void
JvmtiEventControllerPrivate::change_field_watch(jvmtiEvent event_type, bool added) {

  {- -------------------------------------------
  (1) (変数宣言など)
      ---------------------------------------- -}

      int *count_addr;

  {- -------------------------------------------
  (1) 指定された event_type に対応する JvmtiExport 内のフィールドを取得する.
      ---------------------------------------- -}

      switch (event_type) {
      case JVMTI_EVENT_FIELD_MODIFICATION:
        count_addr = (int *)JvmtiExport::get_field_modification_count_addr();
        break;
      case JVMTI_EVENT_FIELD_ACCESS:
        count_addr = (int *)JvmtiExport::get_field_access_count_addr();
        break;
      default:
        assert(false, "incorrect event");
        return;
      }

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

      EC_TRACE(("JVMTI [-] # change field watch - %s %s count=%d",
                event_type==JVMTI_EVENT_FIELD_MODIFICATION? "modification" : "access",
                added? "add" : "remove",
                *count_addr));

  {- -------------------------------------------
  (1) 取得したフィールドに対して, 
      added 引数(追加かクリアか)に応じてインクリメント/デクリメントを行う.

      (必要があれば, JvmtiEventControllerPrivate::recompute_enabled() を呼んで, 
      "truly enabled event" 情報を更新する)
      ---------------------------------------- -}

      if (added) {
        (*count_addr)++;
        if (*count_addr == 1) {
          recompute_enabled();
        }
      } else {
        if (*count_addr > 0) {
          (*count_addr)--;
          if (*count_addr == 0) {
            recompute_enabled();
          }
        } else {
          assert(false, "field watch out of phase");
        }
      }
    }