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

[Andrew Haley]

On 11/24/2015 05:47 PM, Roger Riggs wrote:

> Memory is an increasingly critical resource, we should be giving
> developers more tools to manage their use of memory.  The Weak and
> Soft reference forms of the cleaner make it easier to be aware of
> and respond to increased memory pressure.
> 
> The management of memory is not hypothetical for most large
> applications.  It was one topic that came up in questions at J1.
> Developers do quite a bit of work trying to figure out algorithms
> for working caches, serializing to disk, and having the data
> available when it is needed.  There is a lot of guesswork about how
> GC is working and how it behaves at/near the limits.  SoftReferences
> are a bit of a blunt instrument but they provide evidence an
> application can use to regulate its long term memory use.
> 
> The WeakReference forms, if provided, can be an alternative to the
> ideosyncratic cleanup approaches used in various Weak keyed and weak
> values collections.  If there are other mechanisms contemplated for
> more efficient memory management then perhaps these are not
> necessary but if not the current mechanisms should be easier to use.

By "memory" here, do you mean native memory for buffers, etc?  I'm
guessing so.  If so, I'm not sure that it makes sense to think of this
as a cure for flaky finalization.  We've got a cure for early
finalization now with keepAlive() (or whatever it gets called) but
late (or never) finalization is as far as I can see unfixable.  IMVHO
it makes more sense to encourage developers to get away from lifecycle
maintenance based on reachability.

Unfortunately the current design for ByteBuffers does not allow
unmap(), so large mapped buffers may hang around for a long time.
4724038 says

 "We ... have given this problem a lot of thought ...  We have yet to
 come up with a way to implement an unmap() method that's safe,
 efficient, and plausibly portable across operating systems.  We've
 explored several other alternatives aside from the two described
 above, but all of them were even more problematic.  We'd be thrilled
 if someone could come up with a workable solution, so we'll leave
 this bug open in the hope that it will attract attention from someone
 more clever than we are."

I'm very tempted to take a bite at this, but the above text is rather
forbidding.  I think I know how to do it. (Famous last words?)

Andrew.