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

[Peter Levart]

Hi Zoltan,

On 11/11/2016 10:25 AM, Zoltán Majó wrote:
> Hi,
>
>
> please review the fix for 8169000:
>
> https://bugs.openjdk.java.net/browse/JDK-8169000
> http://cr.openjdk.java.net/~zmajo/8169000/webrev.00/
>
> The bug was filed because different behavior of interpreted and 
> compiled code in HotSpot was observed (different behavior with respect 
> to phantom references).  After discussions with Maurizio C, Alex B, 
> and David H, the best way to address this problem seems to be to 
> update update the documentation of the java.lang.ref to avoid 
> confusion in the future.  David's comment in the bug report [1] 
> accurately and concisely summarizes the reasons for the suggested 
> patch.  For more details please feel free to look at the comments in 
> the bug report.
>
> Thank you!
>
> Best regards,
>
>
> Zoltan
>
> [1] 
> https://bugs.openjdk.java.net/browse/JDK-8169000?focusedCommentId=14021250&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-14021250
>

I think the wording could be even less specific about "detecting" the 
reachability of the reference object. For example:

... If a registered reference becomes unreachable itself, then it *may* 
never be enqueued.


In addition, the situations that describe when the reference *may* not 
be enqueued could be expanded. For example:

... If a registered reference ceases to be strongly reachable itself, 
then it *may* never be enqueued.


The following modified test shows this situation:


public class WeaklyReachablePhantomReference {

     static ReferenceQueue<Object> rq = new ReferenceQueue<>();
     static WeakReference<PhantomReference<Object>> weakRefRef;

     public static void main(final String[] args) throws Exception {
         weakRefRef = new WeakReference<>(
             new PhantomReference<>(
                 new Object(),
                 rq
             )
         );
         // <- here
         System.gc();
         Reference rmRef = rq.remove(1000);
         if (rmRef == null) {
             System.out.println("PhantomReference NOT enqueued");
         } else {
             System.out.println("PhantomReference enqueued");
         }
     }
}


At "<-- here" the PhantomReference object becomes weakly reachable while 
its referent becomes phantom reachable and this is enough for 
PhantomReference to not be enqueued.


Regards, Peter