Missing @PathParam("id")
in the service interface was the problem.
public void getUser(@PathParam("id") Integer id, MethodCallback<GWTUser> callback);
And for example UPDATE:
@PUT
@Path("/update")
@Produces(MediaType.APPLICATION_JSON)
@Consumes(MediaType.APPLICATION_JSON)
public void update(GWTUser user, MethodCallback<UserServiceResponse> callback);
you need to set the RequestBody
@RequestMapping( value = "/update", method = RequestMethod.PUT)
@ResponseStatus(HttpStatus.OK)
public void update(@RequestBody GWTUser gwtUser) {