Now-a-days many web portals are based on Java portlet technology. Since Progress bars for file upload works requires AJAX and there existed no such technology based on the portlet.
Portlets are similar to servlets, however they focus on the presentation layer of the application.
The portlet is indeed very useful for sharing large audio, video, and scientific files. You should just know portal and portlet technology development along with the Web development using Java Servlets and JavaServer Pages (JSPs).
The Apache Commons FileUpload package (referred to as FileUpload) is the basis for the file upload portlet. Version 1.1 of the Apache Commons FileUpload supports file upload in a portlet other than the file upload support in a servlet.
The download packages are available at http://getahead.org/dwr/download. The JAR to download is the best to start with DWR (Direct Web Remoting).