JDK 21 is now in Rampdown Phase One

[Mark Reinhold]

Per the JDK 21 schedule [1], we are now in Rampdown Phase One.

The overall feature set [2] is frozen, except for one JEP that is in
flight to be dropped [3].  No further JEPs will be targeted to this
release.

We’ve forked the main-line source repository to the jdk21 stabilization
repository (https://github.com/openjdk/jdk21).  Any changes pushed to
the main line are now bound for JDK 22, as noted previously [4].

The stabilization repository is open for select bug fixes and, with
approval, late enhancements per JEP 3, the JDK Release Process [5].
If you’re responsible for any of the bugs on the RDP 1 candidate-bug
list [6] then please see JEP 3 for guidance on how to handle them.

In contrast to past practice, in this release we will integrate most
stabilization changes via backports from the main-line repository [7].

  - Most fixes and enhancements intended for the stabilization repository
    will also be applicable to the main-line repository.  Integrate such
    a change into the main-line repository first.  Then, after you have
    obtained any required approvals, backport it to the stabilization
    repository using the Skara `/backport` command [2] or, if necessary,
    by manually opening a backport PR with the title `Backport $HASH`,
    where `$HASH` is the original commit hash.  (The Developers’ Guide
    contains more information on working with backports [8].)

  - Some fixes will be specific to the stabilization repository and not
    applicable to the main-line repository.  Integrate such a change
    directly into the stabilization repository.

In order to make sure that no backports are missed, prior to the RDP 2
and RC phases JDK Contributors will be reminded to review changes that
have been integrated into the main-line repository but not backported
to the stabilization repository.  These JBS queries can be useful:

  - Changes not backported from the main line
    https://bugs.openjdk.org/issues/?filter=44264

  - My changes not backported from the main line
    https://bugs.openjdk.org/issues/?filter=44268

- Mark


[1] https://openjdk.org/projects/jdk/21/#Schedule
[2] https://openjdk.org/projects/jdk/21/#Features
[3] https://mail.openjdk.org/pipermail/jdk-dev/2023-June/007910.html
[4] https://mail.openjdk.org/pipermail/jdk-dev/2023-June/007894.html
[5] https://openjdk.org/jeps/3
[6] https://j.mp/jdk-rdp-1
[7] https://openjdk.org/jeps/3#Integrating-fixes-and-enhancements
[8] https://openjdk.org/guide/#working-with-backports-in-jbs