Spring请求分发DispatcherServlet

        DispatcherServlet是springMVC框架的核心控制器之一，用于请求的分发调度。该控制器在容器的web.xml文件中进行配置。并对中配置的拦截规则进行匹配，最终根据请求的不同交由不同的控制器进行处理。

        DispatcherServlet的继承结构如下图所示：

        请求url匹配成功后，进入DispatcherServlet的doService进行处理。

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        if (this.logger.isDebugEnabled()) {
            String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
            this.logger.debug("DispatcherServlet with name '" + this.getServletName() + "'" + resumed + " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
        }

        Map attributesSnapshot = null;
        if (WebUtils.isIncludeRequest(request)) {
            attributesSnapshot = new HashMap();
            Enumeration attrNames = request.getAttributeNames();

            label108:
            while(true) {
                String attrName;
                do {
                    if (!attrNames.hasMoreElements()) {
                        break label108;
                    }

                    attrName = (String)attrNames.nextElement();
                } while(!this.cleanupAfterInclude && !attrName.startsWith("org.springframework.web.servlet"));

                attributesSnapshot.put(attrName, request.getAttribute(attrName));
            }
        }

        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.getWebApplicationContext());
        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
        request.setAttribute(THEME_SOURCE_ATTRIBUTE, this.getThemeSource());
        FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
        if (inputFlashMap != null) {
            request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
        }

        request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
        request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

        try {
            this.doDispatch(request, response);
        } finally {
            if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) {
                this.restoreAttributesAfterInclude(request, attributesSnapshot);
            }

        }

    }

        从源码中可以看出，DispatcherServlet首先对请求进行判断，如果当前请求是include请求，不是顶层的外部http请求，则该方法遍历该请求并保存一份快照。然后进行一系列的属性设置。设置完之后，调用调度处理函数doDispatch(request, response)

最后将请求还原。

        doDispatch方法的源码如下：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            try {
                ModelAndView mv = null;
                Object dispatchException = null;

                try {
                    processedRequest = this.checkMultipart(request);
                    multipartRequestParsed = processedRequest != request;
                    mappedHandler = this.getHandler(processedRequest);
                    if (mappedHandler == null || mappedHandler.getHandler() == null) {
                        this.noHandlerFound(processedRequest, response);
                        return;
                    }

                    HandlerAdapter ha = this.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 (this.logger.isDebugEnabled()) {
                            this.logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                        }

                        if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {
                            return;
                        }
                    }

                    if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                        return;
                    }

                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }

                    this.applyDefaultViewName(processedRequest, mv);
                    mappedHandler.applyPostHandle(processedRequest, response, mv);
                } catch (Exception var20) {
                    dispatchException = var20;
                } catch (Throwable var21) {
                    dispatchException = new NestedServletException("Handler dispatch failed", var21);
                }

                this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
            } catch (Exception var22) {
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
            } catch (Throwable var23) {
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
            }

        } finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            } else if (multipartRequestParsed) {
                this.cleanupMultipart(processedRequest);
            }

        }
    }

       在doDispatch方法中，首先获取网页的异步管理器，然后调用checkMultipart方法判断当前请求的数据中是否包含文件，然后通过HandlerMapping获取处理器执行链（即拦截器和处理器），并将请求交由处理器执行链进行处理。若未找到处理器执行链，则提示未找到处理器执行链并返回；否则，将请求交由控制器进行处理。

//通过HandlerMapping调用处理器处理请求
 mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
//该方法调用/RequestMappingHandlerAdapter类中的handleInternal实现对对应控制器的调用。
protected ModelAndView handleInternal(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
        this.checkRequest(request);
        ModelAndView mav;
        if (this.synchronizeOnSession) {
            HttpSession session = request.getSession(false);
            if (session != null) {
                Object mutex = WebUtils.getSessionMutex(session);
                synchronized(mutex) {
                    mav = this.invokeHandlerMethod(request, response, handlerMethod);
                }
            } else {
                mav = this.invokeHandlerMethod(request, response, handlerMethod);
            }
        } else {
            mav = this.invokeHandlerMethod(request, response, handlerMethod);
        }

        if (!response.containsHeader("Cache-Control")) {
            if (this.getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
                this.applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
            } else {
                this.prepareResponse(response);
            }
        }

        return mav;
    }

        关于返回的ModelAndView对象的解析方法，将在下一篇文章中进行介绍。