[hibernate-dev] CDI integration in Hibernate ORM and the Application scope

Sanne Grinovero sanne at hibernate.org
Thu Jan 4 11:39:06 EST 2018 

On 4 January 2018 at 14:19, Steve Ebersole <steve at hibernate.org> wrote:
> Well there seems to be some disagreement about that.  I personally think we
> do not need anything other than a pre-shutdown hook so that we can release
> our CDI references.  Sanne seemed to think we needed something more
> "integrated".  I think we should start with the simple and add deeper
> integration (which requires actual CDI changes) only if we see that is
> necessary.  Sanne?

I guess it's totally possible that the current solution you all have
been working on covers most practical use cases and most immediate
user's needs, so that's great, but I wonder if we can clearly document
the limitations which I'm assuming we have (I can't).

I don't believe we can handle all complex dependency graphs that a CDI
user might expect with before & after phases, however I had no time to
prove this with a meaningful example.

If someone with more CDI experience could experiment with complex
dependency graphs then we should be able to better document the
limitations - which I strongly suspect exist - and make a good case to
need the JPA/CDI integration deeper at spec level, however "make it
work as users expect" might not be worthwhile of a spec update, one
could say it's the implementation's job so essentially a problem in
how we deal with integration details.

It's possible that there's no practical need for such a deeper
integration but it makes me a bit nervous to not be able to specify
the limitations to users.

More concrete example: Steve mentions having a "PRE-shutdown hook" to
release our references to managed beans; what if some other beans
depend on these? What if these other beans have wider scopes, like app
scope? Clearly the CDI engine is in the position to figure this out
and might want to initiate a cascade shutdown of such other beans
(which we don't manage directly) so this is essentially initiating a
whole-shutdown (not just a PRE-shutdown).

Vice-versa, same situation can arise during initialization; I'm afraid
this would get hairy quickly, while supposedly any CDI implementation
should have the means to handle ordering details appropriately, so I'd
hope we delegate it all to it to happen during its normal phases
rather than layering outer/inner phases around.

I'm not sure who to ask for a better opinion; I'll add Stuart in CC as
he's the only smart person I know with deep expertise in both
Hibernate and CDI, with some luck he'll say I'm wrong and we're good
:)

