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

[Steve Ebersole]

Scott, how would we register a listener for this event?  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.

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
> >
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