职责链模式在开源代码中的应用

淩亂°似流年 2022-11-29 12:40 85阅读 0赞

职责链模式的作用：把请求从链中的一个对象传到下一个对象，直到请求被响应为止。通过这种方式去除对象之间的耦合。

### **案例一** ###

servlet 2.3 版新增了 Filter 和 FilterChain，其中 FilterChain 就是用了职责链模式。

Filter 用于在 Servlet 响应前，进行预处理、信息校验等，Filter 没有调用 doFilter 方法参数中 chain 的 doFilter 方法，调用链即停止

public interface Filter {
        
        public void init(FilterConfig filterConfig) throws ServletException;
    
        public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException;
    
        public void destroy();
    }

FilterChain 接口中只有一个方法 doFilter，用于控制下一个 Filter 的方法被调用

public interface FilterChain {
        public void doFilter ( ServletRequest request, ServletResponse response ) throws IOException, ServletException;
    }

看下 FilterChain 的简单实现

private static class VirtualFilterChain implements FilterChain {
    
        private final FilterChain originalChain;
    
        private final List<? extends Filter> additionalFilters;
    
        private int currentPosition = 0;
    
        public VirtualFilterChain(FilterChain chain, List<? extends Filter> additionalFilters) {
            this.originalChain = chain;
            this.additionalFilters = additionalFilters;
        }
    
        @Override
        public void doFilter(final ServletRequest request, final ServletResponse response)
                throws IOException, ServletException {
    
            if (this.currentPosition == this.additionalFilters.size()) {
                this.originalChain.doFilter(request, response);
            }
            else {
                this.currentPosition++;
                Filter nextFilter = this.additionalFilters.get(this.currentPosition - 1);
                nextFilter.doFilter(request, response, this);
            }
        }
    }

###    **案例二** ###

Spring 中的拦截器 HandlerInterceptor

HandlerInterceptor 定义了 3 个方法 preHandle、postHandle、afterCompletion，调用时间见注释

public interface HandlerInterceptor {
    	//Called after HandlerMapping determined an appropriate handler object, but before HandlerAdapter invokes the handler
    	boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;
    	
    	//Called after HandlerAdapter actually invoked the handler, but before the DispatcherServlet renders the view
    	void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception;
    	
    	// Callback after completion of request processing, that is, after renderingthe view
    	void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception;
    
    }

![20200820222335142.gif][]

拦截器链 HandlerExecutionChain

public class HandlerExecutionChain {
    
    	//执行链中拦截器的 preHandle 方法，遇到返回 false，执行 triggerAfterCompletion 方法触发
    	boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
    		HandlerInterceptor[] interceptors = getInterceptors();
    		if (!ObjectUtils.isEmpty(interceptors)) {
    			for (int i = 0; i < interceptors.length; i++) {
    				HandlerInterceptor interceptor = interceptors[i];
    				if (!interceptor.preHandle(request, response, this.handler)) {
    					triggerAfterCompletion(request, response, null);
    					return false;
    				}
    				this.interceptorIndex = i;
    			}
    		}
    		return true;
    	}
    	
    	//DispatcherServlet 中 doDispatch 方法调用，调用链中拦截器的 postHandle 方法
    	void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
    		HandlerInterceptor[] interceptors = getInterceptors();
    		if (!ObjectUtils.isEmpty(interceptors)) {
    			for (int i = interceptors.length - 1; i >= 0; i--) {
    				HandlerInterceptor interceptor = interceptors[i];
    				interceptor.postHandle(request, response, this.handler, mv);
    			}
    		}
    	}
    
    	//执行链中截止到 applyPreHandle 返回 false 的 拦截器的 afterCompletion 方法
    	void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex) throws Exception {
    		HandlerInterceptor[] interceptors = getInterceptors();
    		if (!ObjectUtils.isEmpty(interceptors)) {
    			for (int i = this.interceptorIndex; i >= 0; i--) {
    				HandlerInterceptor interceptor = interceptors[i];
    				try {
    					interceptor.afterCompletion(request, response, this.handler, ex);
    				}
    				catch (Throwable ex2) {
    					logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
    				}
    			}
    		}
    	}
    
    }

DispatcherServlet 中对拦截器链的调用

public class DispatcherServlet extends FrameworkServlet {
    	
    	//分发 servlet 请求
    	protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    		.
    		.
    		.
    		//调用拦截器链的 applyPreHandle 方法
    		if (!mappedHandler.applyPreHandle(processedRequest, response)) {
    			return;
    		}
    
    		// Actually invoke the handler.
    		mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
    
    		if (asyncManager.isConcurrentHandlingStarted()) {
    			return;
    		}
    
    		applyDefaultViewName(processedRequest, mv);
    		
    		//调用拦截器链的 applyPostHandle 方法
    		mappedHandler.applyPostHandle(processedRequest, response, mv);
    		.
    		.
    		.
    		processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    	}
    	
    	private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception {
    		.
    		.
    		.
    		//调用拦截器链的 triggerAfterCompletion 方法
    		if (mappedHandler != null) {
    			mappedHandler.triggerAfterCompletion(request, response, null);
    		}
    	}
    
