Questions about Best Practice for multiple PAR

[per.a.norman]

I'm working on a new J2EE-application, and is evaluating EJB3 as persistence layer. I'm quite new to J2EE and have a question about what's best practice regarding my PAR.

Now I've split my @Entity classes into two different PARs. One PAR contains entity A which depends on entity B in the second PAR (with a forreign key in the database). This works fine if I create both object A and B with new in the same Session in the same Stateless bean.

If I instead uses a different Stateless bean to retreive object B, I can't persist entity A (which references entity B) without first calling manager.refresh(b).

The following example will give me an Exception "detached entity passed to persist" on last line:

B entityB = otherStatelessBean.findMyB();
A entityA = new A(entityB);
manager.persist(entityA);

To solve this I can write:

B entityB = otherStatelessBean.findMyB();
manager.refresh(entityB);
A entityA = new A(entityB);
manager.persist(entityA);

The refresh() will create a second (unnecessary) SELECT to the database.

I split my classes because I wanted a modularized application with one PAR and one EJB3 (with session beans) in every module. The above example made me unsure if this is a good idea.

My questions:
* What is best practice when modularizing EJB3-applications?
* Should I split entities or keep them together in one big PAR?
* Is there another solution or pattern to use to solve my problem above?


Thanks in advance!
Per

[epbernard]

related entity beans (ie associated) should be defined in the same persistence unit.
Regarding the issue, what you can do is load virtually B through em.getReference(), and associate it to A, this will not create an extra select.

or use em.merge(a) to avoid the exception and have only 1 api call.