This misses out on the main benefit of using annotations: The associated Java code is "just Java". Testing JAX-RS endpoints is just a matter of instantiating Java objects and calling methods on them. Testing grumpyrest endpoints would require mocking the requestCycle other low-level http-oriented classes.

Here's the JAX-RS equivalent of the demo:

    public class GreetingResource {
        @POST
        @Path("/make-greeting")
        public MakeGreetingResponse greet(final MakeGreetingRequest request) {
            if (request.addendum.isPresent()) {
                return new MakeGreetingResponse("Hello, " + request.name + "! " + request.addendum.getValue());
            } else {
                return new MakeGreetingResponse("Hello, " + request.name + "!");
            }
        }
    }

This is less noisy (no explicit parseBody) and much easier to test - you can just instantiate a GreetingResource and call the method, no mocking required.