RFR: 8268406: Deallocate jmethodID native memory [v2]

[Axel Boldt-Christmas]

On Mon, 16 Jun 2025 17:37:17 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.
>
> Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix formatting errors.

src/hotspot/share/oops/jmethodIDTable.cpp line 126:

> 124: static bool needs_resize(Thread* current) {
> 125:   return ((_jmethodID_counter > (_resize_load_trigger * table_size(current))) &&
> 126:          !_jmethod_id_table->is_max_size_reached());

Should we not just have a separate jmethodID entry count variable we use here instead, that is incremented and decremented on insert and remove. Rather than using the next jmethodID counter which just grows monotonically regardless of any removals.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25267#discussion_r2151348302