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