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

[David Holmes]

Hi Thomas,

On 28/03/2019 5:02 pm, Thomas Stüfe wrote:
> Hi Serguei,
> 
> This is planned to be introduced in jdk 13?
> 
> The only concern I have is that both paths (old and new behavior) should 
> be well tested, and hiding the old behavior behind an optional switch 
> may reduce its test coverage considerably.

The old behaviour is hardly ever used - I'm not even sure we currently 
have a test that tries to use it - so it's already not "well tested" and 
there's no intent here of increasing test coverage for the code we plan 
to remove. The new path will be tested as we do have a test that expects 
to get the error. And Serguei will of course have a simple test that 
checks the flag with both values.

Cheers,
David


> Cheers, Thomas
> 
> (p.s. html format does not work well with the OpenJDK mailing list 
> archive, see 
> https://mail.openjdk.java.net/pipermail/serviceability-dev/2019-March/027612.html)
> 
> On Thu, Mar 28, 2019 at 1:57 AM serguei.spitsyn at oracle.com 
> <mailto:serguei.spitsyn at oracle.com> <serguei.spitsyn at oracle.com 
> <mailto: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
>