Overview

The REST Pluginprovides high level support for the implementation of RESTful resource based web applicationsThe REST plugin can cooperate with the Convention Plugin to support a zero configuration approach to declaring your actions and results, but you can always use the REST plugin with XML style configuration if you like.

If you prefer to see a working code example, instead of reading through an explanation, you can download the struts2 sample apps and check out the struts2-rest-showcase application, a complete WAR file, that demonstrates a simple REST web program.

Features

• Ruby on Rails REST-style URLs
• Zero XML config when used with Convention Plugin
• Built-in serialization and deserialization support for XML and JSON
• Automatic error handling
• Type-safe configuration of the HTTP response
• Automatic conditional GET support

Mapping REST URLs to Struts 2 Actions

The main functionality of the REST plugin lies in the interpretation of incoming request URL's according the RESTful rules. In the Struts 2 framework, this 'mapping' of request URL's to Actions is handled by in implementation of the ActionMapper interface. Out of the box, Struts 2 uses the DefaultActionMapper to map URL's to Actions via the logic you are probably already familiar with.

The REST plugin provides an alternative implementation, RestActionMapper, that provides the RESTful logic that maps a URL to a give action class ( aka 'controller' in RESTful terms ) and, more specifically, to the invocation of a method on that controller class. The following section, which comes from the Javadoc for the class, details this logic.

RESTful URL Mapping Logic

This Restful action mapper enforces Ruby-On-Rails REST-style mappings. If the method is not specified (via '!' or 'method:' prefix), the method is "guessed" at using REST-style conventions that examine the URL and the HTTP method. Special care has been given to ensure this mapper works correctly with the codebehind plugin so that XML configuration is unnecessary.

This mapper supports the following parameters:

• struts.mapper.idParameterName - If set, this value will be the name of the parameter under which the id is stored. The id will then be removed from the action name. Whether or not the method is specified, the mapper will  try to truncate the identifier from the url and store it as a parameter.
• struts.mapper.indexMethodName - The method name to call for a GET request with no id parameter. Defaults to index.
• struts.mapper.getMethodName - The method name to call for a GET request with an id parameter. Defaults to show.
• struts.mapper.postMethodName - The method name to call for a POST request with no id parameter. Defaults to create.
• struts.mapper.putMethodName - The method name to call for a PUT request with an id parameter. Defaults to update.
• struts.mapper.deleteMethodName - The method name to call for a DELETE request with an id parameter. Defaults to destroy.
• struts.mapper.editMethodName - The method name to call for a GET request with an id parameter and the edit view specified. Defaults to edit.
• struts.mapper.newMethodName - The method name to call for a GET request with no id parameter and the new view specified. Defaults to editNew.

The following URL's will invoke its methods:

• GET: /movies => method=index
• GET: /movies/Thrillers => method=show, id=Thrillers
• GET: /movies/Thrillers;edit => method=edit, id=Thrillers
• GET: /movies/Thrillers/edit => method=edit, id=Thrillers
• GET: /movies/new => method=editNew
• POST: /movies => method=create
• PUT: /movies/Thrillers => method=update, id=Thrillers
• DELETE: /movies/Thrillers => method=destroy, id=Thrillers

Or, expressed as a table:

Content Types

In addition to providing mapping of RESTful URL's to Controller ( Action ) invocations, the REST plugin also provides the ability to produce multiple representations of the resource data. By default, the plugin can return the resource in the following content types:

• HTML
• XML 
• JSON

There is nothing configure here, just add the conent type extension to your RESTful URL. The framework will take care of the rest. So, for instance, assuming a Controller called Movies and a movie with the id of superman, the following URL's will all hit the

http://my.company.com/myapp/movies/superman
http://my.company.com/myapp/movies/superman.xml
http://my.company.com/myapp/movies/superman.xhtml
http://my.company.com/myapp/movies/superman.json

Usage

This section will walk you through a quick demo. Here are the steps in the sequence that we will follow.

• Setting Up your Project
• Configuring your Project
• Writing your Controllers

Setting Up

Assuming you have a normal Struts 2 application, all you need to do for this REST demo is to add the following two plugins:

• Struts 2 Rest Plugin
• Struts 2 Convention Plugin

Note, you can download the jars for these plugins from Maven Central

Configuration ( struts.xml )

Just dropping the plugin's into your application may not produce exactly the desired effect. There are a couple of considerations. The first consideration is whether you want to have any non-RESTful URL's coexisting with your RESTful URL's. We'll show two configurations. The first assumes all you want to do is REST. The second assumes you want to keep other non-RESTful URL's alive in the same Struts 2 application.

REST Only Configuration

Instruct Struts to use the REST action mapper:

<constant name="struts.mapper.class" value="rest" />

At this point, the REST mapper has replaced the DefaultActionMapper so all incoming URL's will be interpreted as RESTful URL's.

We're relying on the Convention plugin to find our controllers, so we need to configure the convention plugin a bit:

<constant name="struts.convention.action.suffix" value="Controller"/>
<constant name="struts.convention.action.mapAllMatches" value="true"/>
<constant name="struts.convention.default.parent.package" value="rest-default"/>
<constant name="struts.convention.package.locators" value="example"/>

REST and non-RESTful URL's Together Configuration

If you want to keep using some non-RESTful URL's alongside your REST stuff, then you'll have to provide for a configuration that utilizes to mappers.

