VRaptor3 is a fully-fledged Java Web framework focused on fast development and maintainable code. It intends to make RESTful application development easy. A resource corresponds to a business logic annotated with its path and HTTP method. It runs inside DI Containers (Spring or Pico out of the box) and, therefore, enables full integration with them. Also, using Hibernate is easier: its session is automatically injected by VRaptor, so you just have to use it. VRaptor3 is refactor-friendly, especially in its redirecting configuration. It is also nicely extensible.
RestyGWT is a GWT generator for REST services and JSON encoded data transfer objects. RestyGWT Rest Services allow you to define an asynchronous service API which is then implemented via GWT deferred binding by RestyGWT. It features generation of asynchronous RESTful JSON based service proxies, encoding and decoding of Java objects to JSON, and an easy-to-use REST API.
SerfJ provides a very easy way of developing Java REST Web applications. It helps you to develop your application over an elegant MVC architecture, giving more importance to convention than configuration. This means, for example, you will not need configuration files or annotations in order to specify which view serves a controller's method. However, SerfJ is very flexible, so if you want to jump over those conventions, you can configure the behavior of your applications as you like. The framework tries to meet the JSR 311 specification, but it doesn't follow every point of the specification, because the purpose is to have a very intuitive library, and some some aspects of the specification are out of the scope of SerfJ.