RFR: 8268406: Deallocate jmethodID native memory

[Coleen Phillimore]

On Fri, 16 May 2025 12:18:42 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> This change uses a ConcurrentHashTable to associate Method* with jmethodID, instead of an indirection.  JNI is deprecated in favor of using Panama to call methods, so I don't think we're concerned about JNI performance going forward.  JVMTI uses a lot of jmethodIDs but there aren't any performance tests for JVMTI, but running vmTestbase/nsk/jvmti with in product build with and without this change had no difference in time.
> 
> The purpose of this change is to remove the memory leak when you unload classes: we were leaving the jmethodID memory just in case JVMTI code still had references to that jmethodID and instead of crashing, should get nullptr.  With this change, if JVMTI looks up a jmethodID, we've removed it from the table and will return nullptr.  Redefinition and the InstanceKlass::_jmethod_method_ids is somewhat complicated.  When a method becomes "obsolete" in redefinition, which means that the code in the method is changed, afterward creating a jmethodID from an "obsolete" method will create a new entry in the InstanceKlass table.  This mechanism increases the method_idnum to do this.  In the future maybe we could throw NoSuchMethodError if you try to create a jmethodID out of an obsolete method and remove all this code.  But that's not in this change.
> 
> Tested with tier1-4, 5-7.

Dan, thank you for your first pass.  I've tried to address the things you pointed out.  I think I've used 'null' correctly in comments and strings though.

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PR Comment: https://git.openjdk.org/jdk/pull/25267#issuecomment-2977460666