Types of entity managers
There are two types of entity managers available, depending on the Java environment in which the entity manager is obtained.
Container-managed entity manager
In the Java EE environment, the container manages the entity manager. The entity manager can be injected into a session bean, servlet, or JSP using dependency injection with the @PersistenceContext annotation, as shown next:
@PersistenceContext public EntityManager em;
Alternatively, the entity manager can be looked up using JNDI in the environment referencing context:
@PersistenceContext(name="CatalogEM", unitName="em")
@Resource
SessionContext ctx;
EntityManager em = (EntityManager)ctx.lookup("CatalogEM");
We shall be using the simpler of the two methods—the dependency injection method. Transactions define when entities are synchronized with the database. Container-managed entity managers always use JTA transactions, the transactions of the Java EE server.
Application-managed entity manager
In a Java SE environment...