excludes property of the XmlRenderer class can be used:

import grails.rest.render.xml.*
beans = {
    bookRenderer(XmlRenderer, Book) {
        excludes = ['isbn']
    }
}

Customizing the Converters

As mentioned previously, the default renders use the grails.converters package under the covers. In other words, under the covers they essentially do the following:

import grails.converters.*
…
render book as XML
// or render book as JSON

Why the separation between converters and renderers? Well a renderer has more flexibility to use whatever rendering technology you chose. When implementing a custom renderer you could use Jackson, Gson or any Java library to implement the renderer. Converters on the other hand are very much tied to Grails' own marshalling implementation.

9.1.6.2 Registering Custom Objects Marshallers

XML.registerObjectMarshaller Book, { Book book, XML xml ->
  xml.attribute 'id', book.id
  xml.build {
    title(book.title)
  }
}

You can customize the formatting of an individual value this way too. For example the JodaTime plugin does the following to support rendering of JodaTime dates in JSON output:

JSON.registerObjectMarshaller(DateTime) {
    return it?.toString("yyyy-MM-dd'T'HH:mm:ss'Z'")
}

In the case of JSON it's often simple to use a map to customize output:

JSON.registerObjectMarshaller(Book) {
  def map= [:]
  map['titl'] = it.title
  map['auth'] = it.author
  return map 
}

Registering Custom Marshallers via Spring

Note that if you have many custom marshallers it is recommended you split the registration of these into a separate class:

class CustomMarshallerRegistrar {
    @javax.annotation.PostConstruct
    void registerMarshallers() {
        JSON.registerObjectMarshaller(DateTime) {
            return it?.toString("yyyy-MM-dd'T'HH:mm:ss'Z'")
        }        
    }
}

Then define this class as Spring bean in grails-app/conf/spring/resources.groovy:

beans = {
    myCustomMarshallerRegistrar(CustomMarshallerRegistrar)
}

The PostConstruct annotation will get triggered on startup of your application.

9.1.6.3 Using Named Configurations for Object Marshallers

XML.createNamedConfig('publicApi') {
  it.registerObjectMarshaller(Book) { Book book, XML xml ->
    // do public API
  }
}
XML.createNamedConfig('adminApi') {
  it.registerObjectMarshaller(Book) { Book book, XML xml ->
    // do admin API
  }
}

Then when you use either the render or respond methods you can wrap the call in a named configuration if necessary to customize rendering per request:

XML.use( isAdmin ? 'adminApi' : 'publicApi') {
    render book as XML
}

or

XML.use( isAdmin ? 'adminApi' : 'publicApi') {
    respond book 
}

9.1.6.4 Implementing the ObjectMarshaller Interface

class Book {
    String title
}

By default the output when using:

render book as XML

Would look like:

<book id="1">
   <title>The Stand</title>
</book>

To write a custom marshaller you can do the following:

class BookMarshaller implements ObjectMarshaller<XML> {
  public boolean supports(Object object) {
    return object instanceof Book
  }
  public void marshalObject(Object object, XML converter) {
    Book book  = (Book)object 
    converter.chars book.title
  }
}

And then register the marshaller with:

XML.registerObjectMarshaller(new BookMarshaller())

With the custom ObjectMarshaller in place, the output is now:

<book>The Stand</book>

Customizing the Name of the Root Element

If you wish the customize the name of the surrounding element, you can implement NameAwareMarshaller:

class BookMarshaller implements ObjectMarshaller<XML>,NameAwareMarshaller {
  ...
  String getElementName(Object o) {
    return 'custom-book'
  }
}

With the above change the output would now be:

<custom-book>The Stand</custom-book>

Outputting Markup Using the Converters API or Builder

With the passed Converter object you can explicitly code to the Converters API to stream markup to the response:

public void marshalObject(Object object, XML converter) {
  Book book = (Book)object
  converter.attribute 'id', book.id.toString()
  converter.attribute 'date-released', book.dateReleased.toString()
  converter.startNode 'title'
  converter.chars book.title
  converter.end()
}

