SpringMVC 执行流程解析

注：SpringMVC 版本 5.2.15

在这里插入图片描述
上面这张图许多人都看过，本文试图从源码的角度带大家分析一下该过程。

1. ContextLoaderListener

首先我们从 ContextLoaderListener 讲起，它继承自 ServletContextListener，用于监听 Web 应用程序的启动与停止。ContextLoaderListener 中的 contextInitialized() 方法用于初始化 Web 应用程序上下文。

ContextLoaderListener # contextInitialized

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {.../*** Initialize the root web application context.*/@Overridepublic void contextInitialized(ServletContextEvent event) {initWebApplicationContext(event.getServletContext());}...
}

其中又调用了 initWebApplicationContext() 方法初始化 Web 应用程序上下文

ContextLoader # initWebApplicationContext

public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {if (servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE) != null) {throw new IllegalStateException("Cannot initialize context because there is already a root application context present - " +"check whether you have multiple ContextLoader* definitions in your web.xml!");}servletContext.log("Initializing Spring root WebApplicationContext");Log logger = LogFactory.getLog(ContextLoader.class);if (logger.isInfoEnabled()) {logger.info("Root WebApplicationContext: initialization started");}long startTime = System.currentTimeMillis();try {if (this.context == null) {// 创建并保存应用程序上下文到属性中this.context = createWebApplicationContext(servletContext);}if (this.context instanceof ConfigurableWebApplicationContext) {ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;if (!cwac.isActive()) {if (cwac.getParent() == null) {ApplicationContext parent = loadParentContext(servletContext);cwac.setParent(parent);}// 配置并刷新当前 Web 应用程序上下文configureAndRefreshWebApplicationContext(cwac, servletContext);}}servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);ClassLoader ccl = Thread.currentThread().getContextClassLoader();if (ccl == ContextLoader.class.getClassLoader()) {currentContext = this.context;}else if (ccl != null) {currentContextPerThread.put(ccl, this.context);}if (logger.isInfoEnabled()) {long elapsedTime = System.currentTimeMillis() - startTime;logger.info("Root WebApplicationContext initialized in " + elapsedTime + " ms");}return this.context;}catch (RuntimeException | Error ex) {logger.error("Context initialization failed", ex);servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);throw ex;}
}

该方法中调用了 createWebApplicationContext() 方法创建了 Web 应用程序上下文，并调用 configureAndRefreshWebApplicationContext() 方法配置并刷新当前 Web 应用程序上下文。

ContextLoader # configureAndRefreshWebApplicationContext

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc) {// 为当前 Web 应用程序上下文设置一个 idif (ObjectUtils.identityToString(wac).equals(wac.getId())) {String idParam = sc.getInitParameter(CONTEXT_ID_PARAM);if (idParam != null) {wac.setId(idParam);}else {wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +ObjectUtils.getDisplayString(sc.getContextPath()));}}// 设置 ServletContextwac.setServletContext(sc);String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);if (configLocationParam != null) {wac.setConfigLocation(configLocationParam);}ConfigurableEnvironment env = wac.getEnvironment();if (env instanceof ConfigurableWebEnvironment) {((ConfigurableWebEnvironment) env).initPropertySources(sc, null);}customizeContext(sc, wac);// 刷新当前应用程序上下文wac.refresh();
}

该方法的主要作用是刷新当前应用程序上下文，实际上就是调用了 AbstractApplicationContext # refresh 方法，也就是我们常讲的启动 IOC 容器，这个方法里的具体内容这里就不讲了。

总结：
ContextLoaderListener 的作用是准备好 Web 应用程序上下文，启动 IOC 容器。

2. DispatcherServlet 初始化逻辑

先看一张 DispatcherServlet 的继承关系图
在这里插入图片描述
其中 HttpServletBean 有一个 init() 方法，该方法是一个初始化方法

