Exciting news for all Java developers today, as Google App Engine (GAE) now officially supports Java as a programming language.
The App Engine is able to run Java web applications using the Java 6 virtual machine (JVM). App Engine uses the Java Servlet standard for web applications (JSP files, static files, servlets, web.xml etc.) in the usual WAR file structure.
Download the GAE SDK for Java / Google Plugin for Eclipse
Read about the Google Plugin for Eclipse
Read the App Engine Java Overview
Official Announcement at the GAE Blog
Java on GAE. Its not that easy