The above code results in:

<book id="1" date-released="...">
   <title>The Stand</title>
</book>

You can also use a builder notation to achieve a similar result (although the builder notation does not work for CompileStatic):

public void marshalObject(Object object, XML converter) {
  Book b = (Book)object
  converter.build {
     book(id: b.id) {
        title b.title
     }
  } 
}

Using the convertAnother Method to Recursively Convert Objects

To create more complex responses you can use the convertAnother method to convert associations and other objects:

public void marshalObject(Object object, XML converter) {
  Book book = (Book)object
  converter.startNode 'title'
  converter.chars book.title
  converter.end()
  if (book.authors) {
    converter.startNode 'authors'
    for(author in book.authors) {
      converter.convertAnother author
    }    
    converter.end()
  }
}

9.1.6.5 Implementing a Custom Renderer

package myapp
import grails.rest.render.*
import org.codehaus.groovy.grails.web.mime.MimeType
class BookXmlRenderer extends AbstractRenderer<Book> {
    BookXmlRenderer() {
        super(Book, [MimeType.XML,MimeType.TEXT_XML] as MimeType[])
    }
    void render(Book object, RenderContext context) {
        context.contentType = MimeType.XML.name
        def xml = new groovy.xml.MarkupBuilder(context.writer)
        xml.book(id: object.id, title:object.title)
    }
}

The AbstractRenderer super class has a constructor that takes the class that it renders and the MimeType(s) that are accepted (via the ACCEPT header or file extension) for the renderer.

To configure this renderer, simply add it is a bean to grails-app/conf/spring/resources.groovy:

beans = {
    bookRenderer(myapp.BookXmlRenderer)
}

The result will be that all Book instances will be rendered in the following format:

<book id="1" title="The Stand"/>

Note that if you change the rendering to a completely different format like the above, then you also need to change the binding if you plan to support POST and PUT requests. Grails will not automatically know how to bind data from a custom XML format to a domain class otherwise. See the section on "Customizing Binding of Resources" for further information.

Container Renderers

A grails.rest.render.ContainerRenderer is a renderer that renders responses for containers of objects (lists, maps, collections etc.). The interface is largely the same as the Renderer interface except for the addition of the getComponentType() method, which should return the "contained" type. For example:

class BookListRenderer implements ContainerRenderer<List, Book> {
    Class<List> getTargetType() { List }
    Class<Book> getComponentType() { Book }
    MimeType[] getMimeTypes() { [ MimeType.XML] as MimeType[] }
    void render(List object, RenderContext context) {
        ....
    }
}

9.1.6.6 Using GSP to Customize Rendering

def show(Book book) {
    respond book
}

You could supply a show.xml.gsp file to customize the rendering of the XML:

<%@page contentType="application/xml"%>
<book id="${book.id}" title="${book.title}"/>

9.1.7 Hypermedia as the Engine of Application State

Hypermedia (also called Mime or Media Types) are used to describe the state of a REST resource, and links tell clients how to transition to the next state. The format of the response is typically JSON or XML, although standard formats such as Atom and/or HAL are frequently used.

9.1.7.1 HAL Support

{
    "_links": {
        "self": { "href": "/orders" },
        "next": { "href": "/orders?page=2" },
        "find": {
            "href": "/orders{?id}",
            "templated": true
        },
        "admin": [{
            "href": "/admins/2",
            "title": "Fred"
        }, {
            "href": "/admins/5",
            "title": "Kate"
        }]
    },
    "currentlyProcessing": 14,
    "shippedToday": 20,
    "_embedded": {
        "order": [{
            "_links": {
                "self": { "href": "/orders/123" },
                "basket": { "href": "/baskets/98712" },
                "customer": { "href": "/customers/7809" }
            },
            "total": 30.00,
            "currency": "USD",
            "status": "shipped"
        }, {
            "_links": {
                "self": { "href": "/orders/124" },
                "basket": { "href": "/baskets/97213" },
                "customer": { "href": "/customers/12369" }
            },
            "total": 20.00,
            "currency": "USD",
            "status": "processing"
        }]
    }
}

