From: Christian von Kutzleben <cvkutzleben_at_versant.com>
Date: Thu, 27 Jun 2013 15:41:41 +0200

Hi Scott,

My point is: if it does not work at all at a conceptual level, it does not
make sense to introduce such a feature.

If the container injected proxies were thread-safe, this just would not be
sufficient to protect the internal structures
of the persistence context implementation.

Christian

On Thu, Jun 27, 2013 at 3:14 PM, Scott Marlow <smarlow_at_redhat.com> wrote:

> On 06/27/2013 04:10 AM, Christian von Kutzleben wrote:
>
>> Hi Kevin,
>>
>>
>> There seems to be a lot of conflicting information on the
>> web concerning
>> the thread-safety of injected EMs in runtime components.
>> From a
>> WebSphere perspective, our injected EMs are thread-safe
>> regardless of
>> the runtime component -- EJBs or Servlets. But, there are
>> several
>> references that indicate injected EMs in a Servlet are not
>> thread-safe
>> [1, as an example]. Our JPA spec is not clear on this (or,
>> at least I
>> couldn't find a definitive answer). What's the consensus of
>> the group?
>> Should injected EMs in a Servlet be considered
>> thread-safe? Or, is
>> this an implementation detail left up to each application
>> server?
>>
>>
>> The injected instance implementing EntityManager could of course be made
>> thread-safe,
>> however, this only synchronizes threads accessing the underlying
>> persistence context through
>> the EntityManager interface and threads, accessing the persistence
>> context otherwise would
>> corrupt the data (specifically: field/property access of instances,
>> which cause delayed/lazy loading)
>>
>
> Christian,
>
> IMO, this discussion is only for the container managed entity manager that
> is injected. If we all agree here, in a future spec, the container managed
> entity manager can be required to be thread-safe. The underlying entity
> manager that is used for each entity manager invocation, is not required to
> be thread-safe.
>
> If you implement a persistence provider, you can continue to enjoy
> (mostly) not worrying about thread safety. Providers do need to handle at
> least one concurrency case (in your tx sync callback), you should handle
> transaction rollbacks from a background thread. From section JPA 2.1 7.9.2:
>
> "
> When the JTA transaction rolls back, the provider must detach all managed
> entities if the persistence context is of type SynchronizationType.**SYNCHRONIZED
> or has otherwise been joined to the transaction. Note that the JTA
> transaction may rollback in a background thread (e.g., as a result of
> transaction timeout), in which case the provider should arrange for the
> managed entities to be detached from the persistence context but not
> concurrently while the application is in an EntityManager invocation.
> "
>
> Scott
>
>
>> So IMO only the JPA implementation could provide means for
>> thread-safety, but this is not a required
>> feature, so from my point of view, unless a thread-safe feature would be
>> introduced to JPA and the
>> "injector" enables this, this can not work at all.
>>
>> (Personally, I'm in favor of not introducing such a feature; we had that
>> in the JDO spec and it just
>> creates unnecessary complexity and issues to only support some corner
>> usecases)
>>
>> Christian
>>
>> --
>>
>> *Christian von Kutzleben*
>>
>> Chief Software Engineer
>> Versant GmbH, Subsidiary of Actian Corporation
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-- 
*Christian von Kutzleben*
Chief Software Engineer
Versant GmbH, Subsidiary of Actian Corporation
christian.vonkutzleben_at_actian.com
PHONE +49 40 60990-273
FAX +49 40 60990-113
www.actian.com
Versant GmbH is incorporated in Germany. Company registration number: HRB
54723, Amtsgericht Hamburg. Registered Office: Halenreie 42, 22359 Hamburg,
Germany. Geschäftsührer: Marc Monahan, Fred Gallagher, Volker John
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