(Quick Reference)

13. Web Services - Reference Documentation

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

Version: 1.3.7

Table of Contents

13. Web Services

Web services are all about providing a web API onto your web application and are typically implemented in either SOAP or REST.

13.1 REST

REST is not really a technology in itself, but more an architectural pattern. REST is extremely simple and just involves using plain XML or JSON as a communication medium, combined with URL patterns that are "representational" of the underlying system and HTTP methods such as GET, PUT, POST and DELETE.

Each HTTP method maps to an action. For example GET for retrieving data, PUT for creating data, POST for updating and so on. In this sense REST fits quite well with CRUD.

URL patterns

The first step to implementing REST with Grails is to provide RESTful URL mappings:

static mappings = {
   "/product/$id?"(resource:"product")
}

What this does is map the URI /product onto a ProductController. Each HTTP method such as GET, PUT, POST and DELETE map to unique actions within the controller as outlined by the table below:

MethodAction
GETshow
PUTupdate
POSTsave
DELETEdelete

You can alter how HTTP methods by using the capability of URL Mappings to map to HTTP methods:

"/product/$id"(controller:"product"){
    action = [GET:"show", PUT:"update", DELETE:"delete", POST:"save"]
}

However, unlike the resource argument used previously, in this case Grails will not provide automatic XML or JSON marshaling for you unless you specify the parseRequest argument in the URL mapping:

"/product/$id"(controller:"product", parseRequest:true){
    action = [GET:"show", PUT:"update", DELETE:"delete", POST:"save"]
}

HTTP Methods

In the previous section you saw how you can easily define URL mappings that map specific HTTP methods onto specific controller actions. Writing a REST client that then sends a specific HTTP method is then trivial (example in Groovy's HTTPBuilder module):

import groovyx.net.http.*
import static groovyx.net.http.ContentType.JSON

def http = new HTTPBuilder("http://localhost:8080/amazon")

http.request(Method.GET, JSON) { url.path = '/book/list' response.success = {resp, json -> json.books.each { book -> println book.title } } }

However, issuing a request with a method other than GET or POST from a regular browser is not possible without some help from Grails. When defining a form you can specify an alternative method such as DELETE:

<g:form controller="book" method="DELETE">
	..	
</g:form>

Grails will send a hidden parameter called _method, which will be used as the request's HTTP method. Another alternative for changing the method for non-browser clients is to use the X-HTTP-Method-Override to specify the alternative method name.

XML Marshaling - Reading

The controller implementation itself can use Grails' XML marshaling support to implement the GET method:

import grails.converters.*
class ProductController {
	def show = {
		if(params.id && Product.exists(params.id)) {
			def p = Product.findByName(params.id)
			render p as XML
		}
		else {
			def all = Product.list()
			render all as XML
		}
	}
	..
}

Here what we do is if there is an id we search for the Product by name and return it otherwise we return all Products. This way if we go to /products we get all products, otherwise if we go to /product/MacBook we only get a MacBook.

XML Marshalling - Updating

To support updates such as PUT and POST you can use the params object which Grails enhances with the ability to read an incoming XML packet. Given an incoming XML packet of:

<?xml version="1.0" encoding="ISO-8859-1"?>
<product>
	<name>MacBook</name>
	<vendor id="12">
		<name>Apple</name>
     </vender>
</product>

You can read this XML packet using the same techniques described in the Data Binding section via the params object:

def save = {
	def p = new Product(params['product'])

if(p.save()) { render p as XML } else { render p.errors } }

In this example by indexing into the params object using the key 'product' we can automatically create and bind the XML using the constructor of the Product class. An interesting aspect of the line:

def p = new Product(params['product'])
Is that it requires no code changes to deal with a form submission that submits form data than it does to deal with an XML request. The exact same technique can be used with a JSON request too.

If you require different responses to different clients (REST, HTML etc.) you can use content negotation

The Product object is then saved and rendered as XML, otherwise an error message is produced using Grails' validation capabilities in the form:

<error>
   <message>The property 'title' of class 'Person' must be specified</message>
</error>

REST with JAX-RS

There also is a JAX-RS Plugin which can be used to build web services based on the Java API for RESTful Web Services (JSR 311: JAX-RS).

13.2 SOAP

There are several plugins that add SOAP support to Grails depending on your preferred approach. For Contract First SOAP services there is a Spring WS plugin, whilst if you want to generate a SOAP API from Grails services there are several plugins that do this including:

Most of the SOAP integrations integrate with Grails services via the exposes static property. The below example is taken from the CXF plugin:

class BookService {

static expose=['cxf']

Book[] getBooks(){ Book.list() as Book[] } }

The WSDL can then be accessed at the location: http://127.0.0.1:8080/your_grails_app/services/book?wsdl

For more information on the CXF plug-in refer the documentation on the wiki.

13.3 RSS and Atom

No direct support is provided for RSS or Atom within Grails. You could construct RSS or ATOM feeds with the render method's XML capability. There is however a Feeds plug-in available for Grails that provides a RSS and Atom builder using the popular ROME library. An example of its usage can be seen below:

def feed = {
    render(feedType:"rss", feedVersion:"2.0") {
        title = "My test feed"
        link = "http://your.test.server/yourController/feed"

Article.list().each() { entry(it.title) { link = "http://your.test.server/article/${it.id}" it.content // return the content } } } }