On 06/26/2013 12:28 PM, Kevin Sutter wrote:
> Hi,
> 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?
>
> I understand that EMs by themselves are not considered thread-safe.
> We're clear on that aspect in the spec [sec 7.2]. But, when the
> container is managing the injection of these EMs and the associated
> transactional context, can we consider these EMs to only be accessed by
> a single thread? Like I said, WebSphere has controls and processing in
> place to allow for this usage pattern, but we're not clear whether this
> is a generally accepted pattern or unique to WebSphere? I think this is
> pretty clear when a Transactional context is in place -- in this case,
> the EM should be injectable into a Servlet. But, maybe it becomes more
> murky when a Transactional context is not defined? Is this where the
> concern comes from and it now depends on the QoS provided by the
> application server?
I agree that it becomes murky when a Transactional context is not
defined. For example, to satisfy an EM invocation, the container could
use a *cached* underlying entity manager that is obtained from a
thread-unsafe collection.
Even if containers aren't doing that, they still could. I'm not against
changing the rules (in a future spec), to not allow such thread-unsafe
access in containers.
When we talk about using an *underlying* entity manager in a servlet, I
think that should still be considered a thread-unsafe object.
>
> Customers are getting conflicting information from the specs, web
> references, and jpa provider support... I'm looking for clarification
> on this usage pattern. If something exists, just point me in the right
> direction. Otherwise, maybe we can open a JIRA or something to clarify
> this in the next rev of the spec? Thanks!
> [1]
> https://weblogs.java.net/blog/ss141213/archive/2005/12/dont_use_persis_1.html
>
>
> ----------------------------------------------------------------------------------------------------------------------------------------------------------------
> Kevin Sutter, Java EE and Java Persistence API (JPA) architect
> mail: sutter_at_us.ibm.com, Kevin Sutter/Rochester/IBM
> http://webspherepersistence.blogspot.com/
> phone: tl-553-3620 (office), 507-253-3620 (office)
> http://openjpa.apache.org/