RFR: 8188055: (ref) Add Reference::refersTo predicate [v5]

[Mandy Chung]

On Tue, 20 Oct 2020 05:22:57 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> @kimbarrett  your reworded text is okay.  I think "if it initially had some other referent value" can be dropped.
>> 
>> For a `Reference` constructed with a `null` referent, we can clarify in the spec that such reference object will never
>> get cleared and enqueued.  I suggest to file a separate issue to follow up.
>
>> @kimbarrett your reworded text is okay. I think "if it initially had some other referent value" can be dropped.
>> 
>> For a `Reference` constructed with a `null` referent, we can clarify in the spec that such reference object will never
>> get cleared and enqueued. I suggest to file a separate issue to follow up.
> 
> I don't think that clause can be dropped, because of explicit clearing (by clear() or enqueue()) rather than by the
> GC.  If the reference was constructed with a null referent, ref.refersTo(null) cannot tell whether ref.clear() has been
> called.

> Mandy's comment implied that references with a null referent never get enqueued. Otherwise when would they get
> enqueued? There would be nothing to trigger it.

Sorry I should have been clearer.    What I try to say is that `Reference(null)`
will never be cleared and enqueued by GC since its referent is `null`.
Kim is right that `Reference(null)` can be explicitly cleared and enqueued
via `Reference::enqueue`.    `Reference::clear` on such an "empty" reference
object is essentially a no-op.

> > But the more we discuss this the more I think allowing an initial null referent was a mistake in the first place. :(
> 
> I agree, but here we are. Very hard to know what the compatibility impact of changing that would be.

There are existing use cases depending on `Reference(null)` for example as a special
instance be an empty reference or the head of a doubly-linked list of references.
This was discussed two years ago [1].

[1] https://mail.openjdk.java.net/pipermail/core-libs-dev/2018-July/054325.html

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PR: https://git.openjdk.java.net/jdk/pull/498