[concurrency-interest] Atomicity of clearing of WeakReferences

Gregg Wonderly gregg at cytetech.com
Wed Jul 15 12:13:17 EDT 2009


If you need to cleanup after something is unreferenced, PhantomReference may be 
a good choice.  I used it to construct my ReferenceTracker class ( see 
<http://cs.oswego.edu/pipermail/concurrency-interest/2008-December/005724.html>)

The one warning I will give, is that after doing this, I found that I just could 
not use it in all the places I wanted, because of the Sun JVM "bug" that 
nominates objects for finalization, before ALL references are out of scope.

This causes the objects to be posted to the ReferenceQueue before they are 
actually "dead".  This means that any state you change, which is referenced by 
that object, in finalization, can be seen by the object before all references to 
the object have been used and gone out of scope.  Hans went into a lot of detail 
about this issue in many contexts it seems (see 
<http://gceclub.sun.com.cn/java_one_online/2005/TS-3281/ts-3281.pdf>)as follow 
up to the long trail of posts comming from Doug Lea's post at

<http://cs.oswego.edu/pipermail/concurrency-interest/2009-January/005743.html>

which is the fences API discussion.  Without Fences, or some other mechanics in 
the JIT to automatically manage this issue, it's not much of a real solution 
because you have to always think about scoping to make sure that refences last 
longer than the JIT currently documents them to exist as visible by the GC.  And 
PhantomReference would really help developers to not have to worry so much about 
scoping...

This is a tricky issue I discovered and I'd say that the way it works now is 
wrong, if not an out right bug.  Finalization should not be invoked (directly or 
indirectly through PhantomReference queues etc) until there are no more 
references to the value in registers, on the stack, or in scoped variables etc.

This misbehavior really makes automated reference tracking for limited resources 
nearly impossible when they are used by short lived bits of code that are prime 
for optimization and registerization that appears to hide references from the GC.

Gregg Wonderly


Karnok Dávid wrote:
> Composition over inheritance? Have the WR support for an internal 
> cleanup context object (maybe in an internal WR too)?
> 
>  
> 
> David
> 
>  
> 
>>  If you need to clean up after the referent, you don't have the object 
> around anymore, so you store any state necessary for the cleanup in the 
> WeakReference subclass.
>>
>>  Bob 
> 
> 
> 
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