JAX-RS Content Handlers - RESTful Java with JAX-RS 2.0 - page 98

textResolver() on each individual resolver in the list. If it returns a JAXBContext instance,
it returns that to the original caller; otherwise, it tries the next resolver in this list.

MessageBodyReader

Now that we have written a MessageBodyWriter to convert a Java object into XML and out-
put it as the HTTP response body, let's write an unmarshaller that knows how to convert
HTTP XML request bodies back into a Java object. To do this, we need to use the
javax.ws.rs.ext.MessageBodyReader interface:

public public interface interface MessageBodyReader MessageBodyReader < T > {

boolean boolean isReadable ( Class <?> type , Type genericType ,

Annotation annotations [], MediaType mediaType );

T readFrom ( Class < T > type , Type genericType ,
Annotation annotations [], MediaType mediaType ,
MultivaluedMap < String , String > httpHeaders ,
InputStream entityStream )
throws

throws IOException , WebApplicationException ;

}

The MessageBodyReader interface has only two methods. The isReadable() method is
called by the JAX-RS runtime when it is trying to find a MessageBodyReader to unmarshal
the message body of an HTTP request. The readFrom() method is responsible for creating a
Java object from the HTTP request body.

Implementing a MessageBodyReader is very similar to writing a MessageBodyWriter . Let's
look at how we would implement one:

@Provider
@Consumes ( "application/xml" )
public

public class class JAXBUnmarshaller JAXBUnmarshaller implements implements MessageBodyReader {

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

Annotation annotations [], MediaType mediaType ) {

return return type . isAnnotationPresent ( XmlRootElement . class );

}

Our JAXBUnmarshaller class is annotated with @Provider and @Consumes . The latter an-
notation tells the JAX-RS runtime which media types it can handle. The matching rules for
finding a MessageBodyReader are the same as the rules for matching MessageBodyWriter .