最近终于闲了下来,准备自己记录些东西。网上关于springMVC的资料很多,但关于设计模式的还有限,我就想把springMVC源码中的设计模式抽出来做成一个系列,简单的谈一下其中的实现原理,作为一种学习分享,以后有更深的感悟也会更新。
先从一张图对整个springMVC的运作流程有一个大致的了解,图片侵删。


HandlerExecutionChain
其中我们可以看到,在springMVC中,DispatcherServlet这个核心类中使用到了HandlerExecutionChain这个类,他就是责任链模式实行的具体类。在DispatcherServlet的doDispatch这个方法中,我们可以看到它贯穿了整个请求dispatch的流程:

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);
                // 获取该请求的handler,每个handler实为HandlerExecutionChain,它为一个处理链,负责处理整个请求
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                String method = request.getMethod();
                boolean isGet = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (logger.isDebugEnabled()) {
                        logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                    }
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }
                // 责任链执行预处理方法,实则是将请求交给注册的请求拦截器执行
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }
                // 实际的执行逻辑的部分,也就是你加了@RequestMapping注解的方法
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }
                applyDefaultViewName(processedRequest, mv);
                // 责任链执行后处理方法,实则是将请求交给注册的请求拦截器执行
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            catch (Throwable err) {
                dispatchException = new NestedServletException("Handler dispatch failed", err);
            }
            // 处理返回的结果,触发责任链上注册的拦截器的AfterCompletion方法,其中也用到了HandlerExecutionChain注册的handler来处理错误结果
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            // 触发责任链上注册的拦截器的AfterCompletion方法
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Throwable err) {
            triggerAfterCompletion(processedRequest, response, mappedHandler,
                    new NestedServletException("Handler processing failed", err));
        }
        finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            }
            else {
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

从上面的代码中我们可以看到,HandlerExecutionChain主要负责请求的拦截器的执行和请求的处理,但是他本身不处理请求,只是将请求分配给在链上注册的处理器执行,这是一种责任链的实现方式,减少了责任链本身与处理逻辑之间的耦合的同时,规范了整个处理请求的流程,下面我们看一下上面代码中涉及到的方法在HandlerExecutionChain类中对应的代码。

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;
    }
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);
            }
        }
    }
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);
                }
            }
        }
    }

代码很容易理解,这里不详细说明。需要注意的是,HandlerExecutionChain维护了HandlerInterceptor(拦截器)的集合,可以向其中注册相应的HandlerInterceptor。

总结
个人的理解,责任链模式可以很好的将原本耦合的顺序过程处理的代码和逻辑,解耦成执行的顺序逻辑,和一个个相对应的处理器(责任人),对应的责任链只需要关心责任处理的顺序,而不需要关心具体的处理逻辑,将这些逻辑交给注册的责任人去处理。从springMVC的源码中,我们可以看到这一设计模式的应用,将原本复杂的请求处理逻辑表现的清楚明白。
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版权声明:本文为CSDN博主「春天写下一个bug」的原创文章,遵循CC 4.0 BY-SA版权协议,转载请附上原文出处链接及本声明。
原文链接:https://blog.csdn.net/ljw761123096/java/article/details/79591133