外观模式在开源代码中的应用

阳光穿透心脏的1/2处 2023-03-04 03:27 21阅读 0赞

外观模式的作用：为多个复杂的子系统提供一个一致的接口，使这些子系统更加容易被访问，且不会暴露过多的底层实现细节。

### **案例** ###

Tomcat 中，catalina.jar 中的 RequestFacade 和 ResponseFacade 两个类就使用了外观模式。

servlet-api 中定义了 HttpServlet 抽象类，类中的定义了处理 http get、post、delete... 等请求，返回处理结果，对应需要实现方法 doGet、doPost、doDelete...

这些方法有两个共同的参数：HttpServletRequest、HttpServletResponse

public abstract class HttpServlet extends GenericServlet {
    
        protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException
        {
            String protocol = req.getProtocol();
            String msg = lStrings.getString("http.method_get_not_supported");
            if (protocol.endsWith("1.1")) {
                resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
            } else {
                resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
            }
        }
        
        ...
    }

## ##

HttpServlet 的方法中的 HttpServletRequest、HttpServletResponse 这两个参数实际传入的是 RequestFacade 和 ResponseFacade。

![format_png][]

以 RequestFacade 为例，RequestFacade 通过构造方法持有 Request 对象

public class RequestFacade implements HttpServletRequest {
        protected Request request = null;
        
        public RequestFacade(Request request){
            this.request = request;
        }
        //后面的方法使用 request 对象进行处理
        ...
    }

Request 对象有一个转换为 RequestFacade 对象的方法

public class Request implements HttpServletRequest {
        public HttpServletRequest getRequest() {
            if (this.facade == null) {
                this.facade = new RequestFacade(this);
            }
            return this.facade;
        }
    
        //与tomcat底层组件进行交互
    }

Request 对象的 getRequest 方法 和 Response 对象的 getResponse 方法，在这里被调用

final class StandardWrapperValve extends ValveBase {
    
        public final void invoke(Request request, Response response) throws IOException, ServletException {
            ...
            filterChain.doFilter(request.getRequest(), response.getResponse());
            ...
        }
    
    }

为啥要把 Request 和 Response 包装成 RequestFacade 和 ResponseFacade？

*  RequestFacade 和 ResponseFacade 只提供应用程序所需要的 API
 *  Request、Response 偏底层，且交互复杂，Facade 隐藏了 Request 和 Response 的底层实现细节，降低了底层方法被调用出错的风险

--------------------

### 【Java学习资源】整理推荐 ###

*  [享元模式在开源代码中的应用][Link 1]
 *  [外观模式在开源代码中的应用][Link 2]
 *  [装饰器模式在开源代码中的应用][Link 3]
 *  [桥接模式在开源代码中的应用][Link 4]
 *  [适配器模式在开源代码中的应用][Link 5]
 *  [代理模式在开源代码中的应用][Link 6]
 *  [原型模式在开源代码中的应用][Link 7]
 *  [建造者模式在开源代码中的应用][Link 8]
 *  [工厂模式在开源代码中的应用][Link 9]
 *  [单例模式在开源代码中的应用][Link 10]
 *  [编码规范][Link 11]
 *  [设计模式][Link 12]
 *  [重构][Link 13]
 *  [设计原则][Link 14]
 *  [面向对象到底是什么][Link 15]
 *  [代码质量有哪些评判标准？][Link 16]

--------------------

### 【Java面试题与答案】整理推荐 ###

*  [基础与语法][Link 17]
 *  [集合][Link 18]
 *  [网络编程][Link 19]
 *  [并发编程][Link 20]
 *  [Web][]
 *  [安全][Link 21]
 *  [设计模式][Link 22]
 *  [框架][Link 23]
 *  [算法与数据结构][Link 24]
 *  [异常][Link 25]
 *  [文件解析与生成][Link 26]
 *  [Linux][]
 *  [MySQL][]
 *  [Oracle][]
 *  [Redis][]
 *  [Dubbo][]

[format_png]: /images/20230208/1264c6e234e246f690f5ba8dbc0e6241.png
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