First, you'll need to re-assert the extensions that struts knows about because the rest plugin will have thrown out the default action extension.

  <constant name="struts.action.extension" value="xhtml,,xml,json,action"/>

Next, we will configure the PrefixBasedActionMapper, which is part of the core Struts 2 distribution, to have some URL's routed to the Rest mapper and others to the default mapper.

  <constant name="struts.mapper.class" value="org.apache.struts2.dispatcher.mapper.PrefixBasedActionMapper" />
  <constant name="struts.mapper.prefixMapping" value="/rest:rest,:struts"/>

And, again, we're relying on the Convention plugin to find our controllers, so we need to configure the convention plugin a bit:

<constant name="struts.convention.action.suffix" value="Controller"/>
<constant name="struts.convention.action.mapAllMatches" value="true"/>
<constant name="struts.convention.default.parent.package" value="rest-default"/>
<constant name="struts.convention.package.locators" value="example"/>

Write Your Controller Actions

Once everything is configured, you need to create the controllers. Controllers are simply actions created with the purpose of handling requests for a give RESTful resource. As we saw in the mapping logic above, various REST URL's will hit different methods on the controller. Traditionally, normal Struts 2 actions expose the execute method as their target method. Here's a sample controller for a orders resource. Note, this sample doesn't implement all of the methods that can be hit via the RESTful action mapper's interpretation of URL's.

package org.apache.struts2.rest.example;

public class OrdersController implements ModelDriven<Order> {

    private OrderManager orderManager;
    private String id;
    private Order model;

    // Handles /orders/{id} GET requests
    public HttpHeaders show() {
        model = orderManager.findOrder(id);
        return new DefaultHttpHeaders("show")
            .withETag(model.getUniqueStamp())
            .lastModified(model.getLastModified());
    }

    // Handles /orders/{id} PUT requests
    public String update() {
        orderManager.updateOrder(model);
        return "update";
    }

    // getters and setters
}

In this example, the ModelDriven interface is used to ensure that only my model, the Order object in this case, is returned to the client, otherwise, the whole OrdersController object would be serialized.

You may wonder why the show() method returns a HttpHeaders object and the update() method returns the expected result code String. The REST Plugin adds support for action methods that return HttpHeaders objects as a way for the action to have more control over the response. In this example, we wanted to ensure the response included the ETag header and a last modified date so that the information will be cached properly by the client. The HttpHeaders object is a convenient way to control the response in a type-safe way.

Also, notice we aren't returning the usual "success" result code in either method. This allows us to use the special features of the Codebehind Plugin to intuitively select the result template to process when this resource is accessed with the .xhtml extension. In this case, we can provide a customized XHTML view of the resource by creating /orders-show.jsp and /orders-update.jsp for the respective methods.

Advanced Topics

The following sections describe some of the non-standard bells and whistles that you might need to utilize for your application's more non-standard requirements.

Custom ContentTypeHandlers

If you need to handle extensions that aren't supported by the default handlers, you can create your own ContentTypeHandler implementation and define it in your struts.xml:

<bean name="yaml" type="org.apache.struts2.rest.handler.ContentTypeHandler" class="com.mycompany.MyYamlContentHandler" />

If the built-in content type handlers don't do what you need, you can override the handling of any extension by providing an alternate handler. First, define your own ContentTypeHandler and declare with its own alias. For example:

<bean name="myXml" type="org.apache.struts2.rest.handler.ContentTypeHandler" class="com.mycompany.MyXmlContentHandler" />

Then, tell the REST Plugin to override the handler for the desired extension with yours. In struts.properties, it would look like this:

struts.rest.handlerOverride.xml=myXml

Use Jackson framework as JSON ContentTypeHandler

The default JSON Content Handler is build on top of the JSON-lib. If you prefer to use the Jackson framework for JSON serialisation, you can configure the JacksonLibHandler as Content Handler for your json requests. 

First you need to add the jackson dependency to your web application by downloading the jar file and put it under WEB-INF/lib or by adding following xml snippet to your dependencies section in the pom.xml when you are using maven as build system.

<dependency>
    <groupId>org.codehaus.jackson</groupId>
    <artifactId>jackson-jaxrs</artifactId>
    <version>1.9.13</version>
</dependency>

Now you can overwrite the Content Handler with the Jackson Content Handler in the struts.xml:

<bean type="org.apache.struts2.rest.handler.ContentTypeHandler" name="jackson" class="org.apache.struts2.rest.handler.JacksonLibHandler"/>
<constant name="struts.rest.handlerOverride.json" value="jackson"/>

<!-- Set to false if the json content can be returned for any kind of http method -->
<constant name="struts.rest.content.restrictToGET" value="false"/> 

<!-- Set encoding to UTF-8, default is ISO-8859-1 -->
<constant name="struts.i18n.encoding" value="UTF-8"/> 

Settings

The following settings can be customized. See the developer guide.
For more configuration options see the Convention Plugin Documentation

Resources

• http://www.b-simple.de/documents - Short RESTful Rails tutorial (PDF, multiple languages)
• RESTful Web Services - Highly recommend book from O'Reilly
• Go Light with Apache Struts 2 and REST - Presentation by Don Brown at ApacheCon US 2008

Version History

From Struts 2.1.1+