推荐解释得非常好的两篇博文
http://blog.csdn.net/warmyellow/article/details/5454943
http://www.cnblogs.com/jerrylead/archive/2011/04/18/2020209.html
推荐解释得非常好的两篇博文
http://blog.csdn.net/warmyellow/article/details/5454943
http://www.cnblogs.com/jerrylead/archive/2011/04/18/2020209.html