转载:http://www.cnblogs.com/codingmyworld/archive/2011/09/12/2174255.html
https://my.oschina.net/u/1391648/blog/282892
http://www.jianshu.com/p/9e4d1fab0f36
理解ThreadLocal http://blog.csdn.net/qjyong/article/details/2158097
http://baike.baidu.com/link?url=4c2CzL_ppN-RapAtTOejX2tAWamsqB5EwaPOK2WWYMvXifAthbY13a0-3AEqeb9-wsmGYjhDbc3MI0KdHnr2qX47zCNwjqpJCPXkNpjSyQveHbyA5yeVi8ZqjKS-w-uC