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?
>
> 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
"EE.4.2.3 Transactions and Threads" in the EE 7 platform spec is another
section that *would* need a change:
"
...
* In web components not implementing SingleThreadModel, transactional
resource objects, as well as Java Persistence EntityManager objects,
should not be stored in class instance fields, and should be acquired
and released within the same invocation of the service method.
...
"
>
>
> ----------------------------------------------------------------------------------------------------------------------------------------------------------------
> 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/