ThreadPoolExecutor and finalization

[David Holmes]

Hi Stuart,

Jumping to the end ...

On 1/11/2017 6:37 AM, Stuart Marks wrote:
> On 10/30/17 10:21 AM, Martin Buchholz wrote:
>>> The initiative is to identify and remediate existing uses of 
>>> finalization
>>> in the JDK.
>>
>> I've been skeptical about this initiative as stated.  I would not have
>> deprecated finalize(). We will never remove finalize() from the JDK, 
>> and I
>> don't see how switching TPE from finalize to some other mechanism such as
>> Cleaner has real benefits for users.  There aren't enough instances of 
>> TPE
>> created for finalization to be a real user performance problem.
> 
> Interesting that you say "we will never remove finalize()" ... it is 
> exactly the goal of this initiative to remove finalize() eventually. Or 
> at least to remove the finalization mechanism. It's been a thorn in the 
> side of GC implementors since forever. As Roger stated, the early part 
> of this effort is to remove uses from within the JDK, and to warn 
> external users to start migrating to other facilities. Hence, we've 
> deprecated it and are having this discussion.
> 
> I don't know what the later parts of the transition will look like. 
> Perhaps at some point we deprecate Object.finalize() for removal; 
> perhaps at some point the VM stops calling Object.finalize() even though 
> the method is declared; perhaps at some point we actually remove the 
> Object.finalize() method. All of this will require further discussion, 
> and it will be based on our experience working through these early 
> remediation steps.
> 
>> TPE's spec currently has a finalize deprecation warning, but this is not
>> helpful for users.
>> (a documentation readability regression!)
>> https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/ThreadPoolExecutor.html#finalize-- 
>>
> 
> I'm not sure why you say this isn't helpful. It's clearly not helpful to 
> *clients* of TPE; but since finalize() is protected, the warning is 
> clearly directed at subclasses, and it provides information about 
> migrating away from finalization. Should say something different?

It isn't helpful to people subclassing TPE (or any class that defines 
finalize()) because the general strategies described in 
Object.finalize() can't be applied by the subclass independent of the 
class (TPE here) it is subclassing. Unless TPE provides an alternate 
mechanism, the subclass is stuck with finalize().

David

> s'marks