前文我们有了IOC的源码基础以及SpringMVC的基础，我们便可以进一步深入理解SpringMVC主要实现原理，包含DispatcherServlet的初始化过程和DispatcherServlet处理请求的过程的源码解析。本文是第二篇：DispatcherServlet处理请求的过程的源码解析。@pdai
一个请求发出，经过DispatcherServlet进行了什么样的处理，最后将内容返回的呢？
首先让我们整体看一下Spring Web MVC 处理请求的流程：
核心架构的具体流程步骤如下：
我们以上个demo中这个GET请求为例，请求URL是http://localhost:8080/011_spring_framework_demo_springmvc_war_exploded/user
我们知道servlet处理get请求是doGet方法，所以我们去找DispatcherServlet类结构中的doGet方法。
@Overrideprotected final void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response);}processRequest处理请求的方法如下：
/** * Process this request, publishing an event regardless of the outcome. * <p>The actual event handling is performed by the abstract * {@link #doService} template method. */protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // 计算处理请求的时间 long startTime = System.currentTimeMillis(); Throwable failureCause = null; LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext(); LocaleContext localeContext = buildLocaleContext(request); RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes(); ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes); WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor()); // 初始化context initContextHolders(request, localeContext, requestAttributes); try { // 看这里 doService(request, response); } catch (ServletException | IOException ex) { failureCause = ex; throw ex; } catch (Throwable ex) { failureCause = ex; throw new NestedServletException("Request processing failed", ex); } finally { // 重置context resetContextHolders(request, previousLocaleContext, previousAttributes); if (requestAttributes != null) { requestAttributes.requestCompleted(); } logResult(request, response, failureCause, asyncManager); publishRequestHandledEvent(request, response, startTime, failureCause); }}本质上就是调用doService方法，由DispatchServlet类实现
/** * Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch} * for the actual dispatching. */@Overrideprotected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception { logRequest(request); // 保存下请求之前的参数. Map<String, Object> attributesSnapshot = null; if (WebUtils.isIncludeRequest(request)) { attributesSnapshot = new HashMap<>(); Enumeration<?> attrNames = request.getAttributeNames(); while (attrNames.hasMoreElements()) { String attrName = (String) attrNames.nextElement(); if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) { attributesSnapshot.put(attrName, request.getAttribute(attrName)); } } } // 方便后续 handlers 和 view 要使用它们. request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext()); request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver); request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver); request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource()); if (this.flashMapManager != null) { 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); } RequestPath previousRequestPath = null; if (this.parseRequestPath) { previousRequestPath = (RequestPath) request.getAttribute(ServletRequestPathUtils.PATH_ATTRIBUTE); ServletRequestPathUtils.parseAndCache(request); } try { // 看这里，终于将这个请求分发出去了 doDispatch(request, response); } finally { if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { // Restore the original attribute snapshot, in case of an include. if (attributesSnapshot != null) { restoreAttributesAfterInclude(request, attributesSnapshot); } } if (this.parseRequestPath) { ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request); } }}doDispatch方法是真正处理请求的核心方法
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 { // 判断是不是文件上传类型的request processedRequest = checkMultipart(request); multipartRequestParsed = (processedRequest != request); // 根据request获取匹配的handler. mappedHandler = getHandler(processedRequest); if (mappedHandler == null) { noHandlerFound(processedRequest, response); return; } // 根据handler获取匹配的handlerAdapter HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); // 如果handler支持last-modified头处理 String method = request.getMethod(); boolean isGet = HttpMethod.GET.matches(method); if (isGet || HttpMethod.HEAD.matches(method)) { long lastModified = ha.getLastModified(request, mappedHandler.getHandler()); if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) { return; } } if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // 真正handle处理，并返回modelAndView mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } // 通过视图的prefix和postfix获取完整的视图名 applyDefaultViewName(processedRequest, mv); // 应用后置的拦截器 mappedHandler.applyPostHandle(processedRequest, response, mv); } catch (Exception ex) { dispatchException = ex; } catch (Throwable err) { // As of 4.3, we're processing Errors thrown from handler methods as well, // making them available for @ExceptionHandler methods and other scenarios. dispatchException = new NestedServletException("Handler dispatch failed", err); } // 处理handler处理的结果，显然就是对ModelAndView 或者 出现的Excpetion处理 processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); } catch (Exception ex) { triggerAfterCompletion(processedRequest, response, mappedHandler, ex); } catch (Throwable err) { triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", err)); } finally { if (asyncManager.isConcurrentHandlingStarted()) { // Instead of postHandle and afterCompletion if (mappedHandler != null) { mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); } } else { // Clean up any resources used by a multipart request. if (multipartRequestParsed) { cleanupMultipart(processedRequest); } } }}对于真正的handle方法，我们看下其处理流程
/** * This implementation expects the handler to be an {@link HandlerMethod}. */@Override@Nullablepublic final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { return handleInternal(request, response, (HandlerMethod) handler);}交给handleInternal方法处理，以RequestMappingHandlerAdapter这个HandlerAdapter中的处理方法为例
@Overrideprotected ModelAndView handleInternal(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception { ModelAndView mav; checkRequest(request); // Execute invokeHandlerMethod in synchronized block if required. if (this.synchronizeOnSession) { HttpSession session = request.getSession(false); if (session != null) { Object mutex = WebUtils.getSessionMutex(session); synchronized (mutex) { mav = invokeHandlerMethod(request, response, handlerMethod); } } else { // No HttpSession available -> no mutex necessary mav = invokeHandlerMethod(request, response, handlerMethod); } } else { // No synchronization on session demanded at all... mav = invokeHandlerMethod(request, response, handlerMethod); } if (!response.containsHeader(HEADER_CACHE_CONTROL)) { if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) { applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers); } else { prepareResponse(response); } } return mav;}
然后执行invokeHandlerMethod这个方法，用来对RequestMapping（usercontroller中的list方法）进行处理
/** * Invoke the {@link RequestMapping} handler method preparing a {@link ModelAndView} * if view resolution is required. * @since 4.2 * @see #createInvocableHandlerMethod(HandlerMethod) */@Nullableprotected ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception { ServletWebRequest webRequest = new ServletWebRequest(request, response); try { WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod); ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory); // 重要：设置handler(controller#list)方法上的参数，返回值处理，绑定databinder等 ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod); if (this.argumentResolvers != null) { invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers); } if (this.returnValueHandlers != null) { invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers); } invocableMethod.setDataBinderFactory(binderFactory); invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer); ModelAndViewContainer mavContainer = new ModelAndViewContainer(); mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request)); modelFactory.initModel(webRequest, mavContainer, invocableMethod); mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect); AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response); asyncWebRequest.setTimeout(this.asyncRequestTimeout); WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); asyncManager.setTaskExecutor(this.taskExecutor); asyncManager.setAsyncWebRequest(asyncWebRequest); asyncManager.registerCallableInterceptors(this.callableInterceptors); asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors); if (asyncManager.hasConcurrentResult()) { Object result = asyncManager.getConcurrentResult(); mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0]; asyncManager.clearConcurrentResult(); LogFormatUtils.traceDebug(logger, traceOn -> { String formatted = LogFormatUtils.formatValue(result, !traceOn); return "Resume with async result [" + formatted + "]"; }); invocableMethod = invocableMethod.wrapConcurrentResult(result); } // 执行controller中方法 invocableMethod.invokeAndHandle(webRequest, mavContainer); if (asyncManager.isConcurrentHandlingStarted()) { return null; } return getModelAndView(mavContainer, modelFactory, webRequest); } finally { webRequest.requestCompleted(); }}invokeAndHandle交给UserController中具体执行list方法执行
