Packaging
EJB3 JPA is standardizing POJO persistence. Thus, entities aren't limited to EJB modules; they can be packaged in a Web module, ejb-jar module, library module in the EAR level, or a standard jar file. You can also use entities in Java SE. You must package a descriptor (persistence.xml), like the following, in the archive that contain entities:
oracle.toplink.essentials.PersistenceProvider
jdbc/OracleDS
...
This descriptor identifies the persistence provider, persistent units, and the data source used by a persistent unit. As the name suggests, a persistent unit is a collection of entities that are managed together. If you have a single persistent unit defined in a specific module, you don't have to identify the entity classes in the persistence.xml; it will be dynamically discovered by the persistence provider.