HttpServletBean # init

@Override
public final void init() throws ServletException {// 获取 web.xml 文件中的 <init-param> 中的参数保存到 PropertyValues 中PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);if (!pvs.isEmpty()) {try {// 将 HttpServletBean 对象转换为 BeanWrapper 对象BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);// 创建一个 ResourceLoader 对象ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());// 注册属性编辑器bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));// 初始化 BeanWrapper 对象initBeanWrapper(bw);// 将属性值保存到 BeanWrapper 中bw.setPropertyValues(pvs, true);}catch (BeansException ex) {if (logger.isErrorEnabled()) {logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);}throw ex;}}// Let subclasses do whatever initialization they like.initServletBean();
}

该方法中首先尝试获取 web.xml 文件中的 init-param 值，如果获取到的话，则将它保存到 BeanWrapper 中。最后调用了 initServletBean() 方法

FrameworkServlet # initServletBean

@Override
protected final void initServletBean() throws ServletException {getServletContext().log("Initializing Spring " + getClass().getSimpleName() + " '" + getServletName() + "'");if (logger.isInfoEnabled()) {logger.info("Initializing Servlet '" + getServletName() + "'");}long startTime = System.currentTimeMillis();try {// 初始化 WebApplicationContextthis.webApplicationContext = initWebApplicationContext();// 空方法，可由子类去具体实现initFrameworkServlet();}catch (ServletException | RuntimeException ex) {logger.error("Context initialization failed", ex);throw ex;}if (logger.isDebugEnabled()) {String value = this.enableLoggingRequestDetails ?"shown which may lead to unsafe logging of potentially sensitive data" :"masked to prevent unsafe logging of potentially sensitive data";logger.debug("enableLoggingRequestDetails='" + this.enableLoggingRequestDetails +"': request parameters and headers will be " + value);}if (logger.isInfoEnabled()) {logger.info("Completed initialization in " + (System.currentTimeMillis() - startTime) + " ms");}
}

该方法中主要调用了 initWebApplicationContext() 去初始化 web 应用程序上下文

FrameworkServlet # initWebApplicationContext

protected WebApplicationContext initWebApplicationContext() {WebApplicationContext rootContext =WebApplicationContextUtils.getWebApplicationContext(getServletContext());WebApplicationContext wac = null;if (this.webApplicationContext != null) {wac = this.webApplicationContext;if (wac instanceof ConfigurableWebApplicationContext) {ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;if (!cwac.isActive()) {if (cwac.getParent() == null) {cwac.setParent(rootContext);}// 配置并刷新 web 应用程序上下文configureAndRefreshWebApplicationContext(cwac);}}}if (wac == null) {// 去获取 web 应用程序上下文wac = findWebApplicationContext();}if (wac == null) {// 创建 web 应用程序上下文wac = createWebApplicationContext(rootContext);}if (!this.refreshEventReceived) {synchronized (this.onRefreshMonitor) {// 刷新 web 应用程序上下文onRefresh(wac);}}if (this.publishContext) {String attrName = getServletContextAttributeName();getServletContext().setAttribute(attrName, wac);}return wac;
}

该方法主要是去创建一个新的 web 应用程序上下文，然后调用 onRefresh() 方法

FrameworkServlet # onRefresh

@Override
protected void onRefresh(ApplicationContext context) {initStrategies(context);
}

DispatcherServlet # initStrategies

protected void initStrategies(ApplicationContext context) {// 初始化 文件上传解析器initMultipartResolver(context);// 初始化 本地化解析器initLocaleResolver(context);// 初始化 主题解析器initThemeResolver(context);// 初始化 处理器映射器initHandlerMappings(context);// 初始化 处理器映射适配器initHandlerAdapters(context);// 初始化 异常解析器initHandlerExceptionResolvers(context);// 初始化 请求获取视图名转换器initRequestToViewNameTranslator(context);// 初始化 视图解析器initViewResolvers(context);// 初始化 FlashMap 管理器initFlashMapManager(context);
}

