The most common and widely used entity manager in a Java EE environment is the container-managed entity manager. In this mode, the container is responsible for the opening and closing of the entity manager (this is transparent to the application). It is also responsible for transaction boundaries. A container-managed entity manager is obtained in an application through dependency injection or through JNDI lookup, A container-managed entity manger requires the use of a JTA transaction.

An application-managed entity manager allows you to control the entity manager in application code. This entity manager is retrieved through the EntityManagerFactory API. An application managed entity manager can be either involved in the current JTA transaction (a JTA entity manager), or the transaction may be controlled through the EntityTransaction API (a resource-local entity manager). The resource-local entity manager transaction maps to a direct resource transaction (i. e. in Hibernate's case a JDBC transaction). The entity manager type (JTA or resource-local) is defined at configuration time, when setting up the entity manager factory.

An entity manager is the API to interact with the persistence context. Two common strategies can be used: binding the persistence context to the transaction boundaries, or keeping the persistence context available across several transactions.

The most common case is to bind the persistence context scope to the current transaction scope. This is only doable when JTA transactions are used: the persistence context is associated with the JTA transaction life cycle. When a entity manager is invoked, the persistence context is also opened, if there is no persistence context associated with the current JTA transaction. Otherwise, the associated persistence context is used. The persistence context ends when the JTA transaction completes. This means that during the JTA transaction, an application will be able to work on managed entities of the same persistence context. In other words, you don't have to pass the entity manager's persistence context across your EJB method calls, but simply use dependency injection or lookup whenever you need an entity manager.

You can also use an extended persistence context. This can be combined with stateful session beans, if you use a container-managed entity manager: the persistence context is created when an entity manager is retrieved from dependency injection or JNDI lookup , and is kept until the container closes it after the completion of the Remove stateful session bean method. This is a perfect mechanism for implementing a "long" unit of work pattern. For example, if you have to deal with multiple user interaction cycles as a single unit of work (e.g. a wizard dialog that has to be fully completed), you usually model this as a unit of work from the point of view of the application user, and implement it using an extended persistence context. Please refer to the Hibernate reference manual or the book Hibernate In Action for more information about this pattern. JBoss Seam is a framework tht link together JSF and EJB3 around the notion of conversation and unit of work. For an application-managed entity manager the persistence context is created when the entity manager is created and kept until the entity manager is closed. In an extended persistence context, all modification operations (persist, merge, remove) executed outside a transaction are queued until the persistence context is attached to a transaction. The transaction typically occurs at the user process end, allowing the whole process to be commited or rollbacked. For application-managed entity manager only support the exctended persistence context.

A resource-local entity manager or an entity manager created with EntityManagerFactory.createEntityManager() (application-managed) has a one-to-one relationship with a persistence context. In other situations persistence context propagation occurs.

Persistence context propagation occurs for container-managed entity managers.

In a transaction-scoped container managed entity manager (common case in a Java EE environment), the JTA transaction propagation is the same as the persistence context resource propagation. In other words, container-managed transaction-scoped entity managers retrieved within a given JTA transaction all share the same persistence context. In Hibernate terms, this means all managers share the same session.

Important: persistence context are never shared between different JTA transactions or between entity manager that do not came from the same entity manager factory. There are some noteworthy exceptions for context propagation when using extended persistence contexts: