--简单解释:
http://blog.youkuaiyun.com/liujiahan629629/article/details/8985937
--部分疑惑问题
http://bbs.youkuaiyun.com/topics/390751819
http://blog.youkuaiyun.com/zhengyeqing520/article/details/6091656
http://zhanghua.1199.blog.163.com/blog/static/4644980720111113104642609/