RFR: JDK-8268088: Clarify Method::clear_jmethod_ids() related comments in ClassLoaderData::~ClassLoaderData() [v2]

[Coleen Phillimore]

On Mon, 7 Jun 2021 18:40:57 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Jiangli Zhou has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Remove whitespace.
>>  - Updated based on Evgeny's comments.
>
> src/hotspot/share/classfile/classLoaderData.cpp line 705:
> 
>> 703:   // been derived. After the class is unloaded, the method or field ID becomes
>> 704:   // invalid". In real world usages, the native code may rely on jmethod_ids
>> 705:   // being NULL after class unloading. Hence, it is unsafe to free the memory
> 
> Do we have any use cases of what a real world application can do with a NULL'ed out jmethodID (other than JVM TI will give an JVMTI_ERROR_INVALID_METHODID for it)?
> All or most of the JNI functions will crash with a NULL'ed out jmethodID.  You can catch this error by using -Xcheck:jni.  Maybe we could deprecate this feature of the JVMTI spec and release the memory?

This comment looks like an expanded version of the one I wrote a long time ago and correct.  We were wondering if this is a feature that we actually need.

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PR: https://git.openjdk.java.net/jdk/pull/4383