Enterprise JavaBeans 3.0, 5th Edition

Chapter 5. Persistence: EntityManager

Persistence is a key piece of the Java EE platform. In older versions of Java EE, the EJB specification was responsible for defining this layer. In Java EE 5, persistence has been spun off into its own specification: Java Persistence 1.0. Persistence provides an ease-of-use abstraction on top of JDBC so that your code can be isolated from database, vendor-specific peculiarities and optimizations. It can also be described as an object-to-relational mapping engine (ORM). This means that the Java Persistence API can automatically map your Java objects to and from a relational database. In addition to object mappings, this service also provides a query language that is very SQL-like but is tailored to work with Java objects rather than a relational schema.

chapter-4 showed how to create and interact with an entity bean. For those of you familiar with the older EJB 2.x model for entity beans, you may have noticed that entity beans no longer have a Home interface. So, how are entity beans created? How are they updated and removed? How can you perform queries and such? All of these persistent actions are now performed through the javax.persistence.EntityManager service.

In the new Java Persistence specification, the EntityManager is the central service for all persistence actions. Entities are plain Java objects that are allocated just like any other Java object. They do not become persistent until your code explicitly interacts with the EntityManager ...