后续invoke执行的方法，直接看整个请求流程的调用链即可
执行后获得视图和Model
接下来继续执行processDispatchResult方法，对视图和model（如果有异常则对异常处理）进行处理（显然就是渲染页面了）
/** * Handle the result of handler selection and handler invocation, which is * either a ModelAndView or an Exception to be resolved to a ModelAndView. */private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv, @Nullable Exception exception) throws Exception { boolean errorView = false; // 如果处理过程有异常，则异常处理 if (exception != null) { if (exception instanceof ModelAndViewDefiningException) { logger.debug("ModelAndViewDefiningException encountered", exception); mv = ((ModelAndViewDefiningException) exception).getModelAndView(); } else { Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null); mv = processHandlerException(request, response, handler, exception); errorView = (mv != null); } } // 是否需要渲染视图 if (mv != null && !mv.wasCleared()) { render(mv, request, response); // 渲染视图 if (errorView) { WebUtils.clearErrorRequestAttributes(request); } } else { if (logger.isTraceEnabled()) { logger.trace("No view rendering, null ModelAndView returned."); } } if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { // Concurrent handling started during a forward return; } if (mappedHandler != null) { // Exception (if any) is already handled.. mappedHandler.triggerAfterCompletion(request, response, null); }}接下来显然就是渲染视图了, spring在initStrategies方法中初始化的组件（LocaleResovler等）就派上用场了。
/** * Render the given ModelAndView. * <p>This is the last stage in handling a request. It may involve resolving the view by name. * @param mv the ModelAndView to render * @param request current HTTP servlet request * @param response current HTTP servlet response * @throws ServletException if view is missing or cannot be resolved * @throws Exception if there's a problem rendering the view */protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception { // Determine locale for request and apply it to the response. Locale locale = (this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale()); response.setLocale(locale); View view; String viewName = mv.getViewName(); if (viewName != null) { // We need to resolve the view name. view = resolveViewName(viewName, mv.getModelInternal(), locale, request); if (view == null) { throw new ServletException("Could not resolve view with name '" + mv.getViewName() + "' in servlet with name '" + getServletName() + "'"); } } else { // No need to lookup: the ModelAndView object contains the actual View object. view = mv.getView(); if (view == null) { throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " + "View object in servlet with name '" + getServletName() + "'"); } } // Delegate to the View object for rendering. if (logger.isTraceEnabled()) { logger.trace("Rendering view [" + view + "] "); } try { if (mv.getStatus() != null) { response.setStatus(mv.getStatus().value()); } view.render(mv.getModelInternal(), request, response); } catch (Exception ex) { if (logger.isDebugEnabled()) { logger.debug("Error rendering view [" + view + "]", ex); } throw ex; }}后续就是通过viewResolver进行解析了，这里就不再继续看代码了，上述流程基本上够帮助你构建相关的认知了。
最后无非是返回控制权给DispatcherServlet，由DispatcherServlet 返回响应给用户。
最后的最后我们看下请求的日志：
21:45:53.390 [http-nio-8080-exec-6] DEBUG org.springframework.web.servlet.DispatcherServlet - GET "/011_spring_framework_demo_springmvc_war_exploded/user", parameters={}21:45:53.400 [http-nio-8080-exec-6] DEBUG org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping - Mapped to tech.pdai.springframework.springmvc.controller.UserController#list(HttpServletRequest, HttpServletResponse)22:51:14.504 [http-nio-8080-exec-6] DEBUG org.springframework.web.servlet.view.JstlView - View name 'userList', model {dateTime=Fri Apr 22 21:45:53 CST 2022, userList=[tech.pdai.springframework.springmvc.entity.User@7b8c8dc]}22:51:14.550 [http-nio-8080-exec-6] DEBUG org.springframework.web.servlet.view.JstlView - Forwarding to [/WEB-INF/views/userList.jsp]22:51:44.395 [http-nio-8080-exec-6] DEBUG org.springframework.web.servlet.DispatcherServlet - Completed 200 OK首先， 从Spring框架的整体架构和组成对整体框架有个认知。
其次，通过案例引出Spring的核心（IoC和AOP），同时对IoC和AOP进行案例使用分析。
基于Spring框架和IOC，AOP的基础，为构建上层web应用，需要进一步学习SpringMVC。
Spring进阶 - IoC，AOP以及SpringMVC的源码分析
ConcurrentHashMap<String, Object>；并且BeanDefinition接口中包含了这个类的Class信息以及是否是单例等。那么如何从BeanDefinition中实例化Bean对象呢，这是本文主要研究的内容？