该方法中初始化了 SpringMVC 的九大组件。我以 initHandlerMappings() 方法为例讲解一下，其他组件的获取方式和它基本相同。

DispatcherServlet # initHandlerMappings

/*** Initialize the HandlerMappings used by this class.* <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace,* we default to BeanNameUrlHandlerMapping.*/
private void initHandlerMappings(ApplicationContext context) {this.handlerMappings = null;// 从 ApplicationContext 中去获取 HandlerMapping// 获取不到的话去获取对象名为 handlerMapping 的 HandlerMapping 对象// 再获取不到的话则注册默认的 HandlerMappingif (this.detectAllHandlerMappings) {// 从 ApplicationContext 中获取Map<String, HandlerMapping> matchingBeans =BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);if (!matchingBeans.isEmpty()) {this.handlerMappings = new ArrayList<>(matchingBeans.values());AnnotationAwareOrderComparator.sort(this.handlerMappings);}}else {try {// 获取名为 handlerMapping 的 HandlerMapping 对象HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);this.handlerMappings = Collections.singletonList(hm);}catch (NoSuchBeanDefinitionException ex) {}}if (this.handlerMappings == null) {// 注册默认的 HandlerMappingthis.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);if (logger.isTraceEnabled()) {logger.trace("No HandlerMappings declared for servlet '" + getServletName() +"': using default strategies from DispatcherServlet.properties");}}
}

该方法首先从 ApplicationContext 中去获取 HandlerMapping，获取不到的话去获取对象名为 handlerMapping 的 HandlerMapping 对象，再获取不到的话则注册默认的 HandlerMapping。我们看一下这个默认的 HandlerMapping 是怎么获取的。

DispatcherServlet # getDefaultStrategies

protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {String key = strategyInterface.getName();// 获取逻辑在这String value = defaultStrategies.getProperty(key);if (value != null) {// 获取类名String[] classNames = StringUtils.commaDelimitedListToStringArray(value);List<T> strategies = new ArrayList<>(classNames.length);for (String className : classNames) {try {// 创建 Class 对象Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());Object strategy = createDefaultStrategy(context, clazz);strategies.add((T) strategy);}catch (ClassNotFoundException ex) {throw new BeanInitializationException("Could not find DispatcherServlet's default strategy class [" + className +"] for interface [" + key + "]", ex);}catch (LinkageError err) {throw new BeanInitializationException("Unresolvable class definition for DispatcherServlet's default strategy class [" +className + "] for interface [" + key + "]", err);}}return strategies;}else {return new LinkedList<>();}
}

可以看到会尝试从 defaultStrategies 中获取值。然后将获取到的值通过 Class.forName() 方法去创建对象。那么我们看一下这个 defaultStrategies 是什么。

private static final Properties defaultStrategies;static {try {ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);}catch (IOException ex) {throw new IllegalStateException("Could not load '" + DEFAULT_STRATEGIES_PATH + "': " + ex.getMessage());}
}

private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";

defaultStrategies 是 DispatcherServlet 中的一个属性，类型为 Properties 。在 DispatcherServlet 的静态代码块中加载了 DispatcherServlet.properties 文件到了 defaultStrategies 中。
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DispatcherServlet.properties

org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolverorg.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolverorg.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping,\org.springframework.web.servlet.function.support.RouterFunctionMappingorg.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter,\org.springframework.web.servlet.function.support.HandlerFunctionAdapterorg.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver,\org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolverorg.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslatororg.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolverorg.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager

在 DispatcherServlet.properties 中定义了 HandlerMapping、HandlerAdapter、ViewResolver 等类。它们的获取逻辑是相似的。通过文件中定义的全限定名，然后调用 Class.forName() 方法去创建对象。

3. DispatcherServlet 处理流程

以 get 请求为例。当发送 get 请求时，由 FrameworkServlet # doGet 方法进行处理。

FrameworkServlet # doGet

@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {processRequest(request, response);
}

