Writing applications using Java on Google App Engine provides portability than when developed using Python. While Google App Engine (GAE) uses Google’s magic infrastructure, however, limits the portability if the application uses the Bigtable data store. In Java this tight coupling to the Bigtable data store can be alleviated by making your application use JDO or JPA to access the Bigtable data store. The JDO/JPA implementation in GAE is provided by Data Nucleus Access platform, an open source Java data persistence, analysis and management platform. The Data Nucleus Access Platform supports all popular data stores such as RDBMS and LDAP among others. The other services in GAE shown in the image above such as Memcache and Mail can be accessed  using JCache API, Java Mail API respectively. Coding an application to such standard Java API’s maximize portability to any vendor product that supports these API’s.

Other articles of possible interest:

1. Overview of Java support in Google App Engine
2. What you cannot do using Java in Google App Engine