DWR is Easy Ajax for Java
With Reverse Ajax, DWR allows Java code running on a server to use client side APIs to publish updates to arbitrary groups of browsers. This allows interaction 2 ways - browser calling server and server calling browser. DWR supports Comet, Polling and Piggyback (sending data in with normal requests) as ways to publish to browsers.
DWR provides integration with Dojo, TIBCO GI, Scriptaculous in the browser, and with Spring, Struts, Guice, Hibernate and others on the serve