[concurrency-interest] Object finalization

Boehm, Hans hans.boehm at hp.com
Mon May 14 16:49:59 EDT 2012


But things are considerably worse than that.  References only solve the easy problem.  The most serious problem in my mind is that finalizers can run, and references can be enqueued, while e.g. a method of the object being finalized is still running.  The fact that the method is still running does not ensure that the object itself is still reachable; the method may only need fields that have already been cached in registers to complete.  This affects finalizers and java.lang.ref equally, and explains why probably the large majority of code using either one is broken.

I gave a JavaOne talk about this many years ago (slides at http://www.hpl.hp.com/personal/Hans_Boehm/misc_slides/java_finalizers.pdf).  It is possible, though really ugly and slow to work around the problem.  I don't believe anyone does.  I was told just after I gave the talk that this explained a problem somebody had been trying to track down, so I suspect this actually happens occasionally in the wild, though probably not on x86-32.

Hans

From: concurrency-interest-bounces at cs.oswego.edu [mailto:concurrency-interest-bounces at cs.oswego.edu] On Behalf Of Bob Lee
Sent: Monday, May 14, 2012 11:01 AM
To: dholmes at ieee.org
Cc: concurrency-interest at cs.oswego.edu
Subject: Re: [concurrency-interest] Object finalization

On Sun, May 13, 2012 at 11:22 PM, David Holmes <davidcholmes at aapt.net.au<mailto:davidcholmes at aapt.net.au>> wrote:
I should add the Vitaly's comments prompted me to remember that 'a' and 'b' might refer to objects that themselves have been finalized prior to the current finalizer running. This just reinforces how tricky finalization is.

Indeed, the finalizers can run in any order, independent of the structure of the object graph.

For those who are interested in learning more, I cover that and half a dozen other reasons not to use finalizers in this talk: http://www.parleys.com/#id=2657&st=5

Thanks,
Bob
Square is hiring!<https://squareup.com/jobs>

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