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

[Peter Levart]

Hi Zoltan,


On 11/16/2016 10:27 PM, David Holmes wrote:
> Peter has highlighted the risk with anything but the most minimal of 
> changes - the more you say the more likely you are saying something 
> that is incorrect. Reachability and the GC relationship to it is 
> extremely complex and can't be summarised in a few words.
>
> I would go back to the original simple suggestion to just add "detected":
>
> http://cr.openjdk.java.net/~zmajo/8169000/webrev.00
>
> "If a registered reference is detected as unreachable itself, then it 
> will never be enqueued."
>
> -- 
>
> That removes any expectation that you can determine whether or not it 
> will be unreachable based on looking at the source code or reasoning 
> about the bytecodes.

Yes, but it also gives very little helpful information to a programmer.

Why are we insisting in specifying when it is guaranteed for a Reference 
object to *not* be enqueued. This is not very helpful.

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."

Regards, Peter

>
> Cheers,
> David
> -----
>
>
> On 17/11/2016 2:37 AM, Peter Levart wrote:
>> Hi,
>>
>> On 11/16/2016 05:09 PM, Peter Levart wrote:
>>>>
>>>> http://cr.openjdk.java.net/~zmajo/8169000/webrev.03/
>>>
>>> "If a registered reference ceases to be strongly reachable itself (not
>>> by examining the source code but by looking at the actual state of the
>>> VM at runtime), it will never be enqueued."
>>>
>>> I think this is wrong wording. A Reference object may cease to be
>>> strongly reachable for some time (even by looking at the actual state
>>> of the VM at runtime) and then regain strong reachability and then be
>>> enqueued. If during the period that a Reference object is not strongly
>>> reachable (even by looking at the actual state of the VM at runtime),
>>> GC is not run, the Reference object will not be discovered as not
>>> being strongly reachable and VM will not clear any Soft or Weak
>>> references having the Reference object as a referent and consequently
>>> the program will be able to regain strong reachability to it.
>>
>>
>> A counter example: A Reference object may cease to be strongly reachable
>> by becoming softly reachable. The JVM may even discover it to be softly
>> reachable (looking at the actual state of the VM at runtime), but by
>> policy, VM may also see that is has not been softly reachable long
>> enough and so it will not clear the SoftReference but instead rather
>> enqueue the SoftReference's referent (our PhantomReference object):
>>
>> public class SoftlyReachablePhantomReference {
>>
>>     static ReferenceQueue<Object> rq = new ReferenceQueue<>();
>>     static Reference<PhantomReference<Object>> refRef;
>>
>>     public static void main(final String[] args) throws Exception {
>>         refRef = new SoftReference<>(
>>             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");
>>         }
>>     }
>> }
>>
>>
>> Running with -Xcomp, the above program will print: "PhantomReference
>> enqueued". By just swapping SoftRererence with WeakReference:
>>
>>
>> public class WeaklyReachablePhantomReference {
>>
>>     static ReferenceQueue<Object> rq = new ReferenceQueue<>();
>>     static Reference<PhantomReference<Object>> refRef;
>>
>>     public static void main(final String[] args) throws Exception {
>>         refRef = 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");
>>         }
>>     }
>> }
>>
>>
>> ...the program will print: "PhantomReference NOT enqueued".
>>
>> This is all expected and by the spec.
>>
>> Peter
>>