RFR: 8277444: Race condition on Instrumentation.retransformClasses() and class linking [v3]

[Coleen Phillimore]

On Wed, 27 Aug 2025 06:37:44 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> (I realize this is a tangent, but maybe there is a separate bug here...)
>> 
>>> To have a Class object it must have already undergone the "preparation" part of linking (it is done at the end of loading when we create the class mirror).
>> 
>> If that's what "preparation" means, and Class.forName(name, false, loader) does not link the class, then posting the event here in InstanceKlass::link_class_impl() instead of earlier seems misplaced:
>> 
>> https://github.com/openjdk/jdk/blob/c75534517729b903b63263cf64dc2ff841e3dcb1/src/hotspot/share/oops/instanceKlass.cpp#L1064
>> 
>> Class.forName(name, false, loader) would result in a prepared class but without the event being sent.
>
>> If that's what "preparation" means, and Class.forName(name, false, loader) does not link the class, then posting the event here in InstanceKlass::link_class_impl() instead of earlier seems misplaced
> 
> @dean-long it seems the JVM TI "prepare" event is somewhat misnamed. It states "At this point, class fields, methods, and implemented interfaces are available, and no code from the class has been executed." but that neither describes preparation, nor the rest of linking. You can only access fields and methods after a class has been initialized! But lets take this elsewhere if needed.

Does it fail with the patch?  Sorry for the delay.  @dholmes-ora and I have been discussing how all this works offline, but with time-zone differences, we won't have any agreement until next week.  I wrote a more limited version of the patch I sent and am testing it now.

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PR Comment: https://git.openjdk.org/jdk/pull/26863#issuecomment-3236952904