Open source Java web 2.0 framework

I want to study the source code of a Java web application which makes heavy use of AJAX. These are my preferences:

  1. Source code available for study
  2. Server-side written in Java
  3. Easy to setup with minimal external dependencies
  4. Makes use of AJAX - preferably a single-page application

Tapestry is a modern web application framework in java that has fully integrated ajax functionality/features.

Try google web toolkit. U can create powerful js-based UI (with swing-like api), which interacts with java servlets via gwt-RPC (remote procedure calls on top of ajax).