code: @SuppressWarnings("InfiniteLoopStatement")
    @Override
    public void run() {
      try {
        while (true) {
          try {
            cleanUp(queue.remove());
          } catch (InterruptedException e) { /* ignore */ }
        }
      } catch (ShutDown shutDown) { /* ignore */ }
    }
docs: Loops continuously, pulling references off the queue and cleaning them up.