netty如何知道连接已经关闭,socket心跳,双工?异步?
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/
http://blog.csdn.net/ranjiewen/article/details/52595592
点到点,端对端的概念
https://www.zhihu.com/question/49245486
socket的心跳检测
http://www.blogjava.net/thisliy/archive/2010/02/17/313271.html
socket为什么要用心跳机制,超有用
https://www.cnblogs.com/phpfans/p/4653858.html
https://weibo.com/p/230418b827924e0102waia
全双工,半双工,同步or 异步
http://blog.csdn.net/chun0801/article/details/51965761
模拟心跳
http://blog.csdn.net/z69183787/article/details/52671543
tcp的状态https://blog.csdn.net/bzfys/article/details/73733917
TCP连接的状态详解以及故障排查
https://blog.csdn.net/hguisu/article/details/38700899