Spring 3.0 has support for REST style WebServices, the Spring MVC controllers facilitate the functionality. In this example I will show an example of how to implement a basic REST service that uses XML marshalling to sent information over HTTP. Disclamier: this is not an in depth tutorial for building REST style WebServices.

The Spring MVC controller

The Spring 3.0 REST support relies havily on Spring MVC. We should use the Controller class for implementing a REST style WeService. To declare a Controller I use the Spring annotation based configuration. In this example the ProductRestService class is annotated with @Controller annotation. In order for Spring to pick-up the annotation Spring needs to be configured to scan for annotation (see the Spring configuration section).

REST uses templates that describe the URI to be used to invoke a WebService method. These URI templates can contain variable placeholders which allow for passing information to the WebService. The URI should typically contain all the information required for invoking a WebService method.

The @RequestMapping annotation allowes you to define the URI and HTTP method that are mapped to a method. In this example I have annotated the ProductRestService.getProductById(Long productId) with the @RequestMapping.
The value of the @RequestMapping, in this case: /products/{productId} , defines the URI that is mapped to this method. The productId variable needs to be defined when invoking the method and will be resolved automatically by Spring MVC with the value from the request URI. You can use the @PathVariable to inject the value of the productId variable directly into a method parameter.

The @ResponseBody annotation tells Spring to marshall the return value of the method to the HTTP response body. Spring allowes you to configure HTTP message converters that take care of conversion of the return value to a format which is accepted by the client. In this example the return value will be marshalled to XML using XStream.

package com.oudmaijer.spring.rest;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;

/**
 *  This is an example REST style MVC controller. It serves as an
 *  endpoint for retrieving Product Objects.
 */
@Controller
public class ProductRestService {

    /**
     * This method returns a specific Product. The URI to request a Product is
     * specified in the @RequestMapping.
     *
     * @param productId the identifier of the requested product
     * @return a Product
     */
    @RequestMapping(value="/products/{productId}", method = RequestMethod.GET)
    @ResponseBody
    public Product getProductById(@PathVariable Long productId) {
        Product p = new Product();
        p.setId(productId);
        return p;
    }

}

Spring configuration

The configuration is where all the magic happens. It is important to define the <mvc:annotation-driven /> element at the end of the configuration file or else Spring will not register the marshallingHttpMessageConverter. It took me some time to figure this out ;(

You need to add the MessageConverters to the configuration in order to get the OXM marshalling to work. Spring uses the requests Accept header to determine which converter to use.

Maven2 dependencies

You need to add a couple of Maven2 dependencies to get the project up and running. Below is the entire pom.xml.

Web deployment descriptor: web.xml

Last but not least the web.xml. Since the REST support in Spring is based on Spring MVC you need to define the DispatcherServlet. Make sure to map the correct URL pattern to the DispatcherServlet.

Invoking the service

This example only supports the HTTP GET method. If you want to test or build a client that uses REST WebServices you should use the RestTemplate in Spring. We can easily validate if the example WebService is running by accessing the service through Firefox. This will result in the following response.

For more information on REST support in Spring 3.0 please refer to the Spring reference documentation.

Dit bericht is gepost op 16 January 2010 om 1:50 pm uur en is geplaatst in Algemeen. Je kunt alle reacties op dit bericht volgen via de RSS 2.0 feed. Je kunt een reactie achter laten of trackback vanaf je eigen site.