DispatcherServlet是SpringMVC中的前端控制器,负责接收请求，转发，匹配，数据处理后，还转由页面进行展现，是SpringMVC实现中核心的部分，本篇文章就是分析请求处理的过程。
先看一下入口方法
@Override protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception { if (logger.isDebugEnabled()) { String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : ""; logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed + " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]"); } //如果是Include请求，则保存一份请求参数的快照 //为了执行完请求之后能将参数恢复到原来的值 Map<String, Object> attributesSnapshot = null; if (WebUtils.isIncludeRequest(request)) { attributesSnapshot = new HashMap<String, Object>(); Enumeration<?> attrNames = request.getAttributeNames(); while (attrNames.hasMoreElements()) { String attrName = (String) attrNames.nextElement(); if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) { attributesSnapshot.put(attrName, request.getAttribute(attrName)); } } } // 设置request域中的一些属性 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()); 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 { //分发请求的入口 doDispatch(request, response); } finally { if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { //将参数恢复到原来的值 if (attributesSnapshot != null) { restoreAttributesAfterInclude(request, attributesSnapshot); } } } } 主要的处理流程在doDispatch()方法里，看一下doDispatch()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 { //1.检查是否是文件上传请求 processedRequest = checkMultipart(request); multipartRequestParsed = (processedRequest != request); 2.获取对应请求的handler mappedHandler = getHandler(processedRequest); if (mappedHandler == null || mappedHandler.getHandler() == null) { noHandlerFound(processedRequest, response); return; } //3.获取请求的适配器HandlerAdapter HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); //4.缓存处理 //如果是get或者head类型的请求，缓存时间没有过期则直接返回 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; } } //5.拦截器的预处理 if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } //6.实际的处理器处理请求 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } applyDefaultViewName(request, mv); //7.拦截器的后处理 mappedHandler.applyPostHandle(processedRequest, response, mv); } catch (Exception ex) { dispatchException = ex; } //8.处理请求的结果，视图部分的处理 processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException); } catch (Exception ex) { triggerAfterCompletion(processedRequest, response, mappedHandler, ex); } catch (Error err) { triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err); } finally { if (asyncManager.isConcurrentHandlingStarted()) { if (mappedHandler != null) { mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); } } else { if (multipartRequestParsed) { cleanupMultipart(processedRequest); } } } } 这就是整个处理请求的过程，我们在分析一下里面的主要步骤。先看如何获取请求的HandlerExcetionChain protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { //通过HandlerMapping获取请求对应的Handler for (HandlerMapping hm : this.handlerMappings) { if (logger.isTraceEnabled()) { logger.trace( "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'"); } HandlerExecutionChain handler = hm.getHandler(request); if (handler != null) { return handler; } } return null; } 可以看出是通过HandlerMapping获取请求的Handler，HandlerMapping就是为请求找到对应的Controller。 在看一下如何执行请求的 private ModelAndView invokeHandleMethod(HttpServletRequest request, HttpServletResponse response, HandlerMethod handlerMethod) throws Exception { ServletWebRequest webRequest = new ServletWebRequest(request, response); WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod); ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory); ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory); //创建MVC continer，用于存放所有肯恩够用到的Model和View ModelAndViewContainer mavContainer = new ModelAndViewContainer(); mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request)); modelFactory.initModel(webRequest, mavContainer, requestMappingMethod); mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect); //异步处理相关的设置 AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response); asyncWebRequest.setTimeout(this.asyncRequestTimeout); final 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(); if (logger.isDebugEnabled()) { logger.debug("Found concurrent result value [" + result + "]"); } requestMappingMethod = requestMappingMethod.wrapConcurrentResult(result); } //执行请求 requestMappingMethod.invokeAndHandle(webRequest, mavContainer); if (asyncManager.isConcurrentHandlingStarted()) { return null; } //获取视图返回 return getModelAndView(mavContainer, modelFactory, webRequest); } public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception { //通过反射机制来执行请求 Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs); //设置响应的状态 setResponseStatus(webRequest); if (returnValue == null) { if (isRequestNotModified(webRequest) || hasResponseStatus() || mavContainer.isRequestHandled()) { mavContainer.setRequestHandled(true); return; } } else if (StringUtils.hasText(this.responseReason)) { mavContainer.setRequestHandled(true); return; } mavContainer.setRequestHandled(false); try { this.returnValueHandlers.handleReturnValue( returnValue, getReturnValueType(returnValue), mavContainer, webRequest); } catch (Exception ex) { if (logger.isTraceEnabled()) { logger.trace(getReturnValueHandlingErrorMessage("Error handling return value", returnValue), ex); } throw ex; } } 可以看出最终是通过反射机制来处理请求，最后看一下视图处理部分。主要逻辑在render方法中 protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception { //从request请求中获取local信息 Locale locale = this.localeResolver.resolveLocale(request); //将local信息设置到response里 response.setLocale(locale); //根据ModelAndView中设置的视图名称进行解析，得到相应的视图对象 if (mv.isReference()) { // 需要对视图名进行解析 view = resolveViewName(mv.getViewName(), mv.getModelInternal(), locale, request); if (view == null) { throw new ServletException("Could not resolve view with name '" + mv.getViewName() + "' in servlet with name '" + getServletName() + "'"); } } else { //如果容器中已经存在了则直接使用 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.isDebugEnabled()) { logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'"); } try { //调用view实现对数据进行呈现给客户端 view.render(mv.getModelInternal(), request, response); } catch (Exception ex) { if (logger.isDebugEnabled()) { logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'", ex); } throw ex; } } 至此就分析完了整个DispatchServlet的处理过程。