Java Persistent Objects (JPOX) is a fully-compliant Open Source implementation of the JDO1,JDO2,JDO2.1 and JPA1 specifications for transparent persistence of Java objects. It is Apache 2 licensed and usable with any JDK 1.3 or above. The aim of JPOX from the outset has been to provide the most standards compliant Java persistence engine around. JPOX allows persistence to all popular RDBMS datastores, as well as to DB4O, and in the near future will support other datastores.
This JPOX 1.2.0 release provides compliance with JPA1, as well as a preview of the upcoming JDO2.1 standard. JPOX 1.2 will be the "reference implementation" (RI) for JDO2.1 once it is released (just like JPOX 1.1 was the RI for JDO2.0). The release has been 22 months in the making and there have been many changes during that time (not just for JDO2.1).
For more information see:
http://www.theserverside.com/news/thread.tss?thread_id=48590