14.2、DispatcherServlet初始化过程
DispatcherServlet 本质上是一个 Servlet，所以天然的遵循 Servlet 的生命周期。所以宏观上是 Servlet 生命周期来进行调度。
① 初始化WebApplicationContext
所在类：org.springframework.web.servlet.FrameworkServlet
protected WebApplicationContext initWebApplicationContext() { WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(this.getServletContext()); WebApplicationContext wac = null; if (this.webApplicationContext != null) { // A context instance was injected at construction time -> use it wac = this.webApplicationContext; if (wac instanceof ConfigurableWebApplicationContext) { ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext)wac; if (!cwac.isActive()) { // The context has not yet been refreshed -> provide services such as // setting the parent context, setting the application context id, etc if (cwac.getParent() == null) { // The context instance was injected without an explicit parent -> set // the root application context (if any; may be null) as the parent cwac.setParent(rootContext); } this.configureAndRefreshWebApplicationContext(cwac); } } } if (wac == null) { // No context instance was injected at construction time -> see if one // has been registered in the servlet context. If one exists, it is assumed // that the parent context (if any) has already been set and that the // user has performed any initialization such as setting the context id wac = this.findWebApplicationContext(); } if (wac == null) { // No context instance is defined for this servlet -> create a local one // 创建WebApplicationContext wac = this.createWebApplicationContext(rootContext); } if (!this.refreshEventReceived) { // Either the context is not a ConfigurableApplicationContext with refresh // support or the context injected at construction time had already been // refreshed -> trigger initial onRefresh manually here. synchronized(this.onRefreshMonitor) { this.onRefresh(wac); } } if (this.publishContext) { // Publish the context as a servlet context attribute. // 将IOC容器在应用域共享 String attrName = this.getServletContextAttributeName(); this.getServletContext().setAttribute(attrName, wac); } return wac; }
② 创建WebApplicationContext
所在类：org.springframework.web.servlet.FrameworkServlet
protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) { Class<?> contextClass = this.getContextClass(); if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) { throw new ApplicationContextException("Fatal initialization error in servlet with name '" + this.getServletName() + "': custom WebApplicationContext class [" + contextClass.getName() + "] is not of type ConfigurableWebApplicationContext"); } else { // 通过反射创建 IOC 容器对象 ConfigurableWebApplicationContext wac = (ConfigurableWebApplicationContext)BeanUtils.instantiateClass(contextClass); wac.setEnvironment(this.getEnvironment()); // 设置父容器 wac.setParent(parent); String configLocation = this.getContextConfigLocation(); if (configLocation != null) { wac.setConfigLocation(configLocation); } this.configureAndRefreshWebApplicationContext(wac); return wac; } }
③ DispatcherServlet初始化策略
FrameworkServlet创建WebApplicationContext后，刷新容器，调用onRefresh(wac)，此方法在 DispatcherServlet中进行了重写，调用了initStrategies(context)方法，初始化策略，即初始化 DispatcherServlet的各个组件
所在类：org.springframework.web.servlet.DispatcherServlet
protected void initStrategies(ApplicationContext context) { this.initMultipartResolver(context); this.initLocaleResolver(context); this.initThemeResolver(context); this.initHandlerMappings(context); this.initHandlerAdapters(context); this.initHandlerExceptionResolvers(context); this.initRequestToViewNameTranslator(context); this.initViewResolvers(context); this.initFlashMapManager(context); }
14.3、DispatcherServlet调用组件处理请求
① processRequest()
FrameworkServlet重写HttpServlet中的service()和doXxx()，这些方法中调用了 processRequest(request, response)
所在类：org.springframework.web.servlet.FrameworkServlet
protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { long startTime = System.currentTimeMillis(); Throwable failureCause = null; LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext(); LocaleContext localeContext = this.buildLocaleContext(request); RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes(); ServletRequestAttributes requestAttributes = this.buildRequestAttributes(request, response, previousAttributes); WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor()); this.initContextHolders(request, localeContext, requestAttributes); try { // 执行服务，doService()是一个抽象方法，在DispatcherServlet中进行了重写 this.doService(request, response); } catch (IOException | ServletException var16) { failureCause = var16; throw var16; } catch (Throwable var17) { failureCause = var17; throw new NestedServletException("Request processing failed", var17); } finally { this.resetContextHolders(request, previousLocaleContext, previousAttributes); if (requestAttributes != null) { requestAttributes.requestCompleted(); } this.logResult(request, response, (Throwable)failureCause, asyncManager); this.publishRequestHandledEvent(request, response, startTime, (Throwable)failureCause); } }
② doService()
所在类：org.springframework.web.servlet.DispatcherServlet
