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

Sanne Grinovero sanne at hibernate.org
Mon Apr 25 10:52:04 EDT 2016 

N.B. that chancing the exception types thrown by our native methods is
an API change in my opinion.

So while I think you can have some Hibernate exceptions now extend
some of the types expected by JPA, a fully "clean room" approach is
not something I'd do in a minor release.

On 25 April 2016 at 15:25, Steve Ebersole <steve at hibernate.org> wrote:
> I agree with Gunnar in that the underlying question here is in how we see
> the "Hibernate API" long term.  However I would consider the following
> categories:
>
> Methods which are Hibernate-only
> Methods which Hibernate had historically which share a signature with JPA
> Methods which we added to Hibernate specifically for JPA.
>
>
> Personally, in retrospect, I would say that group (3) unequivocally should
> have been handled by just throwing the JPA exceptions.  In a clean-room impl
> that's what I would do.
>
> So I guess the question is whether we want to look at this as a clean-room
> impl or if we want to constrain ourselves to the legacy exception types.  I
> do wonder if a hybrid approach may be the best, as touched on before.
> Namely, where JPA expects an IllegalArgumentException,
> IllegalStateException, etc we throw those; for the rest we'd have Hibernate
> exception extend PersistenceException.  I think that's a great compromise.
>
> I'd also mention that in the IllegalArgumentException,
> IllegalStateException, etc cases there are a few places where
> HibernateExceptions cover the JPA case already.  E.g. JPA says that if there
> is a problem with the JPQL query (String) used to create a Query an
> IllegalArgumentException should be thrown; Hibernate currently handles this
> by throwing a QuerySyntaxException - we could just make QuerySyntaxException
> extend from IllegalArgumentException rather than Hibernate's
> HibernateException hierarchy.
>
> I think the bottom line is that having Session and EntityManager split
> before caused all kinds of difficult-to-debug complexity that I'd rather
> avoid as we consolidate them together.
>
>
> On Mon, Apr 25, 2016 at 5:37 AM Sanne Grinovero <sanne at hibernate.org> wrote:
>>
>> Gunnar's words seem wise to me: users will need to have the JPA API on
>> classpath anyway, so I don't see why we should have - and maintain -
>> strategies for different kind of exceptions.
>>
>> This might have been useful in the past, but looking forward?
>>
>> If the reasoning is that Hibernate - having more features - could
>> throw more specific and informative exceptions, we could have some
>> Hibernate exceptions to subclass the JPA ones?
>>
>> Would be nice to avoid breaking the expected exception types in a
>> minor, so I'm not sure if you can do that in all cases by subclassing
>> the JPA ones, but I suspect it can get you a long way.
>>
>>
>> On 25 April 2016 at 10:13, Gunnar Morling <gunnar at hibernate.org> wrote:
>> > The strategy approach sounds nice on first thought, but it also adds
>> > complexity.
>> >
>> > I think the underlying question is: What's the long-term strategy around
>> > the "Classic API"? Should it remain in place for all times as a complete
>> > alternative to the JPA API?
>> >
>> > Or should we begin to deprecate it and narrow it down in favour of the
>> > corresponding functionality standardized in JPA, and only functionality
>> > not
>> > present in JPA would be exposed through some kind of unwrap()?
>> >
>> > Without having thought about all the implications too much, I'd lean
>> > towards the latter, in which case I vote for "1. Just move to JPA
>> > expected
>> > exceptions" as part of such larger effort.
>> >
>> > It'd be interesting to run a poll to see some figures of people using
>> > classic vs. JPA.
>> >
>> > --Gunnar
>> >
>> >
>> >
>> > 2016-04-25 10:47 GMT+02:00 andrea boriero <andrea at hibernate.org>:
>> >
>> >> Having a Strategy gives us more flexibility  so +1.
>> >>
>> >> About the expectations I think what Vlad says is reasonable.
>> >>
>> >> On 25 April 2016 at 06:04, Vlad Mihalcea <mihalcea.vlad at gmail.com>
>> >> wrote:
>> >>
>> >> > 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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>> >> > >
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