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Here's a quick cheat sheet of the JPA world:

• A Cache is a copy of data, copy meaning pulled from but living outside the database.
• Flushing a Cache is the act of putting modified data back into the database.
• A PersistenceContext is essentially a Cache. It also tends to have it's own non-shared database connection.
• An EntityManager represents a PersistenceContext (and therefore a Cache)
• An EntityManagerFactory creates an EntityManager (and therefore a PersistenceContext/Cache)

• With <persistence-unit transaction-type="RESOURCE_LOCAL"> you are responsible for EntityManager (PersistenceContext/Cache) creating and tracking...
  • You must use the EntityManagerFactory to get an EntityManager
  • The resulting EntityManager instance is a PersistenceContext/Cache
  • An EntityManagerFactory can be injected via the @PersistenceUnit annotation only (not @PersistenceContext)
  • You are not allowed to use @PersistenceContext to refer to a unit of type RESOURCE_LOCAL
  • You must use the EntityTransaction API to begin/commit around every call to your EntityManger
  • Calling entityManagerFactory.createEntityManager() twice results in two separate EntityManager instances and therefor two separate PersistenceContexts/Caches.
  • It is almost never a good idea to have more than one instance of an EntityManager in use (don't create a second one unless you've destroyed the first)

• With <persistence-unit transaction-type="TRANSACTION"> the container will do EntityManager (PersistenceContext/Cache) creating and tracking...
  • You cannot use the EntityManagerFactory to get an EntityManager
  • You can only get an EntityManager supplied by the container
  • An EntityManager can be injected via the @PersistenceContext annotation only (not @PersistenceUnit)
  • You are not allowed to use @PersistenceUnit to refer to a unit of type TRANSACTION
  • The EntityManager given by the container is a reference to the PersistenceContext/Cache associated with a JTA Transaction.
  • If no JTA transaction is in progress, the EntityManager cannot be used because there is no PersistenceContext/Cache.
  • Everyone with an EntityManager reference to the same unit in the same transaction will automatically have a reference to the same PersistenceContext/Cache
  • The PersistenceContext/Cache is flushed and cleared at JTA commit time

Cache == PersistenceContext

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