[hibernate-dev] Various expectation changes in hibernate-core after consolidating hibernate-entitymanager

[Vlad Mihalcea]

Your second email summarizes my thoughts as well. If we can separate the
exception handling in two separate strategies that are defined during
bootstrap (JPA vs Hibernate),
I think that's the way to go.

There so many projects out there that rely on the exception type being
thrown, and changing those would make it very difficult for them to migrate
to this new version.
But that only affects Hibernate-native projects since, for those who have
been using JPA, they already expect the JPA exceptions anyway.

As for the other behavior discrepancies:

1. "calling EntityManager#close on a closed EntityManager should result in an
exception;" - that's a reasonable default and shouldn't cause too much
trouble.
2. "Another change in expectation is in regards to operations outside of a
transaction" - in JPA we can execute queries outside a transaction, but any
write will fail if there is no transactional context, which is reasonable
for me too. If Hibernate allows writes outside of a transactional context,
that's definitely a thing we should not support anyway.
3. "Asking a Session if is contains (Session/EntityManager#contains) a
 non-entity" - we can handle this with the separate exception handler
strategies to retain both JPA and Hibernate behaviors.
4. "Accessing Session/EntityManager#getTransaction.  JPA says that is
only allowed
for JDBC transactions.  Hibernate always allows it." - I'd choose the
Hibernate behavior because I don;t see how it can cause any issue and it's
an enhancement as well.

Vlad





On Sat, Apr 23, 2016 at 5:29 PM, Steve Ebersole <steve at hibernate.org> wrote:

> Just realized that I should have mentioned an important plan that helps
> understand the idea behind the "exception handling strategy" route.  I plan
> to keep track of how a SessionFactory was bootstrapped in some fashion.  So
> when it was bootstrapped from EntityManagerFactoryBuilder (which JPA
> bootstrap methods leverage) we'd select the "JPA exception handling"
> strategy impl.  When not, we'd use the "legacy Hibernate exception
> handling" strategy.
>
> On Sat, Apr 23, 2016 at 9:21 AM Steve Ebersole <steve at hibernate.org>
> wrote:
>
> > There are a number of "expectation changes" that come about from
> > consolidating hibernate-entitymanger into hibernate-core.  Some we have
> > discussed; some we have not.  Hopefully we can come to a consensus
> regards
> > how to deal with these changes...
> >
> > The first one is different exception types.  We have discussed this
> > before.  For now, in an effort to fix test failures and move forward with
> > developing, I simply changed failing tests to expect the JPA defined
> > exceptions.  I had mentioned, where possible, to to throw a combination
> of
> > the 2 expected exceptions.  Generally this falls into 2 discrete
> categories:
> >
> >
> >    1. JPA expects a PersistenceException and we have historically thrown
> >    a HibernateException
> >    2. JPA expects some form of JDK RuntimeException
> >    (IllegalArgumentException, IllegalStateException, etc) and we have
> >    historically thrown a HibernateException
> >
> > It is unfortunate that Java does not allow exceptions to be defined by
> > means of interfaces; that's the only "clean" way I see to do this - that
> > would have allowed us to define concrete exception classes that extend
> > PersistenceException, IllegalArgumentException, etc and that implement
> HibernateException.
> >
> >
> > So I see 3 potential solutions (feel free to bring up others).
> >
> >    1. Just move to JPA expected exceptions.
> >    2. Have HibernateException extend PersistenceException and just not
> >    worry about the change in expectation in regards to that second
> category.
> >    3. Push exception handling behind a strategy.  This would have to be a
> >    pretty specific strategy for very specific cases.
> >
> > The first and second options are pretty self-explanatory and
> > straight-forward so I won't go into detail there.  Just realize that
> these
> > change the expectation for the user.  They'd have to change their code to
> > catch these JPA-defined exceptions.
> > The other option, I see, is to h
> >
> > The third option is perfect in theory, but it is very tedious.  For
> > example, take the case of trying to perform some operation on a closed
> > Session/EntityManager.  Hibernate historically threw a HibernateException
> > here.  JPA says that should result in an IllegalStateException.  So in
> > SessionImpl#checkOpen, when the Session/EntityManager is closed, we'd
> > call out to the strategy to handle that condition.  Even more, Hibernate
> > (historically) and JPA disagree about which methods getting called on a
> > closed Session/EntityManager should lead to an exception.  For example,
> > JPA says calling EntityManager#close on a closed EntityManager should
> > result in an exception; Hibernate historically did not care if you called
> > Session#close on a closed Session.  So that is a special case, and every
> > one of those special cases would have to be exposed and handled in the
> > exception handling strategy in additional to the general cases.
> >
> > Another change in expectation is in regards to operations outside of a
> > transaction, which I consider a questionable pattern anyway.  Hibernate
> > historically allowed that; JPA explicitly disallows it.  In a way this
> > could fall under the exception discussion above, meaning we could push
> that
> > distinction behind the exception handling strategy.  Or we could decide
> > that we are going to stop supporting that.
> >
> > There are a lot of other highly questionable things I have seen in the
> > tests that JPA explicitly disallows that I think we ought to just stop
> > supporting and opt for the JPA way, although I am open to discussing them
> > if any feels strongly about them.  Some of these include:
> >
> >    - Asking a Session if is contains (Session/EntityManager#contains) a
> >    non-entity.  Hibernate historically would just return false.  JPA
> states
> >    that should be an exception.
> >    - Accessing Session/EntityManager#getTransaction.  JPA says that is
> >    only allowed for JDBC transactions.  Hibernate always allows it.
> >
> > If we go the route of an "exception handling strategy" a lot of the other
> > points I mentioned above could just be pushed behind that strategy.  But
> I
> > really want to start looking critically at what we support today that we
> > maybe really should not be.
> >
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