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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 (2.12.1 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 2.2 3 release is based off of the 2.12.x release stream and will contain some additional features (TBD).