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From "Patrick Linskey" <plins...@bea.com>
Subject RE:
Date Wed, 13 Dec 2006 02:42:28 GMT
> On Dec 12, 2006, at 6:24 PM, Dain Sundstrom wrote:
> 
> > Does JPA guarantee that only one bean will be activated for a  
> > specific pk per transaction?
> 
> Yes.
> 
> > Specifically, will OpenJPA ever create more that one bean for a  
> > specific Class:PK in a single transaction?
> 
> No.

The full story is a little bit more complicated with OpenJPA. JPA guarantees
that object equality (==) comparison must pass for two references that point
to the same database record in the same transaction. IOW, however you
navigate to / query for a given record, you'll get back a reference to the
same Java object in a given transaction.

The complication is that in most circumstances, OpenJPA holds weak
references to all objects except ones with unflushed modifications. The
guarantee I outlined above still holds, since if you hold a reference to an
instance, our weak reference will be maintained as well, and equality tests
will pass. But if you release your reference, the instance may become
garbage-collected, so if you later query for / navigate to the instance, you
might end up with a different object reference.

I bring all this up because it means that OpenJPA does not guarantee that
the return value of System.identityHashCode() will be the same for a given
record over the course of a transaction. So, be sure that you bear this
restriction in mind if you use an IdentityHashMap or some equivalent
structure.

-Patrick
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