Exposing Resources Using HAL

To return HAL instead of regular JSON for a resource you can simply override the renderer in grails-app/conf/spring/resources.groovy with an instance of grails.rest.render.hal.HalJsonRenderer (or HalXmlRenderer for the XML variation):

import grails.rest.render.hal.*
beans = {
    halBookRenderer(HalJsonRenderer, rest.test.Book)
}

With the bean in place requesting the HAL content type will return HAL:

$ curl -i -H "Accept: application/hal+json" http://localhost:8080/myapp/books/1
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/hal+json;charset=ISO-8859-1
{
  "_links": {
    "self": {
      "href": "http://localhost:8080/myapp/books/1",
      "hreflang": "en",
      "type": "application/hal+json"
    }
  },
  "title": ""The Stand""
}

To use HAL XML format simply change the renderer:

import grails.rest.render.hal.*
beans = {
    halBookRenderer(HalXmlRenderer, rest.test.Book)
}

Rendering Collections Using HAL

To return HAL instead of regular JSON for a list of resources you can simply override the renderer in grails-app/conf/spring/resources.groovy with an instance of grails.rest.render.hal.HalJsonCollectionRenderer:

import grails.rest.render.hal.*
beans = {
    halBookCollectionRenderer(HalJsonCollectionRenderer, rest.test.Book)
}

With the bean in place requesting the HAL content type will return HAL:

$ curl -i -H "Accept: application/hal+json" http://localhost:8080/myapp/books
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/hal+json;charset=UTF-8
Transfer-Encoding: chunked
Date: Thu, 17 Oct 2013 02:34:14 GMT
{
  "_links": {
    "self": {
      "href": "http://localhost:8080/myapp/books",
      "hreflang": "en",
      "type": "application/hal+json"
    }
  },
  "_embedded": {
    "book": [
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/myapp/books/1",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "The Stand"
      },
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/myapp/books/2",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "Infinite Jest"
      },
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/myapp/books/3",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "Walden"
      }
    ]
  }
}

Notice that the key associated with the list of Book objects in the rendered JSON is book which is derived from the type of objects in the collection, namely Book. In order to customize the value of this key assign a value to the collectionName property on the HalJsonCollectionRenderer bean as shown below:

import grails.rest.render.hal.*
beans = {
    halBookCollectionRenderer(HalCollectionJsonRenderer, rest.test.Book) {
        collectionName = 'publications'
    }
}

With that in place the rendered HAL will look like the following:

$ curl -i -H "Accept: application/hal+json" http://localhost:8080/myapp/books
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/hal+json;charset=UTF-8
Transfer-Encoding: chunked
Date: Thu, 17 Oct 2013 02:34:14 GMT
{
  "_links": {
    "self": {
      "href": "http://localhost:8080/myapp/books",
      "hreflang": "en",
      "type": "application/hal+json"
    }
  },
  "_embedded": {
    "publications": [
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/myapp/books/1",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "The Stand"
      },
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/myapp/books/2",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "Infinite Jest"
      },
      {
        "_links": {
          "self": {
            "href": "http://localhost:8080/myapp/books/3",
            "hreflang": "en",
            "type": "application/hal+json"
          }
        },
        "title": "Walden"
      }
    ]
  }
}

Using Custom Media / Mime Types

If you wish to use a custom Mime Type then you first need to declare the Mime Types in grails-app/conf/Config.groovy:

grails.mime.types = [
    all:      "*/*",
    book:     "application/vnd.books.org.book+json",
    bookList: "application/vnd.books.org.booklist+json",
    …
]

It is critical that place your new mime types after the 'all' Mime Type because if the Content Type of the request cannot be established then the first entry in the map is used for the response. If you have your new Mime Type at the top then Grails will always try and send back your new Mime Type if the requested Mime Type cannot be established.

