persistence.xml

Overview

The persistence.xml file describes persistence units. It is the deployment descriptor file for persistence using Java Persistence API (JPA). It is used to declare the following.

• Managed persistence classes.
  The managed classes are, for example, those which are annotated using @Entity, @Embeddable or
  @MappedSuperclass.
• Specify object/relation mapping.
  JPA provides several mechanisms to map the java classes to tables in a relational database.
• Configuration information for entity managers and entity manager factory classes.

The persistence.xml file is placed in the META-INF directory of the root of the persistence unit. The schema of the persistence.xml is at this link.

The object/relational mapping information is provided by the following ways.

• Annotations on the managed persistence classes
• One or more XML files contained in the root of the persistence unit
• One or more XML files outside the root of the persistence unit on the classpath and referenced from the
  persistence.xml, or a combination of these.

In Java EE, the root of a persistence unit can be one of the following

• EJB-JAR file
• WEB-INF/classes directory of a WAR file
• A jar file in the WEB-INF/lib directory of a WAR file
• A jar file in the root of the EAR
• A jar file in the EAR library directory
• An application client jar file

The location of the managed persistence classes can be as follows.

• Within the root of the persistence unit.
• Can be specified by reference in the persistence.xml by naming the classes, archives, or mapping XML files
  that are accessible on the application classpath.
• Some combinations of the above methods.