persistence.xml different transaction-type attribu

问题:

In the persistence.xml JPA configuration file, you can have a line like:

<persistence-unit name="com.nz_war_1.0-SNAPSHOTPU" transaction-type="JTA">

or sometimes:

<persistence-unit name="com.nz_war_1.0-SNAPSHOTPU" transaction-type=”RESOURCE_LOCAL”>

My question is:

What is the difference between transaction-type="JTA" and transaction-type=”RESOURCE_LOCAL” ?

I also noticed some persistence.xml files with the transaction-type missing. Is it correct?

回答1:

Defaults

Default to JTA in a JavaEE environment and to RESOURCE_LOCAL in a JavaSE environment.

RESOURCE_LOCAL

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)

JTA

With <persistence-unit transaction-type="JTA"> 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 JTA
• 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