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

[Roger Riggs]

Hi Jason,

The caller that creates the Cleaner does contribute some of its state to 
the new Thread
including the normal inheritance of the ContextClassLoader, 
InheritableThreadLocals, etc.

So yes, if in your context the ThreadFactory would take precautions then 
the thread factory used
for the Cleaner would be no different.

The default factory uses sun.misc.InnocuousThread which explicitly sets
the context classloader to the System class Loader and discards 
inheritable thread locals.

Is there an example of the warning you suggest elsewhere in the JDK?

Thanks, Roger



On 10/2/2015 10:14 AM, Jason Mehrens wrote:
> Roger,
>
> For custom thread factories, do we have to explictly set the CCL for the created thread or should that be a documented as a warning to all who use it?  In web apps that can be a form of a memory leak.
>
> Jason
>
> _______________________________________
> From: core-libs-dev <core-libs-dev-bounces at openjdk.java.net> on behalf of Roger Riggs <Roger.Riggs at Oracle.com>
> Sent: Thursday, October 1, 2015 9:12 AM
> To: Core-Libs-Dev
> Subject: RFR 9: 8138696 : java.lang.ref.Cleaner - an easy to use alternative    to finalization
>
> 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
>