Usage of EntityManagerFactory

[Devaka Cooray]

Can anyone tell me, what is the usage of using @PersistenceUnit annotation, instead of using @PersistenceContext annotation.

When we used @PersistenceUnit, we can get a EntityManagerFactory object as:

@PersistenceUnit(unitName="InventoryManagement")
EntityManagerFactory emf;

In this case, we can get the EntityManager as:

EntityManager em=emf.createEntityManager();

However, we can use the following syntax to get the EntityManager directly, with @PersistenceContext annotation as:

@PersistenceContext(unitName="InventoryManagement")
EntityManager em;

So, why we use the long way with @PersistenceUnit annotation, instead of using @PersistenceContext annotation?
[ November 03, 2008: Message edited by: Devaka Cooray ]

[Christophe Verré]

The EntityManagerFactory, along with the @PersistenceUnit annotation is used to create an Application-managed Persistence Context. The main difference between an application-managed persistence context and a context-managed persistence context is that application-managed persistence contexts are extended in scope, and may span multiple transactions. Using a transaction-scoped container-managed persistence context (like in your last code) limits the scope of the persistence context to the existing transaction.

[Devaka Cooray]

Originally posted by Christophe Verre:
The EntityManagerFactory, along with the @PersistenceUnit annotation is used to create an Application-managed Persistence Context. The main difference between an application-managed persistence context and a context-managed persistence context is that application-managed persistence contexts are extended in scope, and may span multiple transactions. Using a transaction-scoped container-managed persistence context (like in your last code) limits the scope of the persistence context to the existing transaction.

Thanks for the quick reply. However I didn't understand this well. Can you explain me?

[Chaminda Amarasinghe]

application manage persistence context can be shared on multiple transaction since life of container managed persistence context life cycle limited to one transaction,

[Ralph Jaus]

Hi,

The main difference between an application-managed persistence context and a context-managed persistence context is that application-managed persistence contexts are extended in scope, and may span multiple transactions

Can you give an example where an extended persistence context is used in a reasonable way in a stateless session bean or a MDB ?

In stateful session beans it seems to be easier to aquire an extended persistence context via

@PersistenceContext(type=PersistenceContextType.EXTENDED)

[Afzal Hossain]

Hi Friends,
When the PersistenceUnit is used it not only injects the EntityManagerFactory, it also registers a reference to it within the JNDI ENC of the EJB.

thanks