Yeah ...it's easy now to create Portles with AJAX.
Here is good start for creating JSR 168 Portlets with AJAX.
Oracle has written a very good white paper for AJAX Portlet.
The Expert Group is also working on AJAX in Portlets in the coming JSR 286 Portlet Specification.
Let's have a look for Creating Portlets with AJAX.

I had read the document before, but I have no idea of how to build it in JDeveloper.
Which type of portlet should I use, Jave Portlet or Oracle JPDK Portlet ?
Any hints ?
thanks
George (HK)