职责链模式（Chain of Responsibility）

向右看齐 2021-05-18 20:36 357阅读 0赞

### 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。

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zNzk2ODYxMw_size_16_color_FFFFFF_t_70][]

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());

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zNzk2ODYxMw_size_16_color_FFFFFF_t_70 1][]

下面见上述流程简述一下：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zNzk2ODYxMw_size_16_color_FFFFFF_t_70 2][]

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<HandlerAdapter> 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();
    			}
    		}
    	}
    }

![20191115143357772.png][]

这样，当RequestMappingHandler时，会找到合适的适配器，并交给它处理了。当增加适配器的时候，只要增加处理类，不需要修改业务代码。

for (HandlerAdapter ha : handlerAdapterList) {
    			if (ha.supports(handler)) {
    				ha.handler();
    			}
    		}

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[20191115143357772.png]: /images/20210518/65bb06dd611f4ac1b0117ef15bc448dd.png