2.6. Packaging It Up

Now that you know the basic building blocks of JPA, we are ready to organize the pieces into an application that runs in Java SE. The only thing left to discuss is how to put it together so that it runs.

2.6.1. Persistence Unit

The configuration that describes the persistence unit is defined in an XML file called persistence.xml. Each persistence unit is named, so when a referencing application wants to specify the configuration for an entity it needs only to reference the name of the persistence unit that defines that configuration. A single persistence.xml file can contain one or more named persistence unit configurations, but each persistence unit is separate and distinct from the others, and they can be logically thought of as being in separate persistence.xml files.