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From Oliver Deakin <oliver.dea...@googlemail.com>
Subject Re: [classlib][testcase] should weakreference be queued in runFinalization()?
Date Fri, 13 Apr 2007 09:22:38 GMT
I can see what the tester was expecting - if a gc is definitely run, you 
might expect that the weak reference would be cleared. However, it is 
slightly more subtle than that since:
 - gc might not actually be run by the gc() call.
 - the gc might not necessarily determine that the object is weakly 
reachable in its first pass.

References are a very difficult area to test reliably precisely because 
of the non-committal nature of the spec on the gc() and runFnializers() 
methods. Any improvements or innovative approaches you can think of for 
these tests would be good! In it's current state, this test looks invalid.

Regards,
Oliver

Xiao-Feng Li wrote:
> On 4/13/07, Oliver Deakin <oliver.deakin@googlemail.com> wrote:
>> The 5.0 spec for runFinalization() says:
>>
>> "Calling this method suggests that the Java Virtual Machine expend
>> effort toward running the finalize methods of objects that have been
>> found to be discarded but whose finalize methods have not yet been run."
>>
>> and for gc():
>>
>> "Calling the gc method suggests that the Java Virtual Machine expend
>> effort toward recycling unused objects"
>>
>> The key word in both those specs is /suggests/. There is *no* guarantee
>> that any finalizers are run or that a gc actually occurs when these
>> calls are made - it is only a hint to the VM.
>>
>> If a test is expecting these calls to definitely gc and run finalizers,
>> then IMO the test is in error.
>
> Yes, I have the seem opinion. And both gc() and runFinalization()
> actually say nothing about weakreference. Don't know why they are used
> to test References.
>
> Thanks,
> xiaofeng
>
>> Regards,
>> Oliver
>>
>>
>> Xiao-Feng Li wrote:
>> > In classlib tests "gc.PhantomReferenceTest" and
>> > "tests.api.java.lang.ref.ReferenceTest", they expect weakreference
>> > objects be queued after System.runFinalization(). Is this correct? In
>> > my understanding of the spec, there is no requirement on this
>> > behavior.
>> >
>> > The tests do like this:
>> >
>> > =========================
>> > //wr is the weakreference, whose referent is only weakly reachable.
>> > //rq is the reference queue
>> >
>> > System.gc();
>> > System.runFinalization();
>> >
>> > ref = rq.poll();
>> >
>> > assertTrue("Unexpected ref2", ref == wr);
>> > assertNotNull("Object not garbage collected.", ref);
>> > assertNull("Object could not be reclaimed.", ref.get());
>> > =========================
>> >
>> > After runFinalization(), it requires the queue has the weakreference.
>> > Actually it has requirement on System.gc() as well, requiring it to
>> > identify the weakly reachable object accurately.
>> >
>> > In my understanding of the spec, this kind of tests are wrong. It
>> > forces the GC to do something not required by spec.
>> >
>> > How do you think?
>> >
>> > Thanks,
>> > xiaofeng
>> >
>>
>> -- 
>> Oliver Deakin
>> Unless stated otherwise above:
>> IBM United Kingdom Limited - Registered in England and Wales with 
>> number 741598.
>> Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire 
>> PO6 3AU
>>
>>
>
>

-- 
Oliver Deakin
Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598. 
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU


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