JPA: question about merging an entity before removing it

Question

I know I have to merge the entity before removing it, but I never thought I have to do it inside EJB. First I have these:

e = (Event) scholarBean.merge(e);
schol         


        

Answer 1

I know I have to merge the entity before removing it

Not exactly. The object passed to remove has to be an entity and must not be detached. That's different.

but I never thought I have to do it inside EJB. First I have these (...)

Let's see what you're doing:

1: e = (Event) scholarBean.merge(e); 
2: scholarBean.remove(e);

So in 1:, you call an EJB (very likely with a transaction-scoped Persistence Context) that merges the entity. But then the method ends, the transaction commits, the Persistence Context gets closed, making the returned entity detached again.

And in 2:, you pass the (still) detached entity to an EJB and tries to remove it, which is not allowed. And KaBOOM!

So then I bring those two lines inside my session bean, and it works. Any idea why?

It works because you're now working within the scope of the persistence context associated to the JTA transaction and you're thus really passing a managed entity to remove.

Answer 2

I had the same transaction-issues when this was used in a servlet. When using the EJB-service-bean from a MDB it worked fine, since the transaction was started before the EJB was called, but when the EJB-call came from a servlet, there was no running transaction. I solved this in my webapp by creating a filter that starts and commis a UserTransaction. Then every call to the EJB-methods joins my UserTransaction instead of starting it's own transaction.

Answer 3

...and you can even combine those:

Like so:

    public void deleteManCheck(ManCheck manCheck) {
    em.remove(em.merge(manCheck));
}