职责链模式（Chain of Responsibility）设计模式

职责链模式：
该模式避免了请求与多个处理者之间的耦合。说直白点，就是从多个处理者中选择一个，我们平时的想法是if else判断，但当处理者过多的时候就过于复杂，且增加或删除处理者，会改变业务代码，不满足开闭原则。

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从结构图可以看出，我们对处理者与处理者之间指定了顺序，如果该处理者处理不了就交给下一个。当处理者之间的顺序不要求的时候也可以直接用集合代替。
下面以SpringMVC中的doDispatch方法中，getHandlerAdapter方法对职责链进行分析。

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception { HttpServletRequest processedRequest = request; HandlerExecutionChain mappedHandler = null; boolean multipartRequestParsed = false; WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); try { ModelAndView mv = null; Exception dispatchException = null; try { processedRequest = checkMultipart(request); multipartRequestParsed = (processedRequest != request); // Determine handler for the current request. mappedHandler = getHandler(processedRequest); if (mappedHandler == null || mappedHandler.getHandler() == null) { noHandlerFound(processedRequest, response); return; }// Determine handler adapter for the current request. HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

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【职责链模式（Chain of Responsibility）】下面见上述流程简述一下：

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MVC根据外部请求取到合适的handler，再通过handler寻找合适的adapter，那怎么寻找呢？这里用到了职责链模式
定义HandlerAdapter接口：

public interface HandlerAdapter { //是否支持该handler boolean supports(Object handler); //请求处理 void handler(); }

具体的适配器实现HandlerAdapter接口：

public class AnnotationMethodHandlerAdapter implements HandlerAdapter { @Override public boolean supports(Object handler) { return (handler instanceof AnnotationMethodHandler); } @Override public void handler() { System.out.println("请求交给AnnotationMethodHandlerAdapter处理"); } }

public class RequestMappingHandlerAdapter implements HandlerAdapter { @Override public boolean supports(Object handler) { return (handler instanceof RequestMappingHandler); } @Override public void handler() { System.out.println("请求交给RequestMappingHandlerAdapter处理"); } }

public class SimpleControllerHandlerAdapter implements HandlerAdapter { @Override public boolean supports(Object handler) { return (handler instanceof SimpleControllerHandler); } @Override public void handler() { System.out.println("请求交给SimpleControllerHandlerAdapter处理"); } }

用户使用时：

public class Main { public static void main(String[] args) { //前期预处理，假设链上有这三种adapter List handlerAdapterList = new ArrayList<>(); handlerAdapterList.add(new RequestMappingHandlerAdapter()); handlerAdapterList.add(new SimpleControllerHandlerAdapter()); handlerAdapterList.add(new AnnotationMethodHandlerAdapter()); //假如请求的handler为RequestMappingHandler RequestMappingHandler handler = new RequestMappingHandler(); //根据职责链匹配查找，这里没有指定职责链的顺序，也是可以的 for (HandlerAdapter ha : handlerAdapterList) { if (ha.supports(handler)) { ha.handler(); } } } }