From: Christian von Kutzleben <cvkutzleben_at_versant.com>
Date: Thu, 27 Jun 2013 10:10:08 +0200

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)

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
christian.vonkutzleben_at_actian.com
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