return;
      }
      synchronized (this) {
        if (closed) {
          return;
        }
        closed = true;
      }
      for (Map.Entry<Closeable, Executor> entry : entrySet()) {
        closeQuietly(entry.getKey(), entry.getValue());
      }
      clear();
      if (whenClosed != null) {
        whenClosed.countDown();
      }
    }