Summary

Entities are classes that need to be persisted, usually in a relational database. The persistence aspects of EJB 3 have been packaged as a separate specification, the Java Persistence API (JPA), so that applications that do not need EJB container services can still persist their entities to a relational database. Persistence services are handled by a persistence engine. In this chapter we make use of the Toplink persistence engine that comes bundled with the GlassFish application server.

Any Java class, or POJO, can be converted to an entity using metadata annotations. We described by means of an example the default rules for mapping an entity to a relational database table.

We introduced the EntityManager service, which provides methods for persisting, finding, querying, removing and updating entities. We saw examples of the EntityManager.persist() and EntityManager.find() methods. We introduced the concept of a persistence context, which is the set of managed entity instances.

We looked at Ant scripts for packaging and deploying an application which uses entities.

We examined strategies for generating primary keys. Finally we looked at examples of overriding default rules for mapping entities to relational tables.