jmethodIDs are basically handles of Method*.

In generated JVMTI functions such as jvmti_GetMethodDeclaringClass, we will first check if a jmethodID is valid (i.e., if it contains a non-NULL Method*). If so, we will dereference the Method*:

===================================
jvmti_GetMethodDeclaringClass(jvmtiEnv* env,
            jmethodID method,
            jclass* declaring_class_ptr) {
  if(JvmtiEnv::get_phase(env)!=JVMTI_PHASE_START && JvmtiEnv::get_phase()!=JVMTI_PHASE_LIVE) {
    return JVMTI_ERROR_WRONG_PHASE;
  }
  Thread* this_thread = Thread::current_or_null(); 
  if (this_thread == NULL || !this_thread->is_Java_thread()) {
    return JVMTI_ERROR_UNATTACHED_THREAD;
  }
  JavaThread* current_thread = this_thread->as_Java_thread();
  MACOS_AARCH64_ONLY(ThreadWXEnable __wx(WXWrite, current_thread));
  ThreadInVMfromNative __tiv(current_thread);
  VM_ENTRY_BASE(jvmtiError, jvmti_GetMethodDeclaringClass , current_thread)
  debug_only(VMNativeEntryWrapper __vew;)
  PreserveExceptionMark __em(this_thread);
  JvmtiEnv* jvmti_env = JvmtiEnv::JvmtiEnv_from_jvmti_env(env);
  if (!jvmti_env->is_valid()) {
    return JVMTI_ERROR_INVALID_ENVIRONMENT;
  }
  jvmtiError err;
  Method* checked_method = Method::checked_resolve_jmethod_id(method);
  if (checked_method == NULL) {
      return JVMTI_ERROR_INVALID_METHODID;
  }
  if (declaring_class_ptr == NULL) {
      return JVMTI_ERROR_NULL_POINTER;
  }

  <<<<<<< ZGC can concurrently deallocate the checked_method

  err = jvmti_env->GetMethodDeclaringClass(checked_method, declaring_class_ptr);  <<<<< crash
  return err;
}


===================================
 
This in not compatible with GCs that perform concurrent class deallcation (such as ZGC). If <method> belongs to a class that has been collected (see JDK-8268088), but not yet deallocated, <checked_method> could be non-NULL, but will be concurrently deallocated by ZGC. So by the time we call into jvmti_env->GetMethodDeclaringClass, <checked_method> already points to bad memory.

Note: ZGC performs metadata deallocation outside of a safepoint. See JDK-8267879 for a case where ClassLoaderData::~ClassLoaderData() is called by ZGC while a safepoint is still in session.