Java webpart like asp.net?
I don't know of any JSF or JSP implementations, and I don't think they even exist.
It is possible to create such a thing with GWT from Google and GWT-Dnd . They allow you to create javascript web applications, but you write them in Java.
Here is an example generated with GWT-DND: http://www.wibokr.com
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Haven't seen any frameworks that directly support this kind of functionality outside of the very core, but there are many frameworks out there that allow you to design such a system if you like. Of the other answers, only GWT and Portlet are valid at the moment.
If you want to find something that is compatible, you should start looking for web frameworks that support either the portlet API specifications ( 1.0 is JSR-168 , 2.0 is JSR-286 ). Otherwise, you can just roll your own web based framework that you are comfortable with, no matter what GWT , Seam , Wicket , Tapestry , Vaadin ...
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