SpringMVC的DispatcherServlet是如何处理请求的整个流程？

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在前文，我们掌握了IOC的源码基础及SpringMVC的基本原理。接下来，我们将深入理解SpringMVC主要实现原理，包括DispatcherServlet的初始化过程及处理请求的流程解析。

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

• Spring框架系列(14) - SpringMVC实现原理之DispatcherServlet处理请求的过程
  • DispatcherServlet处理请求的过程？
    • 回顾整理处理流程
    • doGet入口
    • 请求分发
    • 映射和适配器处理
    • 视图渲染
  • 更多文章

DispatcherServlet处理请求的过程？

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？

回顾整理处理流程

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：

1. 首先用户发送请求——>DispatcherServlet，前端控制器收到请求后自己不进行处理，而是委托给其他的解析器进行
   处理，作为统一访问点，进行全局的流程控制；
2. DispatcherServlet——>HandlerMapping， HandlerMapping 将会把请求映射为 HandlerExecutionChain 对象（包含一
   个Handler 处理器（页面控制器）对象、多个HandlerInterceptor 拦截器）对象，通过这种策略模式，很容易添加新
   的映射策略；
3. DispatcherServlet——>HandlerAdapter，HandlerAdapter 将会把处理器包装为适配器，从而支持多种类型的处理器，
   即适配器设计模式的应用，从而很容易支持很多类型的处理器；
4. HandlerAdapter——>处理器功能处理方法的调用，HandlerAdapter 将会根据适配的结果调用真正的处理器的功能处
   理方法，完成功能处理；并返回一个ModelAndView 对象（包含模型数据、逻辑视图名）；
5. ModelAndView 的逻辑视图名——> ViewResolver，ViewResolver 将把逻辑视图名解析为具体的View，通过这种策
   略模式，很容易更换其他视图技术；
6. View——>渲染，View 会根据传进来的Model 模型数据进行渲染，此处的Model 实际是一个Map 数据结构，因此
   很容易支持其他视图技术；
7. 返回控制权给DispatcherServlet，由DispatcherServlet 返回响应给用户，到此一个流程结束。

doGet入口

我们以上个demo中这个GET请求为例，请求URL是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。

@Override protected 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. */ @Override protected 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); } } } 请求分发

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); } } } } 映射和适配器处理

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 @Nullable public 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

@Override protected 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) */ @Nullable protected 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

视图渲染

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 [pdai.tech)