Chapter 15. JAX-RS Content Negotiation

The HTTP protocol has built-in content negotiation headers that allow the client and server to specify the type of content that they transfer, and the type of content they prefer to receive. The server declares content preferences via the @Produces and @Consumes headers.
@Consumes is an array of media types that a particular resource or resource method consumes. For example: 
         @Consumes("text/*")
         @Path("/library")
         public class Library {

         @POST
         public String stringBook(String book) {...}


         @Consumes("text/xml")
         @POST
         public String jaxbBook(Book book) {...}
When a client makes a request, JAX-RS first locates all methods that match the path. It then sorts objects based on the content-type header sent by the client. If a client sends the following: 
         POST /library
         content-type: text/plain

         this is a nice book
The stringBook() method is invoked, because it matches the default text/* media type. If the client sends XML, like so: 
         POST /library
         content-type: text/xml

         <book name="EJB 3.0" author="Bill Burke"/>
Then the jaxbBook() method is invoked.
@Produces is used to map a client request and match it with the client's Accept header. The Accept HTTP header is sent by the client, and defines the media types that the client prefers to receive from the server. 
         @Produces("text/*")
         @Path("/library")
         public class Library {

         @GET
         @Produces("application/json")
         public String getJSON() {...}


         @GET
         public String get() {...}
So, if the client sends: 
      
         GET /library
         Accept: application/json
The getJSON() method would be invoked.
@Consumes and @Produces can support multiple media types by presenting them in a list. The client's Accept header can also list multiple media types to receive. More specific media types are selected first. The Accept header (or @Produces or @Consumes) can also specify weighted preferences that will match requests with resource methods. (This is best explained in Section 14.1 of RFC 2616.) RESTEasy provides support for this more complex method of content negotiation.
An alternative method used by JAX-RS is a combination of media-type, content-language, and content encoding, in addition to etags, last modified headers, and other pre-conditions. This is a more complex form of content negotiation, performed programmatically by the application developer via the javax.ws.rs.Variant, VarianListBuilder, and Request objects. Request is injected using the @Context annotation. (For more information, read the JavaDoc.)