Under the transaction persistence context model, an EntityManager begins a new persistence context with each transaction, and ends the context when the transaction commits or rolls back. Within the transaction, entities you retrieve through the EntityManager or via Queries are managed entities. They can access datastore resources to lazy-load additional persistent state as needed, and only one entity may exist for any persistent identity.

When the transaction completes, all entities lose their association with the EntityManager and become detached. Traversing a persistent field that wasn't already loaded now has undefined results. And using the EntityManager or a Query to retrieve additional objects may now create new instances with the same persistent identities as detached instances.

If you use an EntityManager with a transaction persistence context model outside of an active transaction, each method invocation creates a new persistence context, performs the method action, and ends the persistence context. For example, consider using the EntityManager.find method outside of a transaction. The EntityManager will create a temporary persistence context, perform the find operation, end the persistence context, and return the detached result object to you. A second call with the same id will return a second detached object.

When the next transaction begins, the EntityManager will begin a new persistence context, and will again start returning managed entities. As you'll see in Chapter 8, EntityManager, you can also merge the previously-detached entites back into the new persistence context.

EntityManager em; // Injected
...

// Outside a transaction:

// Each operation occurs in a separate persistence context and returns 
// a new detached instance.
Magazine mag1 = em.find (Magazine.class, magId);
Magazine mag2 = em.find (Magazine.class, magId);
assertTrue (mag2 != mag1);
...

// Transaction begins:

// Within a transaction, a subsequent lookup doesn't return any of the
// detached objects.  However, two lookups within the same transaction
// return the same instanc, because the persistence context spans the
// transaction.
Magazine mag3 = em.find (Magazine.class, magId);
assertTrue (mag3 != mag1 && mag3 != mag2);
Magazine mag4 = em.find (Magazine.class (magId);
assertTrue (mag4 == mag3);
...

// Transaction commits:

// Once again, each operation returns a new instance.
Magazine mag5 = em.find (Magazine.class, magId);
assertTrue (mag5 != mag3);

An EntityManager using an extended persistence context maintains the same persistence context for its entire lifecycle. Whether inside a transaction or not, all entities returned from the EntityManager are managed, and the EntityManager never creates two entity instances to represent the same persistent identity. Entities only become detached when you finally close the EntityManager (or when they are serialized).

EntityManagerFactory emf = ...
EntityManager em = emf.createEntityManager (PersistenceContextType.EXTENDED);

// The persistence context is active for the entire life of the EM, so there is
// only one entity for a given persistent identity.
Magazine mag1 = em.find (Magazine.class, magId);
Magazine mag2 = em.find (Magazine.class, magId);
assertTrue (mag2 == mag1);

em.getTransaction ().begin ();

// The same persistence context is active within the transaction.
Magazine mag3 = em.find (Magazine.class, magId);
assertTrue (mag3 == mag1);
Magazine mag4 = em.find (Magazine.class (magId);
assertTrue (mag4 == mag1);

em.getTransaction.commit ();

// When the transaction commits, the instance is still managed.
Magazine mag5 = em.find (Magazine.class, magId);
assertTrue (mag5 == mag1);

// The instance finally becomes detached when the EM closes.
em.close ();