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4.3. EXTENDED Persistence Context

All application managed entity manager and container managed persistence contexts defined as such are EXTENDED. This means that the persistence context type goes beyond the transaction life cycle. We should then understand what happens to operations made outside the scope of a transaction.
In an EXTENDED persistence context, all read only operations of the entity manager can be executed outside a transaction (find(), getReference(), refresh(), and read queries). Some modifications operations can be executed outside a transaction, but they are queued until the persistence context join a transaction: this is the case of persist(), merge(), remove(). Some operations cannot be called outside a transaction: flush(), lock(), and update/delete queries.

4.3.1. Container Managed Entity Manager

When using an EXTENDED persistence context with a container managed entity manager, the lifecycle of the persistence context is binded to the lifecycle of the Stateful Session Bean. Plus if the entity manager is created outside a transaction, modifications operations (persist, merge, remove) are queued in the persistence context and not executed to the database.
When a method of the stateful session bean involved or starting a transaction is later called, the entity manager join the transaction. All queued operation will then be executed to synchronize the persistence context.
This is perfect to implement the entitymanager-per-conversation pattern. A stateful session bean represents the conversation implementation. All intermediate conversation work will be processed in methods not involving transaction. The end of the conversation will be processed inside a JTA transaction. Hence all queued operations will be executed to the database and commited. If you are interested in the notion of conversation inside your application, have a look at JBoss Seam. Jboss Seam emphasizes the concept of conversation and entity manager lifecycle and bind EJB3 and JSF together.