Proposal: Improve java.lang.ref processing in Java SE 8 and JDK 8

[Volker Simonis]

Hi Iris,

Thanks for the heads-up. Is there any evidence where this has caused
compatibility issues?

Also, I see that JDK-8175797 [1] (which is the implementation
CCC-8175797) introduced the property
`jdk.lang.ref.disableClearBeforeEnqueue` which can be used to maintain
backward compatibility with the old behavior. However, this property
wasn't really functional until it was fixed by JDK-8178832. Do you
plan to downport JDK-8178832 as well?

Also, more generally, what is the "critical issue" that justifies a
new Java 8 MR? I expect there are a lot of fixes in later Java
specifications and it would be interesting to know when it is
appropriate and justified to downport them and create a new Java 8 MR?

Thank you and best regards,
Volker

[1] https://bugs.openjdk.java.net/browse/JDK-8175797
[2] https://bugs.openjdk.java.net/browse/JDK-8178832

On Wed, Apr 13, 2022 at 6:12 PM Iris Clark <iris.clark at oracle.com> wrote:
>
> A critical issue has been identified in the Java SE 8 Platform, pertaining to
> the cloning of java.lang.ref.Reference objects.
>
> While investigating this issue, two related issues were identified in the
> processing of Reference objects.
>
> The issues are:
>
>   - (Critical) The Java SE 8 Specification does not clearly define the
>     semantics of cloning a Reference object.  During Java SE 11 development,
>     it was determined that a Reference object cannot be meaningfully cloned
>     since its reachability may change during garbage collection reference
>     processing.  Thus, Reference.clone() was specified in Java SE 11 to always
>     throw a CloneNotSupportedException. [0]
>
>   - (Related) The Java SE 8 Specification does not prohibit the referent of a
>     phantom reference from being retrieved between the time that the reference
>     is enqueued and the time it is cleared by the garbage collector.  To
>     eliminate this possibility, the specification of PhantomReference was
>     updated in Java SE 9 to automatically clear reference objects when they
>     are enqueued.  This update better aligned PhantomReference with
>     SoftReference and WeakReference. [1]
>
>   - (Related) The Java SE 8 Specification allows an application to explicitly
>     add a Reference object to its registered queue before it is cleared by the
>     garbage collector.  In Java SE 9, the specification of Reference.enqueue()
>     was modified to always clear the reference object before it is enqueued.
>     This aligns application code invocations of this method with the behaviour
>     of garbage collectors. [2]
>
> To resolve these issues for future JDK updates, I'll shortly propose a
> Maintenance Release of the Java SE 8 Platform JSR [3] in the JCP to backport
> these three changes from Java SE 9 and 11.  This will require updates to the
> Specification and the Reference Implementation (RI), which I and my colleagues
> at Oracle will provide.  The TCK does not require updates.  I expect the
> Maintenance Release process to complete by July 2022, in time for these
> changes to be merged into the October security releases of JDK 8.
>
> Following the practice introduced in the previous Maintenance Release
> (February 2020), we want to reduce risk by basing the open-source RI on
> OpenJDK 8u41 [4], the RI for the previous Maintenance Release, rather than the
> most recent OpenJDK 8 Updates release.  We propose to label the RI build as
> "8u42" in order to convey that it is outside the contemporary train of update
> releases, and (like the 8u41 build before it) is neither meant for production
> use nor updated with security fixes.
>
> If it's not too much work then we'll also contribute the changes required by
> the MR to the next appropriate OpenJDK 8 Updates release, most likely 8u352.
>
> Comments?
>
> Iris
>
> [0]: https://bugs.openjdk.java.net/browse/JDK-8202260
> [1]: https://bugs.openjdk.java.net/browse/CCC-8071507
> [2]: https://bugs.openjdk.java.net/browse/CCC-8175797
> [3]: https://jcp.org/en/jsr/detail?id=337
> [4]: https://hg.openjdk.java.net/jdk8u/jdk8u41