缓存文件有效期

http://blog.csdn.net/xwren362922604/article/details/8539066

 http://blog.sina.com.cn/s/blog_71ce775e0101elue.html

http://qk13warcraft.blog.163.com/blog/static/15754934420129294439199/

http://www.awnlab.com/archives/1942.html