Transparent persistence. Powerful querying. Use the API your prefer, to a very wide range of datastores. The most standards-compliant Java persistence product. Open Source software, under the Apache 2 license. Persisting Java objects is now easy.
Providing access to industry leading RDBMS, map stores such as Cassandra and HBase, the Neo4j graph store, spreadsheets in Excel or OpenDocument formats, JSON formatted Amazon and Google Storage options, the popular MongoDB JSON-like document store, as well as ubiquitous LDAP and others.
Java standard since 2006, providing easy persistence to RDBMS datastores. Comes with its own "SQL-like" JPQL query language, so you query your data in a language similar to what your datastore understands.
Java standard since 2001, providing transparent persistence and datastore agnosticity. Comes with its own "Java syntax" JDOQL query language so you can use the Java syntax you already know to query your data.
Explore the codebase on GitHub and contribute new features or fix bugs. All code is available under the very flexible Apache 2 License so you have every opportunity to offer your skills to the project. Follow the DataNucleus blog or Twitter (@datanucleus) and receive updates when features are added, or bugs are fixed that could impact on your application(s).