Apache OpenJPA is a Java persistence project at The Apache Software Foundation that can be used as a stand-alone POJO persistence layer or integrated into any Java EE compliant container and many other lightweight frameworks, such as Tomcat and Spring.

The 1.x releases (1.2.2 is the latest) are a production ready, feature-rich, compliant implementation of the Java Persistence API (JPA) 1.0 part of the JSR-220 Enterprise Java Beans 3.0 specification, which pass the Sun JPA 1.0b Technology Compatibility Kit.

The 2.x releases (2.2.0 is the latest) are a production ready, compliant implementation of the JSR-317 Java Persistence 2.0 specification, which is backwards compatible to the JPA 1.0 specification and passes the Sun JPA 2.0 Technology Compatibility Kit.

The upcoming 2.3 release is based off of the 2.2.x release stream and will contain some additional features (TBD).