[9] RFR (XS): 8169000: Define reference reachability more precisely in java.lang.ref package

[David Holmes]

On 17/11/2016 10:00 AM, Peter Levart wrote:
> Hi David,
>
>
> On 11/17/2016 12:35 AM, David Holmes wrote:
>>> Why are we insisting in specifying when it is guaranteed for a Reference
>>> object to *not* be enqueued. This is not very helpful.
>>
>> I disagree. The whole point of the statement was to make it clear that
>> for a referent to be enqueued the referee reference type must itself
>> remain reachable. The problem with the statement was that it could be
>> misconstrued as indicating when a referent _must_ not be enqueued.
>
> The original text is correct: "If a registered reference becomes
> unreachable itself, then it will never be enqueued".
>
> And we must actually read it as "then the reference  _must_ not be
> enququed".

You are presuming the reader understands precisely what "becomes 
unreachable" means. It does not mean "I looked at the source code and at 
this point the reference is unused therefore it is unreachable and 
therefore the referent must never be enqueued". It means that the VM 
detected that the reference was unreachable. Hence the clarification.

David

> The transitioning of reachability from unreachable to strongly reachable
> is not allowed. If a registered reference becomes unreachable then it
> _must_ not be enqueued as enqueueing would transition it to strongly
> reachable from unreachable and break the spec.
>
> Regards, Peter
>
>>> A more helpful information for a programmer would be to specify when a
>>> Reference object is guaranteed to *be* enqueued. Guarantee is given only
>>> if the Reference object is kept strongly reachable. If it ceases to be
>>> strongly reachable, it may never be enqueued, but it is not guaranteed
>>> that it will not be enqueued either.
>>>
>>> So this is still my favorite:
>>>
>>> "If a registered reference ceases to be strongly reachable for any
>>> period of time, it may become unreachable and never be enqueued. In
>>> order to guarantee for a registered reference to be enqueued, it must be
>>> kept strongly reachable at all times."
>>
>> That seems redundant to me. Given the reachability definitions it
>> reduces to "If a registered reference becomes unreachable it may never
>> be enqueued."
>>
>> David
>