ThreadPoolExecutor and finalization

[Roger Riggs]

Hi David,

On 10/30/2017 3:31 AM, David Holmes wrote:
> Hi Andrej,
>
> On 30/10/2017 5:02 PM, Andrej Golovnin wrote:
>> Hi David,
>>
>>> On 30. Oct 2017, at 01:40, David Holmes <david.holmes at oracle.com> 
>>> wrote:
>>>
>>> Hi Roger,
>>>
>>> On 30/10/2017 10:24 AM, Roger Riggs wrote:
>>>> Hi,
>>>> With the deprecation of Object.finalize its time to look at its 
>>>> uses too see if they can be removed or mitigated.
>>>
>>> So the nice thing about finalize was that it followed a 
>>> nice/clean/simple OO model where a subclass could override, add 
>>> their own cleanup and then call super.finalize(). With finalize() 
>>> deprecated, and the new mechanism being Cleaners, how do Cleaners 
>>> support such usages?
>>
>> Instead of ThreadPoolExecutor.finalize you can override 
>> ThreadPoolExecutor.terminated.
>
> True. Though overriding shutdown() would be the semantic equivalent of 
> overriding finalize(). :)
>
> In the general case though finalize() might be invoking a final method.
>
> Anyway I'm not sure we can actually do something to try to move away 
> from use of finalize() in TPE. finalize() is only deprecated - it is 
> still expected to work as it has always done. Existing subclasses that 
> override finalize() must continue to work until some point where we 
> say finalize() is not only deprecated but obsoleted (it no longer does 
> anything). So until then is there actually any point in doing 
> anything? Does having a Cleaner and a finalize() method make sense? 
> Does it aid in the transition?
As you observe, the alternatives directly using PhantomRefs or the 
Cleanup do not provide
as nice a model.  Unfortunately, that model has been recognized to have 
a number of
issues [1].  Finalization is a poor substitute for explicit shutdown, it 
is unpredictable and unreliable,
and not guaranteed to be executed.

ThreadPoolExecutor has a responsibility to cleanup any native resources 
it has allocated (threads)
and it should be free to use whatever mechanism is appropriate. 
Currently, the spec for finalize
does not give it that freedom.

The initiative is to identify and remediate existing uses of 
finalization in the JDK.
The primary concern is about subclasses that reply on the current spec.
If I'm using grepcode correctly[2], it does not show any subclasses of 
ThreadPoolExecutor that
override finalize; so it may be a non-issue.

Regards, Roger

[1] https://bugs.openjdk.java.net/browse/JDK-8165641

[2] 
http://grepcode.com/search/usages?type=method&id=repository.grepcode.com%24java%24root@jdk%24openjdk@8u40-b25@java%24util%24concurrent@ThreadPoolExecutor@finalize%28%29&k=d