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception { this.logRequest(request); // Keep a snapshot of the request attributes in case of an include, // to be able to restore the original attributes after the include. Map<String, Object> attributesSnapshot = null; if (WebUtils.isIncludeRequest(request)) { attributesSnapshot = new HashMap(); Enumeration<?> attrNames = request.getAttributeNames(); label116: while(true) { String attrName; do { if (!attrNames.hasMoreElements()) { break label116; } attrName = (String)attrNames.nextElement(); } while(!this.cleanupAfterInclude && !attrName.startsWith("org.springframework.web.servlet")); attributesSnapshot.put(attrName, request.getAttribute(attrName)); } } // Make framework objects available to handlers and view objects. 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()); 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 { // 处理请求和响应 this.doDispatch(request, response); } finally { // Restore the original attribute snapshot, in case of an include. if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) { this.restoreAttributesAfterInclude(request, attributesSnapshot); } if (this.parseRequestPath) { ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request); } } }
③ doDispatch()
所在类：org.springframework.web.servlet.DispatcherServlet
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; Exception dispatchException = null; try { processedRequest = this.checkMultipart(request); multipartRequestParsed = processedRequest != request; // Determine handler for the current request. /* mappedHandler：调用链 包含handler、interceptorList、interceptorIndex handler：浏览器发送的请求所匹配的控制器方法 interceptorList：处理控制器方法的所有拦截器集合 interceptorIndex：拦截器索引，控制拦截器afterCompletion()的执行 */ mappedHandler = this.getHandler(processedRequest); if (mappedHandler == null) { this.noHandlerFound(processedRequest, response); return; } // Determine handler adapter for the current request. // 通过控制器方法创建相应的处理器适配器，调用所对应的控制器方法 HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler()); // Process last-modified header, if supported by the handler. 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; } } // 调用拦截器的preHandle() if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // Actually invoke the handler. // 由处理器适配器调用具体的控制器方法，最终获得ModelAndView对象 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } this.applyDefaultViewName(processedRequest, mv); // 调用拦截器的postHandle() mappedHandler.applyPostHandle(processedRequest, response, mv); } catch (Exception var20) { dispatchException = var20; } catch (Throwable var21) { // 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", var21); } // 后续处理：处理模型数据和渲染视图 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()) { // Instead of postHandle and afterCompletion if (mappedHandler != null) { mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); } } else if (multipartRequestParsed) { // Clean up any resources used by a multipart request. this.cleanupMultipart(processedRequest); } } }
④ processDispatchResult()
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv, @Nullable Exception exception) throws Exception { boolean errorView = false; if (exception != null) { if (exception instanceof ModelAndViewDefiningException) { this.logger.debug("ModelAndViewDefiningException encountered", exception); mv = ((ModelAndViewDefiningException)exception).getModelAndView(); } else { Object handler = mappedHandler != null ? mappedHandler.getHandler() : null; mv = this.processHandlerException(request, response, handler, exception); errorView = mv != null; } } // Did the handler return a view to render? if (mv != null && !mv.wasCleared()) { // 处理模型数据和渲染视图 this.render(mv, request, response); if (errorView) { WebUtils.clearErrorRequestAttributes(request); } } else if (this.logger.isTraceEnabled()) { this.logger.trace("No view rendering, null ModelAndView returned."); } // Concurrent handling started during a forward if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { if (mappedHandler != null) { // Exception (if any) is already handled.. // 调用拦截器的afterCompletion() mappedHandler.triggerAfterCompletion(request, response, (Exception)null); } } }
14.4、SpringMVC的执行流程
1.用户向服务器发送请求，请求被SpringMVC 前端控制器 DispatcherServlet捕获。
2.DispatcherServlet对请求URL进行解析，得到请求资源标识符（URI），判断请求URI对应的映射：
不存在
再判断是否配置了 mvc:default-servlet-handler
如果没配置，则控制台报映射查找不到，客户端展示404错误
如果有配置，则访问目标资源（一般为静态资源，如：JS,CSS,HTML），找不到客户端也会展示404错误
存在
1.根据该URI，调用HandlerMapping获得该Handler配置的所有相关的对象（包括Handler对象以及 Handler对象对应的拦截器），最后以HandlerExecutionChain执行链对象的形式返回。
2.DispatcherServlet 根据获得的Handler，选择一个合适的HandlerAdapter。
3.如果成功获得HandlerAdapter，此时将开始执行拦截器的preHandler(…)方法【正向】
4.提取Request中的模型数据，填充Handler入参，开始执行Handler（Controller)方法，处理请求。 在填充Handler的入参过程中，根据你的配置，Spring将帮你做一些额外的工作：
HttpMessageConveter： 将请求消息（如Json、xml等数据）转换成一个对象，将对象转换为指定 的响应信息
数据转换：对请求消息进行数据转换。如String转换成Integer、Double等
数据格式化：对请求消息进行数据格式化。 如将字符串转换成格式化数字或格式化日期等
数据验证： 验证数据的有效性（长度、格式等），验证结果存储到BindingResult或Error中
5.Handler执行完成后，向DispatcherServlet 返回一个ModelAndView对象。
6.此时将开始执行拦截器的postHandle(…)方法【逆向】。
7.根据返回的ModelAndView（此时会判断是否存在异常：如果存在异常，则执行 HandlerExceptionResolver进行异常处理）选择一个适合的ViewResolver进行视图解析，根据Model 和View，来渲染视图。
8.渲染视图完毕执行拦截器的afterCompletion(…)方法【逆向】。
9.将渲染结果返回给客户端。
