RFR 9: 8138696 : java.lang.ref.Cleaner - an easy to use alternative to finalization

[Peter Levart]

Hi Roger,

I now think that this is the right move. The simplified API is really 
only useful for PhantomReference(s). Since Cleaner.Cleanable interface 
does not provide access to the referent, it would be tempting for users 
to do two things at once for the same referent with Weak or Soft 
reachability:

- register cleanup function with: Cleaner.registerWeak(referent, () -> 
...); and
- wrap the referent with: WeakReference<Referent> wr = new 
WeakReference<>(referent);

...to get both features: obtainable referent and automatically managed 
cleanup.

This would of course work, but would not be very efficient. In 
Phantom-only world, they will not be tempted to do such thing.

Regards, Peter

On 12/02/2015 09:23 PM, Roger Riggs wrote:
> Please review the java.lang.ref.Cleaner and tests following the 
> recommendation to simplify the public
> interface to support only phantom cleanup.
>
> Webrev:
>   http://cr.openjdk.java.net/~rriggs/webrev-cleaner-8138696/
>
> Javadoc:
>    http://cr.openjdk.java.net/~rriggs/cleaner-doc/index.html
>
> Thanks, Roger
>
>
> On 11/23/2015 5:32 PM, mark.reinhold at oracle.com wrote:
>> ( Finally getting back to this, after too many weeks of travel ... )
>>
>> 2015/10/20 11:28 -0700, roger.riggs at oracle.com:
>>> Sorry for the silence, JavaOne preparations and the availability of
>>> folks who wanted to review have stretched things out.
>>>
>>> The Cleaner API was very simple and saw feature creep as the ideas for
>>> how it might be used were explored.  There are concerns about
>>> committing to supporting subclassable CleanableReferences in all
>>> future JDK versions before there had been a chance to see how if they
>>> would be useful and necessary to address the need to reduce the use of
>>> finalization within the OpenJDK and beyond.
>>>
>>> ...
>>>
>>> Updated Javadoc:
>>>     http://cr.openjdk.java.net/~rriggs/cleaner-doc/
>>>
>>> Updated Webrev:
>>> http://cr.openjdk.java.net/~rriggs/webrev-cleaner-8138696/
>> I'm very happy to see this API return to something like its original
>> simple form, but I think it can be simplified even further.
>>
>> We have a very strong need for phantom-ref-based cleaners, so as to
>> discourage people from relying upon flaky finalization. The arguments in
>> support of the weak and soft forms have, by contrast, been rather weak
>> (and soft?).  I don't think it's right to bake methods into a core API
>> based on just a couple of hypothetical use cases.  I'd much rather see
>> the Cleaner::{phantom,soft,weak}Cleanable methods reduced to a single
>> register method,
>>
>>      Cleaner.Cleanable register(Object, Runnable);
>>
>> which would create the phantom form only.  If strong justification for
>> the other forms arise then we can generalize this later, either to
>> distinct register{Soft,Weak} methods or, perhaps, to a method that takes
>> a type token.
>>
>> - Mark
>