The new language on Google App Engine: Java™

App Engine now supports the standards that make Java tooling great. (We’re working on the tooling too, with Google Plugin for Eclipse). It provides the current App Engine API’s and wraps them with standards where relevant, like the Java Servlet API, JDO and JPA, javax.cache, and javax.mail. It also provides a secure sandbox that’s powerful enough to run your code safely on Google’s servers, while being flexible enough for you to break abstractions at will.
(Link: The new language on Google App Engine: Java™)