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1.2.3. Persistence context scope

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 that 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 extended 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.