Thanks,
Sanne


>
> On Thu, Jan 4, 2018 at 7:58 AM Scott Marlow <smarlow at redhat.com> wrote:
>>
>> I can arrange to keep access to the specific
>> ExtendedBeanManager/LifecycleListener, that is not difficult.
>>
>> What changes do we need from the CDI implementation?
>>
>>
>> On Jan 3, 2018 4:36 PM, "Steve Ebersole" <steve at hibernate.org> wrote:
>>
>> If you have access to the specific ExtendedBeanManager/LifecycleListener,
>> that should already be enough.  Those things are already properly scoped to
>> the SessionFactory, unless you are passing the same instance to multiple
>> SessionFactory instances.
>>
>> On Wed, Jan 3, 2018 at 10:09 AM Scott Marlow <smarlow at redhat.com> wrote:
>>>
>>> On Tue, Jan 2, 2018 at 2:42 PM, Steve Ebersole <steve at hibernate.org>
>>> wrote:
>>>>
>>>> Scott, how would we register a listener for this event?
>>>
>>>
>>> If we want a standard solution, we could ask for an earlier CDI
>>> pre-destroy listener.
>>>
>>>> The problem we have had with most CDI "listeners" so far is that they
>>>> are non-contextual, meaning there has been no way to link that back to a
>>>> specific SessionFactory..  If I can register this listener with a reference
>>>> back to the Sessionfactory, this should actually be fine.
>>>
>>>
>>> I could pass the EMF to the
>>> org.hibernate.jpa.event.spi.jpa.ExtendedBeanManager.LifecycleListener, if
>>> that helps.
>>>
>>>>
>>>>
>>>> On Tue, Jan 2, 2018 at 1:39 PM Scott Marlow <smarlow at redhat.com> wrote:
>>>>>
>>>>> On Wed, Dec 20, 2017 at 9:48 AM, Sanne Grinovero <sanne at hibernate.org>
>>>>> wrote:
>>>>>
>>>>> > Any dependency injection framework will have some capability to
>>>>> > define
>>>>> > the graph of dependencies across components, and such graph could be
>>>>> > very complex, with details only known to the framework.
>>>>> >
>>>>> > I don't think we can solve the integration by having "before all
>>>>> > others" / "after all others" phases as that's too coarse grained to
>>>>> > define a full graph; we need to find a way to have the DI framework
>>>>> > take in consideration our additional components both in terms of DI
>>>>> > consumers and providers - then let the framework wire up things in
>>>>> > the
>>>>> > order it prefers. This is also to allow the DI engine to print
>>>>> > appropriate warnings for un-resolvable situations with its native
>>>>> > error handling, which would resolve in more familiar error messages.
>>>>> >
>>>>> > If that's not doable *or a priority* then all we can do is try to
>>>>> > make
>>>>> > it clear enough that there will be limitations and hopefully describe
>>>>> > these clearly. Some of such limitations might be puzzling as you
>>>>> > describe.
>>>>> >
>>>>> >
>>>>> >
>>>>> > On 20 December 2017 at 12:50, Yoann Rodiere <yoann at hibernate.org>
>>>>> > wrote:
>>>>> > > Hello all,
>>>>> > >
>>>>> > > TL;DR: Application-scoped beans cannot be used as part of the
>>>>> > > @PreDestroy
>>>>> > > method of ORM-instantiated CDI beans, and it's a bit odd because
>>>>> > > they can
>>>>> > > be used as part of the @PostConstruct method.
>>>>> > >
>>>>> > > I've been testing the CDI integration in Hibernate ORM for the past
>>>>> > > few
>>>>> > > days, trying to integrate it into Search. I think I've discovered
>>>>> > something
>>>>> > > odd: when CDI-managed beans are destroyed, they cannot access other
>>>>> > > Application-scoped CDI beans anymore. Not sure whether this is a
>>>>> > > problem
>>>>> > or
>>>>> > > not, so maybe we should discuss it a bit before going forward with
>>>>> > > the
>>>>> > > current behavior.
>>>>> > >
>>>>> > > Short reminder: scopes define when CDI beans are created and
>>>>> > > destroyed.
>>>>> > > @ApplicationScoped is pretty self-explanatory: created when the
>>>>> > application
>>>>> > > starts and destroyed when it stops. Some other scopes are a bit
>>>>> > > more
>>>>> > > convoluted: @Singleton basically means created *before* the
>>>>> > > application
>>>>> > > starts and destroyed *after* the application stops (and also means
>>>>> > > "this
>>>>> > > bean shall not be proxied"), @Dependent means created when an
>>>>> > > instance is
>>>>> > > requested and destroyed when the instance is released, etc.
>>>>> > >
>>>>> > > The thing is, Hibernate ORM is typically started very early and
>>>>> > > shut down
