JEP 132: More-prompt finalization (Re: Another take on Finalization)

Peter Levart peter.levart at gmail.com
Mon Jun 8 20:20:38 UTC 2015



On 06/08/2015 09:47 PM, Kim Barrett wrote:
> On Jun 6, 2015, at 11:02 AM, Peter Levart <peter.levart at gmail.com> wrote:
>> I understand that it would be desirable for a finalizable object to be made "untracked" as soon as it is manually cleaned-up. This would most certainly give a relief to GC as it could reclaim such untracked objects immediately like normal objects without pushing them through all the finalization steps.
>>
>> Such feature would need new API. Here's my take on such API incorporated in my prototype. This feature is mostly independent of other changes in the prototype and could be provided stand-alone. I choose to incorporate it into the prototype to compare it's overhead with classical finalization in unchanged and changed JDKs:
>>
>>      http://cr.openjdk.java.net/~plevart/misc/JEP132/ReferenceHandling/webrev.03/
> While changes are needed to do this sort of thing, it seems to me a better change would be to not use finalize() at all,

You mean FinalReference, right? This is the only remaining Reference 
type that isn't cleared automatically by GC. So if a XxxReference is 
cleared when it is discovered with referent being xxx-ly-reachable and 
later enqueued by ReferenceHandler thread, then GC has less work with 
it? Why is automatically cleared Reference more lightweight than 
FinalReference? Because GC can reclaim such referent right away in the 
same GC cycle as hooking it on the pending list?

> but instead use PhantomReference-based cleanup. It provides exactly that kind of behavior, and has other benefits besides.
>
> There is some discussion about doing exactly that for File{Input,Output}Stream (see https://bugs.openjdk.java.net/browse/JDK-8080225).
>

sun.misc.Cleaner is already such thing (a PhantomReference). Would it be 
of interest to have a public API similar to sun.misc.Cleaner, but with 
the following difference:

- public "Cleaners" would be executed by same thread(s) as finalize() 
methods. This would detach their execution from ReferenceHandler thread 
which must dispatch all references and must not be disturbed by user code.

Regards, Peter

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