void
JvmtiEventControllerPrivate::set_extension_event_callback(JvmtiEnvBase *env,
                                                          jint extension_event_index,
                                                          jvmtiExtensionEvent callback)
{

  {- -------------------------------------------
  (1) (assert)
      ---------------------------------------- -}

      assert(Threads::number_of_threads() == 0 || JvmtiThreadState_lock->is_locked(), "sanity check");

  {- -------------------------------------------
  (1) (トレース出力)
      ---------------------------------------- -}

      EC_TRACE(("JVMTI [*] # set extension event callback"));

  {- -------------------------------------------
  (1) (assert)
      ---------------------------------------- -}

      // extension events are allocated below JVMTI_MIN_EVENT_TYPE_VAL
      assert(extension_event_index >= (jint)EXT_MIN_EVENT_TYPE_VAL &&
             extension_event_index <= (jint)EXT_MAX_EVENT_TYPE_VAL, "sanity check");


  {- -------------------------------------------
  (1) 
      ---------------------------------------- -}

      // As the bits for both standard (jvmtiEvent) and extension
      // (jvmtiExtEvents) are stored in the same word we cast here to
      // jvmtiEvent to set/clear the bit for this extension event.
      jvmtiEvent event_type = (jvmtiEvent)extension_event_index;

      // Prevent a possible race condition where events are re-enabled by a call to
      // set event callbacks, where the DisposeEnvironment occurs after the boiler-plate
      // environment check and before the lock is acquired.
      // We can safely do the is_valid check now, as JvmtiThreadState_lock is held.
      bool enabling = (callback != NULL) && (env->is_valid());
      env->env_event_enable()->set_user_enabled(event_type, enabling);

      // update the callback
      jvmtiExtEventCallbacks* ext_callbacks = env->ext_callbacks();
      switch (extension_event_index) {
        case EXT_EVENT_CLASS_UNLOAD :
          ext_callbacks->ClassUnload = callback;
          break;
        default:
          ShouldNotReachHere();
      }

  {- -------------------------------------------
  (1) env->env_event_enable()->_event_callback_enabled の値を以下のように変更する.
      (これは, 「env 引数で指定された JvmtiEnvBase オブジェクト中の JvmtiEnvEventEnable オブジェクトの
       _event_callback_enabled フィールドに格納されている JvmtiEventEnabled オブジェクト」, ということ.
       つまり, コールバックを設定しているイベント一覧を示す JvmtiEventEnabled オブジェクトのこと.)

      enabling 引数が true であれば, 
      env->env_event_enable()->_event_callback_enabled の値を
      「現在の値に event_type 引数で指定された種別を示すビットを加えた値」に変更する.

      逆に enabling 引数が false であれば, 

      「現在の値から event_type 引数で指定された種別を示すビットをクリアした値」に変更する.

      ---------------------------------------- -}

      // update the callback enable/disable bit
      jlong enabled_bits = env->env_event_enable()->_event_callback_enabled.get_bits();
      jlong bit_for = JvmtiEventEnabled::bit_for(event_type);
      if (enabling) {
        enabled_bits |= bit_for;
      } else {
        enabled_bits &= ~bit_for;
      }
      env->env_event_enable()->_event_callback_enabled.set_bits(enabled_bits);

  {- -------------------------------------------
  (1) JvmtiEventControllerPrivate::recompute_enabled() を呼んで, 
      "truly enabled event" 情報を更新する.
      ---------------------------------------- -}

      recompute_enabled();
    }