A persistence unit defines a set of all entity classes that are managed by EntityManager instances in an application. This set of entity classes represents the data contained within a single data store.
Persistence units are defined by the persistence.xml configuration file. The JAR file or directory whose META-INF directory contains persistence.xml is called the root of the persistence unit. The scope of the persistence unit is determined by the persistence unit’s root.
Each persistence unit must be identified with a name that is unique to the persistence unit’s scope.
Persistent units can be packaged as part of a WAR or EJB JAR file, or can be packaged as a JAR file that can then be included in an WAR or EAR file.
If you package the persistent unit as a set of classes in an EJB JAR file, persistence.xml should be put in the EJB JAR’s META-INF directory.
If you package the persistence unit as a set of classes in a WAR file, persistence.xml should be located in the WAR file’s WEB-INF/classes/META-INF directory.
If you package the persistence unit in a JAR file that will be included in a WAR or EAR file, the JAR file should be located:
In the WEB-INF/lib directory of a WAR.
In the top-level of an EAR file.
In the EAR file’s library directory.
persistence.xml defines one or more persistence units. The following is an example persistence.xml file.
<persistence> <persistence-unit name="OrderManagement"> <description>This unit manages orders and customers. It does not rely on any vendor-specific features and can therefore be deployed to any persistence provider. </description> <jta-data-source>jdbc/MyOrderDB</jta-data-source> <jar-file>MyOrderApp.jar</jar-file> <class>com.widgets.Order</class> <class>com.widgets.Customer</class> </persistence-unit> </persistence>
This file defines a persistence unit named OrderManagement, which uses a JTA-aware data source jdbc/MyOrderDB. The jar-file and class elements specify managed persistence classes: entity classes, embeddable classes, and mapped superclasses. The jar-file element specifies JAR files that are visible to the packaged persistence unit that contain managed persistence classes, while the class element explicitly names managed persistence classes.
The jta-data-source (for JTA-aware data sources) and non-jta-data-source (non-JTA-aware data sources) elements specify the global JNDI name of the data source to be used by the container.