...
The JPA spec requires some type of monitoring of Entity objects, but the spec does not define how to implement this monitoring. Some JPA providers auto-generate new subclasses or proxy objects that front the user's Entity objects at runtime, while others use byte-code weaving technologies to enhance the actual Entity class objects at build time. OpenJPA supports both enhancement methods, but strongly suggests only using the build time enhancement. The following Entity Enhancement page includes more details on both enhancement types, along with examples on how to setup build time enhancement in ANT, Maven and Eclipse environments.
Tools
OpenJPA provides several design-time and runtime tools, to perform such tasks as entity enhancement, schema mapping, generating metamodel classes and to help migrate from other JPA providers. Please checkout the Tools page for more details.
FAQ
Some common questions concerning the history, architecture and usage of OpenJPA can be found on the FAQ page.
...