参考别人的博客:
http://blog.youkuaiyun.com/caolaosanahnu/article/details/7172542
http://blog.youkuaiyun.com/smilingleo/article/details/2637511
http://blog.youkuaiyun.com/caolaosanahnu/article/details/7172542
http://blog.youkuaiyun.com/smilingleo/article/details/2637511