Asynchronous communication has become more important nowadays, especially as data become more voluminous every day. 

One of the simplest cool new features in JAX-RS 2 is asynchronous processing. What's really nice is that async processing has been added both to client and server side APIs.

For instance:

@Path("/test/operation")
@Stateless
public class TestResource {
    @GET 
    @Produce("application/json")
    @Asynchronous
    public void operation(@Suspended AsyncResponse asyncResponse) {
        asyncResponse.resume(someLongOperation()) 
    }
} 

There are two key annotations used here.

@Asynchronous: This informs the container to release client requests and start processing other incoming requests. When the worker finishes, the processing method will call the container to send the result to the client.

@Suspended: This informs the container to suspend and resume communication while the worker thread, someLongOperaton in this case, is finished.

At the client side, we could also use InvocationCallback.