>>>>> > > very late in the CDI lifecycle - at least within WildFly. So when
>>>>> > Hibernate
>>>>> > > starts, CDI Application-scoped beans haven't been instantiated yet,
>>>>> > > and
>>>>> > it
>>>>> > > turns out that when Hibernate ORM shuts down, CDI has already
>>>>> > > destroyed
>>>>> > > Application-scoped beans.
>>>>> > >
>>>>> > > Regarding startup, Steve and Scott solved the problem by delaying
>>>>> > > bean
>>>>> > > instantiation to some point in the future when the Application
>>>>> > > scope is
>>>>> > > active (and thus Application-scoped beans are available). This
>>>>> > > makes it
>>>>> > > possible to use Application-scoped beans within ORM-instantiated
>>>>> > > beans as
>>>>> > > soon as the latter are constructed (i.e. within their
>>>>> > > @PostConstruct
>>>>> > > methods).
>>>>> > > However, when Hibernate ORM shuts down, the Application scope has
>>>>> > > already
>>>>> > > been terminated. So when ORM destroys the beans it instantiated,
>>>>> > > those
>>>>> > > ORM-instantiated beans cannot call a method on referenced
>>>>> > > Application-scoped beans (CDI proxies will throw an exception).
>>>>> > >
>>>>> > > All in all, the only type of beans we can currently use in a
>>>>> > > @PreDestroy
>>>>> > > method of an ORM-instantiated bean is @Dependent beans. @Singleton
>>>>> > > beans
>>>>> > > will work, but only because they are not proxied and thus you can
>>>>> > > cheat
>>>>> > and
>>>>> > > use them even after they have been destroyed... which I definitely
>>>>> > wouldn't
>>>>> > > recommend.
>>>>> > >
>>>>> > > I see two ways to handle the issue:
>>>>> > >
>>>>> > >    1. We don't change anything, and simply document somewhere that
>>>>> > > beans
>>>>> > >    instantiated as part of the CDI integration are instantiated
>>>>> > > within
>>>>> > the
>>>>> > >    Application scope, but are destroyed outside of it. And we
>>>>> > > suggest
>>>>> > that any
>>>>> > >    bean used in @PostDestroy method in an ORM-instantiated bean
>>>>> > (directly or
>>>>> > >    not) must have either a @Dependent scope, or a @Singleton scope
>>>>> > > and no
>>>>> > >    @PostDestroy method.
>>>>> > >    2. We implement an "early shut-down" somehow, which would bring
>>>>> > forward
>>>>> > >    bean destruction to some time when the Application scope is
>>>>> > > still
>>>>> > active.
>>>>> >
>>>>>
>>>>> org.hibernate.jpa.event.spi.jpa.ExtendedBeanManager mentions that we
>>>>> could
>>>>> look at introducing a beanManagerDestroyed notification, if that is
>>>>> useful
>>>>> and we can find a way to implement it
>>>>> (javax.enterprise.spi.BeforeShutdown
>>>>> [1] is not early enough to meet your requirements).
>>>>>
>>>>> Scott
>>>>>
>>>>> [1]
>>>>>
>>>>> https://docs.oracle.com/javaee/7/api/javax/enterprise/inject/spi/BeforeShutdown.html
>>>>>
>>>>>
>>>>> > >
>>>>> > > #1 may be enough for now, even though the behavior feels a bit odd,
>>>>> > > and
>>>>> > > forces users to resort to less-than-ideal practices (using a
>>>>> > > @Singleton
>>>>> > > bean after it has been destroyed).
>>>>> > >
>>>>> > > #2 would require changes in WildFly and may be a bit complex. In
>>>>> > > particular, if we aren't careful, Application-scoped beans may not
>>>>> > > be
>>>>> > able
>>>>> > > to use Hibernate ORM from within their @PreDestroy methods... Which
>>>>> > > is
>>>>> > > probably not a good idea. So we would have to find a solution
>>>>> > > together
>>>>> > with
>>>>> > > the WildFly team. Also to be considered: Hibernate Search would
>>>>> > > have to
>>>>> > be
>>>>> > > shut down just before the "early shut-down" of Hibernate ORM
>>>>> > > occurs,
>>>>> > > because Hibernate Search cannot function at all without the beans
>>>>> > > it
>>>>> > > retrieves from the CDI context.
>>>>> > >
>>>>> > > Thoughts?
>>>>> > >
>>>>> > >
>>>>> > > Yoann Rodière
>>>>> > > Hibernate NoORM Team
>>>>> > > yoann at hibernate.org
>>>>> > > _______________________________________________
>>>>> > > hibernate-dev mailing list
>>>>> > > hibernate-dev at lists.jboss.org
>>>>> > > https://lists.jboss.org/mailman/listinfo/hibernate-dev
>>>>> >
>>>>> _______________________________________________
>>>>> hibernate-dev mailing list
>>>>> hibernate-dev at lists.jboss.org
>>>>> https://lists.jboss.org/mailman/listinfo/hibernate-dev
>>
>>
>

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