I have used DWR plus jQuery for online store project, and it is easy to use, fast to create your web 2.0 project. Here is basic concept.

What is DWR?
1. Is a Java and JavaScript open source library which allows you to write Ajax web applications (Hides low-level XMLHttpRequest handling).
2. Specifically designed with Java technology in mind. (like Java-RPC)
3. Allows JavaScript code in a browser to use Java methods running on a web server just as if they were in the browser.

Why DWR?
1. Without DWR, you would have to create many
Web application endpoints (servlets) that need to
be address’able via URI’s.

2. What happens if you have several methods in a
class on the server that you want to invoke from
the browser?
– Each of these methods need to be addressable via URI
whether you are using XMLHttpRequest directory or clientside
only toolkit such as Dojo or Prototype.
– You would have to map parameters and return values to
HTML input form parameters and responses yourself.

3. DWR comes with some JavaScript utility functions

How Does DWR Work?
1. DWR dynamically generates a matching client-side Javascript class from a backend Java class
2. The generated JavaScript class handles remoting details between the browser and the backend server

1. DWR
2. Introduction to DWR
3. Some examples of Developing Rich Internet Applications by DWR2, please look at doc1, and doc2