Then override the renderer to return HAL using the custom Mime Types:

import grails.rest.render.hal.*
import org.codehaus.groovy.grails.web.mime.*
beans = {
    halBookRenderer(HalJsonRenderer, rest.test.Book, new MimeType("application/vnd.books.org.book+json", [v:"1.0"]))
    halBookListRenderer(HalJsonCollectionRenderer, rest.test.Book, new MimeType("application/vnd.books.org.booklist+json", [v:"1.0"]))
}

In the above example the first bean defines a HAL renderer for a single book instance that returns a Mime Type of application/vnd.books.org.book+json. The second bean defines the Mime Type used to render a collection of books (in this case application/vnd.books.org.booklist+json).

With this in place issuing a request for the new Mime Type returns the necessary HAL:

$ curl -i -H "Accept: application/vnd.books.org.book+json" http://localhost:8080/myapp/books/1
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/vnd.books.org.book+json;charset=ISO-8859-1

{
  "_links": {
    "self": {
      "href": "http://localhost:8080/myapp/books/1",
      "hreflang": "en",
      "type": "application/vnd.books.org.book+json"
    }
  },
  "title": ""The Stand""
}

Customizing Link Rendering

An important aspect of HATEOAS is the usage of links that describe the transitions the client can use to interact with the REST API. By default the HalJsonRenderer will automatically create links for you for associations and to the resource itself (using the "self" relationship).

However you can customize link rendering using the link method that is added to all domain classes annotated with grails.rest.Resource or any class annotated with grails.rest.Linkable. For example, the show action can be modified as follows to provide a new link in the resulting output:

def show(Book book) {
    book.link rel:'publisher', href: g.link(resource:"publisher", params:[bookId: book.id])
    respond book
}

Which will result in output such as:

{
  "_links": {
    "self": {
      "href": "http://localhost:8080/myapp/books/1",
      "hreflang": "en",
      "type": "application/vnd.books.org.book+json"
    }
    "publisher": {
        "href": "http://localhost:8080/myapp/books/1/publisher",
        "hreflang": "en"
    }
  },
  "title": ""The Stand""
}

The link method can be passed named arguments that match the properties of the grails.rest.Link class.

9.1.7.2 Atom Support

<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
 <title>Example Feed</title>
 <link href="http://example.org/"/>
 <updated>2003-12-13T18:30:02Z</updated>
 <author>
   <name>John Doe</name>
 </author>
 <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>
 <entry>
   <title>Atom-Powered Robots Run Amok</title>
   <link href="http://example.org/2003/12/13/atom03"/>
   <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
   <updated>2003-12-13T18:30:02Z</updated>
   <summary>Some text.</summary>
 </entry>
</feed>

To use Atom rendering again simply define a custom renderer:

import grails.rest.render.atom.*
beans = {
    halBookRenderer(AtomRenderer, rest.test.Book)
    halBookListRenderer(AtomCollectionRenderer, rest.test.Book)
}

9.1.7.3 Vnd.Error Support

By default when a validation error occurs when attempting to POST new resources then the errors object will be sent back allow with a 422 respond code:

$ curl -i -H "Accept: application/json"  -H "Content-Type: application/json" -X POST -d "" http://localhost:8080/myapp/books
HTTP/1.1 422 Unprocessable Entity
Server: Apache-Coyote/1.1
Content-Type: application/json;charset=ISO-8859-1
{"errors":[{"object":"rest.test.Book", "field":"title", "rejected-value":null, "message":"Property [title] of class [class rest.test.Book] cannot be null"}]}

If you wish to change the format to Vnd.Error then simply register grails.rest.render.errors.VndErrorJsonRenderer bean in grails-app/conf/spring/resources.groovy:

beans = {
    vndJsonErrorRenderer(grails.rest.render.errors.VndErrorJsonRenderer)
    // for Vnd.Error XML format
    vndXmlErrorRenderer(grails.rest.render.errors.VndErrorXmlRenderer)
}

Then if you alter the client request to accept Vnd.Error you get an appropriate response:

$ curl -i -H "Accept: application/vnd.error+json,application/json" -H "Content-Type: application/json" -X POST -d "" http://localhost:8080/myapp/books
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Content-Type: application/vnd.error+json;charset=ISO-8859-1
[
    {
        "logref": ""book.nullable"",
        "message": "Property [title] of class [class rest.test.Book] cannot be null",
        "_links": {
            "resource": {
                "href": "http://localhost:8080/rest-test/books"
            }
        }
    }
]

