ThreadPoolExecutor and finalization

[David Holmes]

On 31/10/2017 1:02 AM, David Lloyd wrote:
> On Mon, Oct 30, 2017 at 9:43 AM, Roger Riggs <Roger.Riggs at oracle.com> wrote:
>> ThreadPoolExecutor has a responsibility to cleanup any native resources it
>> has allocated (threads) and it should be free to use whatever mechanism is appropriate.
> 
> I wonder though whether TPE.finalize() is ever actually hit in
> practice: TPE is (by definition) a thread pool, and every live thread
> in that pool has (by way of TPE.Worker) a strong reference to the TPE
> itself via an outer class reference which is passed around in enough
> places to make me think that it would never actually be collectable in
> a real-world situation, unless all core threads were allowed to time
> out.

The docs for TPE cover this in detail: [1]

Finalization
     A pool that is no longer referenced in a program AND has no 
remaining threads will be shutdown automatically. If you would like to 
ensure that unreferenced pools are reclaimed even if users forget to 
call shutdown(), then you must arrange that unused threads eventually 
die, by setting appropriate keep-alive times, using a lower bound of 
zero core threads and/or setting allowCoreThreadTimeOut(boolean).

David
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[1] 
https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/ThreadPoolExecutor.html