JAX-RS Content Handlers - RESTful Java with JAX-RS 2.0 - page 93

of the output. Finally, the writeTo() method does all the heavy lifting and writes the content
out to the HTTP response buffer. Let's implement this interface to support JAXB:

@Provider
@Produces ( "application/xml" )
public public class class JAXBMarshaller JAXBMarshaller implements implements MessageBodyWriter {

public public boolean boolean isWriteable ( Class <?> type , Type genericType ,

Annotation annotations [], MediaType mediaType ) {

return return type . isAnnotationPresent ( XmlRootElement . class );

}

We start off the implementation of this class by annotating it with the
@javax.ws.rs.ext.Provider annotation. This tells JAX-RS that this is a deployable JAX-
RS component. We must also annotate it with @Produces to tell JAX-RS which media types
this MessageBodyWriter supports. Here, we're saying that our JAXBMarshaller class sup-
ports application/xml .

The isWriteable() method is a callback method that tells the JAX-RS runtime whether or
not the class can handle writing out this type. JAX-RS follows this algorithm to find an ap-
propriate MessageBodyWriter to write out a Java object into the HTTP response:

1. First, JAX-RS calculates a list of MessageBodyWriters by looking at each writer's
@Produces annotation to see if it supports the media type that the JAX-RS resource
method wants to output.
2. This list is sorted, with the best match for the desired media type coming first. In oth-
er words, if our JAX-RS resource method wants to output application/xml and we
have three MessageBodyWriters (one produces application/* , one supports any-
thing */* , and the last supports application/xml ), the one producing application/
xml will come first.
3. Once this list is calculated, the JAX-RS implementation iterates through the list in or-
der, calling the MessageBodyWriter.isWriteable() method. If the invocation re-
turns true , that MessageBodyWriter is used to output the data.

The isWriteable() method takes four parameters. The first one is a java.lang.Class that
is the type of the object that is being marshalled. We determine the type by calling the
getClass() method of the object. In our example, we use this parameter to find out if our
object's class is annotated with the @XmlRootElement annotation.

The second parameter is a java.lang.reflect.Type . This is generic type information
about the object being marshalled. We determine it by introspecting the return type of the
JAX-RS resource method. We don't use this parameter in our JAXBMar-