my idea is DWR for you becuase :
Direct Web Remoting (DWR) is designed to be used with Java, one of the most widely used object-oriented languages. Java is not limited to any particular platform, but its main use has been server-side rather than desktop applications.

Here is my experience hope that helps:
I was developing Struts base web application and looking for an Suggest feature of Ajax frame work. I found that Struts-Layout support this feature very easily, it provides a simple action so you can extend it and code it. It will take just 10 line of codes on action and one jsp tag.
struts.application-servers.com/suggest/index.html
Advantages:
Fully integrated with Struts
It supports UTF-8 chars
I have tested in IE 5.0+ and Fire Fox 1.1 and it worked fine
Disadvantages:
If you are not using Struts then it is not worth to use it.
Only supports Suggest Feature, may be enough for many form base applications but it is still the limitation.

DWR is so good, I had also some successful experience with it
you may also try one of the JSF AJAX solution, one comes with JSC and netbeans give it a try u can start AJAX and JSF with minimum knowledge