https://stackoverflow.com/questions/10240694/java-socket-api-how-to-tell-if-a-connection-has-been-closed
http://fanyi.baidu.com/#en/zh/
http://blog.qiusuo.im/blog/2014/03/19/tcp-timeout/
点到点，端对端的概念
https://www.zhihu.com/question/49245486
netty如何知道连接已经关闭
原文地址：https://www.cnblogs.com/wth21-1314/p/8417662.html