RFC (csr): 8221528: Introduce compatibility mode with VM option -XX:AllowRedefinitionToAddOrDeleteMethods

[Volker Simonis]

Hi Serguei,

can you please detail a little bit on the reasons why you finally
decided to change the implementation to conform to the spec instead of
just updating the spec to endorse a behavior which is there since
quite some time?

I read trough the comment thread of "JDK-8192936" and it seems to me
that right until your last comment from today, the general conses has
been to change the specs rather than the implementation. I understand
from the various comments in that issue that changing the specs is not
trivial but still doable. Unfortunately, some of the referenced
material in that issue is not available outside Oracle (i.e.
wiki.se.oracle.com) - it would be nice if you could make these
information available in the OpenJDK wiki or somewhere else.

I think that adding private methods is a feature widely used by a lot
of tools and removing it will break them. Deprecating this feature in
a non-LTS release and removing it after "a couple" of (non-LTS)
releases won't help a lot because many productive systems won't use
these releases anyway.

Thank you and best regards,
Volker

On Thu, Mar 28, 2019 at 1:57 AM serguei.spitsyn at oracle.com
<serguei.spitsyn at oracle.com> wrote:
>
> Hi All,
>
> This is request for comments and potentially reviews for the following CSR:
>   https://bugs.openjdk.java.net/browse/JDK-8221528
>
>
> Problem
>
> Now, the implementation of JVMTI RedefineClasses and RetransformClasses allows to
> add/delete private static and private final instance methods in the new class versions.
>
> It means that current implementation does not follow the JVM TI spec which explicitly states:
>
> "The redefinition must not add, remove or rename fields or methods, change the signatures
>  of methods, change modifiers, or change inheritance."
>
> For details, please, see the spec:
>
> https://docs.oracle.com/en/java/javase/12/docs/specs/jvmti.html#RedefineClasses
> https://docs.oracle.com/en/java/javase/12/docs/specs/jvmti.html#RetransformClasses
>
> The decision was made to align the implementation with the spec.
> For reference, please, see the https://bugs.openjdk.java.net/browse/JDK-8192936.
>
> This decision is going to cause some inconveniences to the customers.
>
>
> Solution
>
> The solution is to introduce a compatibility mode with new command-line VM option:
>    -XX:{+|-}AllowRedefinitionToAddOrDeleteMethods
>
> New option will enable old behavior and allow the JVM TI RedefineClasses and RetransformClasses
> to add/delete private static and private final instance methods in the new class versions.
> Without this option the old behavior will be disabled.
>
> New option is deprecated right away.
> The plan is to keep it for a couple of releases to allow customers (tool vendors)
> to remove dependency on old behavior from their tools.
>
> Thanks,
> Serguei