Direct Web Remoting DWR is a Java library that enables Java on the server and JavaScript in a browser to interact and call each other as simply as possible. DWR is Easy Ajax for Java

DWR will generate the JavaScript to allow web browsers to securely call into Java code almost as if it was running locally. It can marshal virtually any data including collections, POJOs, XML and binary data like images and PDF files. All that is required is a security policy that defines what is allowed.
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