JAX-RS jersey ExceptionMappers User-Defined Exception

You need to annotate your exception mapper with @Provider, otherwise it will never get registered with the JAX-RS runtime.

@Provider
public class UserNotFoundMapper implements
        ExceptionMapper<UserNotFoundException> {
    @Override
    public Response toResponse(UserNotFoundException ex) {
        return Response.status(404).entity(ex.getMessage()).type("text/plain")
                .build();
    }
}