//processor.sendResponse
  protected[network] def sendResponse(response: RequestChannel.Response) {
    trace(s"Socket server received response to send, registering for write and sending data: $response")
    val channel = selector.channel(response.responseSend.destination)
    // `channel` can be null if the selector closed the connection because it was idle for too long
    if (channel == null) {
      warn(s"Attempting to send response via channel for which there is no open connection, connection id $id")
      response.request.updateRequestMetrics()
    }
    else {
      selector.send(response.responseSend)
      inflightResponses += (response.request.connectionId -> response)
    }

因此，通道被选择器关闭，因为它空闲的时间太长