Example ex06_2: Creating a Content Handler

For this example, we’re going to create something entirely new. The Chapter 6 example of a content handler is a reimplementation of JAXB support. It is suitable for that chapter because it illustrates both the writing of a MessageBodyReader and a MessageBodyWriter and demonstrates how the ContextResolver is used. For ex06_2, though, we’re going to keep things simple.

In ex06_2, we’re going to rewrite ex06_1 to exchange Java objects between the client and server instead of XML. Java objects, you ask? Isn’t this REST? Well, there’s no reason a Java object can’t be a valid representation of a resource! If you’re exchanging Java objects, you can still realize a lot of the advantages of REST and HTTP. You still can do content negotiation (described in Chapter 9) and HTTP caching (described in Chapter 11).

The Content Handler Code

For our Java object content handler, we’re going to write one class that is both a MessageBodyReader and a MessageBodyWriter:

@Provider
@Produces("application/example-java")
@Consumes("application/x-java-serialized-object")
public class JavaMarshaller
                     implements MessageBodyReader, MessageBodyWriter
{

The JavaMarshaller class is annotated with @Provider, @Produces, and @Consumes, as required by the specification. The media type used by the example to represent a Java object is application/example-java:²²

   public boolean isReadable(Class type, Type genericType,
   Annotation[] annotations, MediaType mediaType)
   {
      return Serializable.class.isAssignableFrom(type);
   }

   public boolean isWriteable(Class type, Type genericType,
                      Annotation[] annotations, MediaType mediaType)
   {
      return Serializable.class.isAssignableFrom(type);
   }

For the isReadable() and isWriteable() methods, we just check to see if our Java type implements the java.io.Serializable interface:

   public Object readFrom(Class type, Type genericType,
                       Annotation[] annotations, MediaType mediaType,
                          MultivaluedMap httpHeaders,
                            InputStream is)
                     throws IOException, WebApplicationException
   {
      ObjectInputStream ois = new ObjectInputStream(is);
      try
      {
         return ois.readObject();
      }
      catch (ClassNotFoundException e)
      {
         throw new RuntimeException(e);
      }
   }

The readFrom() method uses basic Java serialization to read a Java object from the HTTP input stream:

   public long getSize(Object o, Class type,
                     Type genericType, Annotation[] annotations,
                      MediaType mediaType)
   {
      return −1;
   }

The getSize() method returns –1. It is impossible to figure out the exact length of our marshalled Java object without serializing it into a byte buffer and counting the number of bytes. We’re better off letting the servlet container figure this out for us. The getSize() method has actually been deprecated in JAX-RS 2.0.

   public void writeTo(Object o, Class type,
                        Type genericType, Annotation[] annotations,
                         MediaType mediaType,
                          MultivaluedMap httpHeaders, OutputStream os)
                          throws IOException, WebApplicationException
   {
      ObjectOutputStream oos = new ObjectOutputStream(os);
      oos.writeObject(o);
   }

Like the readFrom() method, basic Java serialization is used to marshal our Java object into the HTTP response body.

The Resource Class

The CustomerResource class doesn’t change much from ex06_2:

@Path("/customers")
public class CustomerResource
{
...

@POST
   @Consumes("application/example-java
")
   public Response createCustomer(Customer customer)
   {
      customer.setId(idCounter.incrementAndGet());
      customerDB.put(customer.getId(), customer);
      System.out.println("Created customer " + customer.getId());
      return Response.created(URI.create("/customers/"
                                + customer.getId())).build();

   }
...
}

The code is actually exactly the same as that used in ex06_1, except that the @Produces and @Consumes annotations use the application/example-java media type.

The Application Class

The ShoppingApplication class needs to change a tiny bit from the previous examples:

public class ShoppingApplication extends Application {
   private Set<Object> singletons = new HashSet<Object>();
   private Set<Class<?>> classes = new HashSet<Class<?>>();

   public ShoppingApplication() {
      singletons.add(new CustomerResource());
      classes.add(JavaMarshaller.class);
   }

   @Override
   public Set<Class<?>> getClasses() {
      return classes;
   }

   @Override
   public Set<Object> getSingletons() {
      return singletons;
   }
}

For our Application class, we need to register the JavaMarshaller class. If we don’t, the JAX-RS runtime won’t know how to handle the application/example-java media type.

The Client Code

The client code isn’t much different from ex06_1. We just need to modify the javax.ws.rs.client.Entity construction to use the application/example-java media type. For example, here’s what customer creation looks like:

public class CustomerResourceTest
{
   @Test
   public void testCustomerResource() throws Exception
   {
...
      Response response = client.target
              ("http://localhost:8080/services/customers")
              .request().post(Entity.entity
                             (newCustomer, "application/example-java"));

The static Entity.entity() method is called, passing in the plain Customer Java object along with our custom media type.

Build and Run the Example Program

Perform the following steps:

1. Open a command prompt or shell terminal and change to the ex06_2 directory of the workbook example code.
2. Make sure your PATH is set up to include both the JDK and Maven, as described in Chapter 17.
3. Perform the build and run the example by typing maven install.

²² The media type application/x-java-serialized-object should actually be used, but as of the second revision of this book, RESTEasy now has this type built in.