[jboss-user] [JBoss Seam] - Transactions in Perstistence Context

[bfo81]

Sorry for my late answer :).

Those transactions are independent from each other. But step by step:

During the existence of an Extended Persistence Context there can be multiple Java Transactions (JTA).

And each Java Transaction will encapsulate write accesses to the database in a database transaction, if necessary (but I'm not sure about that). Don't know what about read accesses ;).

Usually (i.e. in a Stateless or Stateful Bean with default transaction stuff) changes to entities get written to the database at the end of a method. And those changes are encapsulated in a transaction (which is created by the EJB framework).

You might even write changes explicitely to the database with em.flush() before a method ends. Then the SQL-Statements are created immediately and submitted to the database, so you can react to exceptions if necessary (e.g. when violating db constraints).

But I must confess, I am not 100% sure at which point in time a transaction starts and ends. My imagination is like this:
- before methode starts: em.getTransaction().start().
- after method ends: em.getTransaction().commit() (or rollback() if setRollbackOnly() was called or a RuntimeException occured).
- and whenever there are Java transactions running, a database transaction is started before the first db access (e.g. set autocommit=0 in MySQL or begin tran in MS SQL). And after writing was finished, a database commit (or rollback) is called.

GAVIN? Could you take as on a ride into the deep misterys of the Java Transaction API and its interactions with the EntityManager here? Only a short comment wether my findings are correct :)

Plus some more questions I have:

- If a rollback was set (setRollbackOnly() or RuntimeException) there won't be any SQL statements sent to the database at the end of a method. If you called em.flush() before, then statements were already created and only then you need a database rollback, right?

- Ist there a SQL commit called at the end of em.flush()? If yes, and if there are e.g. constraints violated, the database rolls back the changed stuff automatically and throws an exception, right?

- How does the EntityManager know which entities changed? Does he hold a copy of every managed entity in memory and compare it to the actual entities, field by field?!? Or are there some interceptors doing some observer stuff on the entity?



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