9.1.8 Customizing Binding of Resources

<?xml version="1.0" encoding="UTF-8"?>
<book>
    <title>The Stand</title>
    <authorName>Stephen King</authorName>
</book>

class BookController {
    def createBook() {
        def book = new Book()
        book.properties = request
        // …
    }
}

If the root element of the XML document contains an id attribute, the id value will be used to retrieve the corresponding persistent instance from the database and then the rest of the document will be bound to the instance. If no corresponding record is found in the database, the command object reference will be null.

<?xml version="1.0" encoding="UTF-8"?>
<book>
    <title>The Stand</title>
    <authorName>Stephen King</authorName>
</book>

Command objects will automatically be bound with the body of the request:

class BookController {
    def createBook(BookCommand book) {
        // …
    }
}
class BookCommand {
    String title
    String authorName
}

If the command object type is a domain class and the root element of the XML document contains an id attribute, the id value will be used to retrieve the corresponding persistent instance from the database and then the rest of the document will be bound to the instance. If no corresponding record is found in the database, the command object reference will be null.

<?xml version="1.0" encoding="UTF-8"?>
<book id="42">
    <title>Walden</title>
    <authorName>Henry David Thoreau</authorName>
</book>

class BookController {
    def updateBook(Book book) {
        // The book will have been retrieved from the database and updated
        // by doing something like this:
        //
        // book == Book.get('42')
        // if(book != null) {
        //    book.properties = request
        // }
        //
        // the code above represents what the framework will
        // have done. There is no need to write that code.
        // ...
    }
}

The data binding depends on an instance of the DataBindingSource interface created by an instance of the DataBindingSourceCreator interface. The specific implementation of DataBindingSourceCreator will be selected based on the contentType of the request. Several implementations are provided to handle common content types. The default implementations will be fine for most use cases. The following table lists the content types which are supported by the core framework and which DataBindingSourceCreator implementations are used for each. All of the implementation classes are in the org.codehaus.groovy.grails.web.binding.bindingsource package.

In order to provide your own DataBindingSourceCreator for any of those content types, write a class which implements DataBindingSourceCreator and register an instance of that class in the Spring application context. If you are replacing one of the existing helpers, use the corresponding bean name from above. If you are providing a helper for a content type other than those accounted for by the core framework, the bean name may be anything that you like but you should take care not to conflict with one of the bean names above.

The DataBindingSourceCreator interface defines just 2 methods:

package org.grails.databinding.bindingsource
import org.codehaus.groovy.grails.web.mime.MimeType
import org.grails.databinding.DataBindingSource
/**
 * A factory for DataBindingSource instances
 * 
 * @since 2.3
 * @see DataBindingSourceRegistry
 * @see DataBindingSource
 *
 */
interface DataBindingSourceCreator {
    /**
     * return All of the {link MimeType} supported by this helper
     */
    MimeType[] getMimeTypes()
    /**
     * Creates a DataBindingSource suitable for binding bindingSource to bindingTarget
     * 
     * @param mimeType a mime type
     * @param bindingTarget the target of the data binding
     * @param bindingSource the value being bound
     * @return a DataBindingSource
     */
    DataBindingSource createDataBindingSource(MimeType mimeType, Object bindingTarget, Object bindingSource)
}

AbstractRequestBodyDataBindingSourceCreator is an abstract class designed to be extended to simplify writing custom DataBindingSourceCreator classes. Classes which extend AbstractRequestbodyDatabindingSourceCreator need to implement a method named createBindingSource which accepts an InputStream as an argument and returns a DataBindingSource as well as implementing the getMimeTypes method described in the DataBindingSourceCreator interface above. The InputStream argument to createBindingSource provides access to the body of the request.

The code below shows a simple implementation.

