void
JvmtiEventController::set_user_enabled(JvmtiEnvBase *env, JavaThread *thread, jvmtiEvent event_type, bool enabled) {

  {- -------------------------------------------
  (1) JvmtiEventControllerPrivate::set_user_enabled() を呼び出す.

      (なお, HotSpot の起動中(= Threads::number_of_threads() が 0)であれば
       (他にスレッドがいないので) この呼び出し時に排他処理は必要ない.
       起動中でなければ, JvmtiThreadState_lock で排他した状態で呼び出す.)
      ---------------------------------------- -}

      if (Threads::number_of_threads() == 0) {
        // during early VM start-up locks don't exist, but we are safely single threaded,
        // call the functionality without holding the JvmtiThreadState_lock.
        JvmtiEventControllerPrivate::set_user_enabled(env, thread, event_type, enabled);
      } else {
        MutexLocker mu(JvmtiThreadState_lock);
        JvmtiEventControllerPrivate::set_user_enabled(env, thread, event_type, enabled);
      }
    }