    }

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

### 【Java学习资源】整理推荐 ###

*  [职责链模式在开源代码中的应用][Link 1]
 *  [命令模式在开源代码中的应用][Link 2]
 *  [策略模式在开源代码中应用][Link 3]
 *  [模板方法模式在开源代码中应用][Link 4]
 *  [组合模式在开源代码中的应用][Link 5]
 *  [享元模式在开源代码中的应用][Link 6]
 *  [外观模式在开源代码中的应用][Link 7]
 *  [装饰器模式在开源代码中的应用][Link 8]
 *  [桥接模式在开源代码中的应用][Link 9]
 *  [适配器模式在开源代码中的应用][Link 10]
 *  [代理模式在开源代码中的应用][Link 11]
 *  [原型模式在开源代码中的应用][Link 12]
 *  [建造者模式在开源代码中的应用][Link 13]
 *  [工厂模式在开源代码中的应用][Link 14]
 *  [单例模式在开源代码中的应用][Link 15]
 *  [编码规范][Link 16]
 *  [设计模式][Link 17]
 *  [重构][Link 18]
 *  [设计原则][Link 19]
 *  [面向对象到底是什么][Link 20]
 *  [代码质量有哪些评判标准？][Link 21]

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

### 【Java面试题与答案】整理推荐 ###

*  [基础与语法][Link 22]
 *  [集合][Link 23]
 *  [网络编程][Link 24]
 *  [并发编程][Link 25]
 *  [Web][]
 *  [安全][Link 26]
 *  [设计模式][Link 27]
 *  [框架][Link 28]
 *  [算法与数据结构][Link 29]
 *  [异常][Link 30]
 *  [文件解析与生成][Link 31]
 *  [Linux][]
 *  [MySQL][]
 *  [Oracle][]
 *  [Redis][]
 *  [Dubbo][]

[20200820222335142.gif]: /images/20221124/aa0900fc587046e795745d9c72f14f6c.png
[Link 1]: https://www.javanav.com/val/dc8fdb1954b1438ca55148e3c62547bd.html
[Link 2]: https://www.javanav.com/val/37ea584b5a9646f88e3916d13037e13f.html
[Link 3]: https://www.javanav.com/val/21eda51d16904f2798dc30974fe0fed9.html
[Link 4]: https://www.javanav.com/val/acd6815d8bcd45ce9db38659a049e778.html
[Link 5]: https://www.javanav.com/val/12f35b774b514c47a219bd860148f179.html
[Link 6]: https://www.javanav.com/val/72ac6d1ddd2d46b2a5312ac9b3050986.html
[Link 7]: https://www.javanav.com/val/f4a27593b46b46bc89d77d9f92c0f50c.html
[Link 8]: https://www.javanav.com/val/40716af16cda47ffae5fcf4090e69e14.html
[Link 9]: https://www.javanav.com/val/75bfae90731d40c781926a0dbec9edca.html
[Link 10]: https://www.javanav.com/val/f35536fdc771448f8d70574d830b7616.html
[Link 11]: https://www.javanav.com/val/24468130d26e460187c9280b90146649.html
[Link 12]: https://www.javanav.com/val/ffd6d3d1ceed414daafeb3ce10794c60.html
[Link 13]: https://www.javanav.com/val/e8afb788fdc54712aaf981c705796b46.html
[Link 14]: https://www.javanav.com/val/6f367ed6ee894adb830987f11536141c.html
[Link 15]: https://www.javanav.com/val/838b98837f574aa6a09c840921984707.html
[Link 16]: https://www.javanav.com/val/2b9f8790e8564bc0a2cb36357a66502f.html
[Link 17]: https://www.javanav.com/val/90b430ff0cbc4ef997e64cf7a0e1e745.html
[Link 18]: https://www.javanav.com/val/d9095cb3712040c0bee8054732321d6d.html
[Link 19]: https://www.javanav.com/val/9916617e210c4e0eae8649874a2cedd5.html
[Link 20]: https://www.javanav.com/val/5a834b46cfd945f7be9c4ea652306fd4.html
[Link 21]: https://www.javanav.com/val/0cea6c7c082043ab83ad87a9c5a48d9d.html
[Link 22]: https://www.javanav.com/interview/93b0069472fd479393006c0e73043fc4.html
[Link 23]: https://www.javanav.com/interview/4147d95e693945e7a50345fd33ec817e.html
[Link 24]: https://www.javanav.com/interview/68a57ffc78de4133bb890bdd502ac418.html
[Link 25]: https://www.javanav.com/interview/36a24fa3258e45c89873f474ce914912.html
[Web]: https://www.javanav.com/interview/4e27cb63adb440c6825a159d35cf4010.html
[Link 26]: https://www.javanav.com/interview/03affb2ce3f04fda8853f714e5764906.html
[Link 27]: https://www.javanav.com/interview/07a32a9ff17a4006a53550ce658cafed.html
[Link 28]: https://www.javanav.com/interview/5c466456e9b54ab0a749b5948e51f1d3.html
[Link 29]: https://www.javanav.com/interview/943f917041ee4f05add24539f5b861b2.html
[Link 30]: https://www.javanav.com/interview/d3f058cfc05d4c6dadd821b622e84d36.html
[Link 31]: https://www.javanav.com/interview/67a50996c28f418e83b109ea42fc3eff.html
[Linux]: https://www.javanav.com/interview/00cccb81729540d1923bd0a0ef49533a.html
[MySQL]: https://www.javanav.com/interview/d311e78502a845f1a4bb307bd6aec2b3.html
[Oracle]: https://www.javanav.com/interview/5f29e9677edf4a16be3dce30d257c9ab.html
[Redis]: https://www.javanav.com/interview/066c62b3af4d4827b2243a822c3084a9.html
[Dubbo]: https://www.javanav.com/interview/9a3e7a7740324757abaab953f7ebc455.html