// MyCustomDataBindingSourceCreator.groovy in
// src/groovy/com/demo/myapp/databinding
package com.demo.myapp.databinding
import org.codehaus.groovy.grails.web.mime.MimeType
import org.grails.databinding.DataBindingSource
import org...databinding.SimpleMapDataBindingSource
import org...databinding.bindingsource.AbstractRequestBodyDataBindingSourceCreator
/**
 * A custom DataBindingSourceCreator capable of parsing key value pairs out of
 * a request body containing a comma separated list of key:value pairs like:
 *
 * name:Herman,age:99,town:STL
 *
 */
class MyCustomDataBindingSourceCreator extends AbstractRequestBodyDataBindingSourceCreator {
    @Override
    public MimeType[] getMimeTypes() {
        [new MimeType('text/custom+demo+csv')] as MimeType[]
    }
    @Override
    protected DataBindingSource createBindingSource(InputStream inputStream) {
        def map = [:]
        def reader = new InputStreamReader(inputStream)
        // this is an obviously naive parser and is intended
        // for demonstration purposes only.
        reader.eachLine { line ->
            def keyValuePairs = line.split(',')
            keyValuePairs.each { keyValuePair ->
                if(keyValuePair?.trim()) {
                    def keyValuePieces = keyValuePair.split(':')
                    def key = keyValuePieces[0].trim()
                    def value = keyValuePieces[1].trim()
                    map[key] = value
                }
            }
        }
        // create and return a DataBindingSource which contains the parsed data
        new SimpleMapDataBindingSource(map)
    }
}

An instance of MyCustomDataSourceCreator needs to be registered in the spring application context.

// grails-app/conf/spring/resources.groovy
beans = {
    myCustomCreator com.demo.myapp.databinding.MyCustomDataBindingSourceCreator
    // …
}

With that in place the framework will use the myCustomCreator bean any time a DataBindingSourceCreator is needed to deal with a request which has a contentType of "text/custom+demo+csv".

9.2 SOAP

SOAP Clients

To call SOAP web services there are generally 2 approaches taken, one is to use a tool to generate client stubs, the other is to manually construct the SOAP calls. The former can be easier to use, but the latter provides more flexibility / control.

The CXF client plugin uses the CXF framework, which includes a wsdl2java tool for generating a client. There is nothing Groovy/Grails specific here in the generated code as it simply provides a Java API which you can invoke to call SOAP web services.

See the documentation on the CXF client plugin for further information.

Alternatively, if you prefer more control over your SOAP calls the WS-Lite library is an excellent choice and features a Grails plugin. You have more control over the SOAP requests sent, and since Groovy has fantastic support for building and parsing XML it can be very productive approach.

Below is an example of a SOAP call with wslite:

withSoap(serviceURL: 'http://www.holidaywebservice.com/Holidays/US/Dates/USHolidayDates.asmx') {
    def response = send {
        body {
            GetMothersDay(xmlns: 'http://www.27seconds.com/Holidays/US/Dates/') {
                year(2011)
            }
        }
    }
    println response.GetMothersDayResponse.GetMothersDayResult.text()
}

It is not recommended that you use the GroovyWS library, it pulls in many dependencies which increases the likelihood of conflicts. The WSlite library provides a far simpler and easier to use solution.

SOAP Servers

Again, Grails does not have direct support for exposing SOAP web services, however if you wish to expose a SOAP service from your application then the CXF plugin (not to be confused with the cxf-client plugin), provides an easy way to do so.

Typically it involves taking a Grails service and adding 'expose'-style configuration, such as the below:

static expose = EndpointType.JAX_WS_WSDL
  //your path (preferred) or url to wsdl
  static wsdl = 'org/grails/cxf/test/soap/CustomerService.wsdl'

Please refer to the documentation of the plugin for more information.

9.3 RSS and Atom

def feed() {
    render(feedType: "rss", feedVersion: "2.0") {
        title = "My test feed"
        link = "http://your.test.server/yourController/feed"
        for (article in Article.list()) {
            entry(article.title) {
                link = "http://your.test.server/article/${article.id}"
                article.content // return the content
            }
        }
    }
}