Spring Integration

Spring is an open source framework similar to EJB. Like EJB, it provides a great abstraction for transactions, persistence, and security. Further explanation of Spring is beyond the scope of this book. If you want more information on it, check out Spring: A Developer’s Notebook by Bruce A. Tate and Justin Gehtland (O’Reilly). Most JAX-RS implementations have their own proprietary support for Spring and allow you to write Spring beans that are JAX-RS web services. If portability is not an issue for you, I suggest that you use the integration with Spring provided by your JAX-RS implementation.

There is a simple, portable way to integrate with Spring that we can talk about in this chapter. What you can do is write an Application class that loads your Spring XML files and then registers your Spring beans with JAX-RS through the getSingletons() method. First, let’s define a Spring bean that represents a customer database. It will pretty much look like the CustomerResource bean described in EJB Integration:

@Path("/customers")
public interface CustomerResource {

   @GET
   @Produces("application/xml")
   public String getCustomers();

   @GET
   @Produces("application/xml")
   @Path("{id}")
   public String getCustomer(@PathParam("id") int id);
}

In this example, we first create an interface for our CustomerResource that is annotated with JAX-RS annotations:

public class CustomerResourceBean implements CustomerResource {

   public String getCustomers() {...}
   public String getCustomer(int id) {...}
}

Our Spring bean class, CustomerResourceBean, simply implements the CustomerResource interface. Although you can opt to not define an interface and use JAX-RS annotations directly on the bean class, I highly suggest that you use an interface. Interfaces work better in Spring when you use features like Spring transactions.

Now that we have a bean class, we should declare it within a Spring XML file called spring-beans.xml (or whatever you want to name the file):

<beans xmlns="http://www.springframework.org/schema/beans"
   <bean id="custService"
         class="com.shopping.restful.services.CustomerResourceBean"/>
</beans>

Place this spring-beans.xml file within your WAR’s WEB-INF/classes directory or within a JAR within the WEB-INF/lib directory. For this example, we’ll put it in the WEB-INF/classes directory. We will find this file through a class loader resource lookup later on when we write our Application class.

Next we write our web.xml file:

<web-app>
    <context-param>
        <param-name>spring-beans-file</param-name>
        <param-value>META-INF/applicationContext.xml</param-value>
    </context-param>
</web-app>

In our web.xml file, we define a <context-param> that contains the classpath location of our Spring XML file. We use a <context-param> so that we can change this value in the future if needed. We then need to wire everything together in our Application class:

@ApplicationPath("/")
public class ShoppingApplication extends Application
{
   protected ApplicationContext springContext;

   @Context
   protected ServletContext servletContext;

   public Set<Object> getSingletons()
   {
      try
      {
         InitialContext ctx = new InitialContext();
         String xmlFile = (String)servletContext.getInitParameter
                          ("spring-beans-file");
         springContext = new ClassPathXmlApplicationContext(xmlFile);
      }
      catch (Exception ex)
      {
         throw new RuntimeException(ex);
      }
      HashSet<Object> set = new HashSet();
      set.add(springContext.getBean("customer"));
      return set;
   }

}

In this Application class, we look up the classpath location of the Spring XML file that we defined in the <context-param> of our web.xml deployment descriptor. We then load this XML file through Spring’s ClassPathXmlApplicationContext. This will also create the beans defined in this file. From the Spring ApplicationContext, we look up the bean instance for our CustomerResource using the ApplicationContext.getBean() method. We then create a HashSet and add the CustomerResource bean to it and return it to be registered with the JAX-RS runtime.