RFR 9: 8138696 : java.lang.ref.Cleaner - an easy to use alternative to finalization

Daniel Fuchs daniel.fuchs at oracle.com
Wed Oct 21 13:58:18 UTC 2015

Hi Roger, Peter,

On 21/10/15 15:38, Roger Riggs wrote:
> Hi Peter,
> I've always assumed that the arguments could not be gc'd at least until
> the method returns
> in part because the caller was holding references.  But I suppose in a
> completely inlined
> case an optimizer might try to make more relaxed conclusions.
> Since the new object has not yet been published, the reachability of
> objects in a constructor
> must have some additional rules to keep objects alive until it is
> published.

I've recently been bitten by a test that did the rough
equivalent of this:

    Object o = new Object();
    ReferenceQueue<Object> queue = new ReferenceQueue<>();
    WeakReference<Object> ref = new WeakReference(o, queue);
    o = null;
    WeakReference<?> wr = queue.remove();

The issue was that the reference 'ref' was sometime
eagerly garbage collected before 'o', and the call to
queue.remove() sometimes never returned.
I should have known (because it had  happened to me before)
but these kind of issues are quite easy to miss, and have a
propensity to wait for some weeks after integration before
making themselves known...

I do hope that method parameters are not eagerly gc'ed before
the method returns - that would be yet another thing to
easily miss :-(

best regards,

-- daniel

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