App Engine applications can be implemented using the Java programming language, and other languages that use the Java Virtual Machine. The App Engine Java runtime environment includes the Java 6 JVM and interfaces to the App Engine services that conform to industry standards, such as servlets and JDO/JPA. You can also use raw APIs to the services for implementing other interfaces.
This developer guide introduces the standard interfaces and explains how to use them with App Engine, as well as the raw APIs and development tools included in the App Engine Java SDK.
GAE The Java Runtime Environment