[hibernate-dev] HHH-12353: have getReference stick to the Java API it promises?

[Sanne Grinovero]

Issue HHH-12353 was just reported and I have to agree it looks dodgy.

Essentially invoking:

 entityManager.getReference( Person.class )

might not return a type compatible with Person, when Person is final
and annotated with @Proxy(proxyClass = AnotherTypeX.class), then
AnotherTypeX might be returned and this is not necessarily a subtype
of Person.

That's ok as I understand we hardly have alternatives, but the
EntityManager API for getReference mandates this signature:

 <T> T getReference(Class<T> entityClass, Object primaryKey)

so we're not actually returning the expected type T in all
circumstances, confusing compilers and tooling into giving the wrong
suggestions.

I see some options:
a# disallow using @Proxy on a final entity - seems reasonable since
the docs warn against pitfalls.
b# throw an exception on Person.class being used as parameter, as the
user should actually use the proxytype?
c# somehow have the proxy still extend the final entity?

Option b# seems problematic as it's not obvious to users either, and
we would need to return a proxy reference even when there actually is
a managed Person instance in the current session.

Option c# is probably unrealistic and problematic too, as it would
require enhancing of the final class and I guess we can't rely on that
consistently.

In summary, I believe we should throw a bootstrap exception requiring
to remove the `final` modifier on the parent entity. Or challenge the
specification :(

Thanks,
Sanne