看到了几篇讲的不错的博客,这里收集起来 不明白的地方互相参考
https://blog.csdn.net/fangqingan_java/article/details/9320769
https://blog.csdn.net/michael019/article/details/48714303
https://blog.csdn.net/waljl/article/details/51144351
原文：https://www.cnblogs.com/wangshaowei/p/8964105.html