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

[Peter Levart]

Hi Roger,

This is a nice and clean API. I like the trick with embedding a private 
CleanerImpl in Cleaner and registering Cleaner itself as a tracked 
object so that automatic thread termination can be performed in a safe 
manner. You say the scope of this is not to replace internal usages of 
sun.misc.Cleaner. But if they ever get replaced, there are two 
differences between the implementations to note:

- sun.misc.Cleaner thunks get executed by a ReferenceHandler thread 
directly and bypass ReferenceQueue-ing. This might be OK for internal 
use, but not appropriate for public API. I doubt this sun.misc.Cleaner 
exception in ReferenceHandler is necessary though.
- With sun.misc.Cleaner one has to create a minimum of 2 additional 
objects per tracked object: the Cleaner object and a Runnable thunk. 
This API requires creation of 3 objects per tracked object: the Cleanup 
object, the internal Reference object and a Runnable thunk.

Do you think Cleaner will not be used in scenarios where this additional 
footprint matters?

It might be possible to merge the roles of Cleanup and Reference into 
one object, like this:

http://cr.openjdk.java.net/~plevart/jdk9-dev/Cleaner/Cleaner.java

Making Cleanup an interface, internal Reference(s) can implement it. One 
can cast a Cleanup instance to a Reference and invoke it's other 
methods, but this can be prevented by throwing 
UnsupportedOperationException from them, so nobody is tempted to employ 
this implementation detail.

There might be utility in exposing Cleanup (or better named Cleanable) 
References as public abstract classes so that a footprint sensitive 
application (for example in a data structure with many elements) is 
possible. By subclassing such an abstract reference and implementing the 
abstract method, a single object is required per tracked object. Here I 
derived a proof of concept from your code:

     http://cr.openjdk.java.net/~plevart/jdk9-dev/Cleaner/webrev.01/

The simple API is unchanged and there is additional low-level API that 
can be used if footprint matters.

What do you think?

Regards, Peter


On 10/01/2015 04:12 PM, Roger Riggs wrote:
> Please review a proposal for public Cleaner API:
>
> A Cleaner is proposed to provide an easy to use alternative to 
> finalization. The service would provide easy registration and 
> cancellation of cleanup functions for objects. Applications create a 
> cleanup service for their own use and the service terminates when it 
> is no longer in use.
>
> Finalization has a long history of issues both in usage and 
> performance. PhantomReferences have been proposed as the alternative 
> GC based mechanism for cleaning functions but it has been left as an 
> exercise to the developer to construct the necessary mechanisms to 
> handle ReferenceQueues, handle threading issues and robust termination.
>
> The Cleaner performs cleaning functions when objects are unreachable 
> as found by garbage collection using the existing mechanisms of 
> PhantomReference, WeakReference, SoftReferences, and ReferenceQueues. 
> It manages a thread that dequeues references to unreachable objects 
> and invokes the corresponding cleaning function. Registered cleaning 
> functions can be cleared if no longer needed, can be invoked 
> explicitly to perform the cleanup immediately, or be invoked when the 
> object is not reachable (as detected by garbage collection) and 
> handled by a cleanup thread.
>
> The java.lang.ref package is proposed for the Cleaner because it is 
> complementary to the reference classes and reference queues and to 
> make it easy to find.
>
> It is not a goal to replace all uses of finalization or 
> sun.misc.Cleaner in the JDK.
> Investigation will evaluate if and in what cases the Cleaner can 
> replace finalization.
> A subsequent task will examine uses of finalization and propose 
> specific changes
> on a case by base basis.
>
> Please review and comment:
>
> Javadoc:
>   http://cr.openjdk.java.net/~rriggs/cleaner-doc/
>
> Webrev:
>    http://cr.openjdk.java.net/~rriggs/webrev-cleaner-8138696/
>
> Issue:
>    https://bugs.openjdk.java.net/browse/JDK-8138696
>
> Thanks, Roger
>