三层架构解析

https://wenku.baidu.com/view/8db023fda0116c175f0e48df.html

https://wenku.baidu.com/view/52935769a98271fe910ef99e.html

https://wenku.baidu.com/view/f67e2712302b3169a45177232f60ddccda38e6a4.html?re=view