该方法中又调用了 processRequest() 方法

FrameworkServlet # processRequest

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {long startTime = System.currentTimeMillis();Throwable failureCause = null;// 获取一个 LocaleContext 对象LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();// 构建一个新的 LocaleContext 对象LocaleContext localeContext = buildLocaleContext(request);// 获取一个 RequestAttributes 对象RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();// 构建一个新的 ServletRequestAttributes 对象ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());// 初始化资源持有者// 将 localeContext 保存到 LocaleContextHolder 中// 将 requestAttributes 保存到 RequestContextHolder 中initContextHolders(request, localeContext, requestAttributes);try {doService(request, response);}catch (ServletException | IOException ex) {failureCause = ex;throw ex;}catch (Throwable ex) {failureCause = ex;throw new NestedServletException("Request processing failed", ex);}finally {resetContextHolders(request, previousLocaleContext, previousAttributes);if (requestAttributes != null) {requestAttributes.requestCompleted();}logResult(request, response, failureCause, asyncManager);publishRequestHandledEvent(request, response, startTime, failureCause);}
}

该方法中新建了一个 LocaleContext 对象和一个 ServletRequestAttributes 对象，并保存到了 LocaleContextHolder 与 RequestContextHolder 中。尤其是 RequestContextHolder 对象，我们可以通过它去获取 HttpServletRequest、HttpServletResponse 等对象。最后调用了 doService() 方法。

DispatcherServlet # doService

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {logRequest(request);// 保留 request attributes 的快照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));}}}// Make framework objects available to handlers and view objects.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);}try {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);}}}
}

该方法中又调用了 doDispatch() 方法

DispatcherServlet # doDispatch

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 {// 检查是否是上传文件的请求processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// 获取 HandlerExecutionChain // 该方法中确定了用哪个处理器处理当前请求// 并添加了拦截器mappedHandler = getHandler(processedRequest);// 没有找到合适的处理器if (mappedHandler == null) {noHandlerFound(processedRequest, response);return;}// 获取 HandlerAdapterHandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// 获取请求方法String method = request.getMethod();boolean isGet = "GET".equals(method);if (isGet || "HEAD".equals(method)) {long lastModified = ha.getLastModified(request, mappedHandler.getHandler());if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {return;}}// 执行拦截器的前置处理if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// 处理请求并返回 ModelAndView 对象mv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}applyDefaultViewName(processedRequest, mv);// 执行拦截器的后置处理mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;}catch (Throwable err) {dispatchException = new NestedServletException("Handler dispatch failed", err);}// 处理最终的结果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()) {if (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}}else {if (multipartRequestParsed) {cleanupMultipart(processedRequest);}}}
}

接下来我们看一下 getHandler() 方法

DispatcherServlet # getHandler

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {if (this.handlerMappings != null) {// 遍历所有的 HandlerMapping 去获取 HandlerExecutionChain for (HandlerMapping mapping : this.handlerMappings) {HandlerExecutionChain handler = mapping.getHandler(request);if (handler != null) {return handler;}}}return null;
}

该方法中遍历了所有的 HandlerMapping 去获取 Handler

AbstractHandlerMapping # getHandler

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {// 获取合适的 HandlerObject handler = getHandlerInternal(request);if (handler == null) {// 没有找到合适的 Handler，则使用默认的 Handlerhandler = getDefaultHandler();}if (handler == null) {return null;}// 获取到了该 Handler 的字符串名，则通过字符串名去获取对应的对象if (handler instanceof String) {String handlerName = (String) handler;handler = obtainApplicationContext().getBean(handlerName);}// 为当前请求添加拦截器HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);if (logger.isTraceEnabled()) {logger.trace("Mapped to " + handler);}else if (logger.isDebugEnabled() && !request.getDispatcherType().equals(DispatcherType.ASYNC)) {logger.debug("Mapped to " + executionChain.getHandler());}if (hasCorsConfigurationSource(handler) || CorsUtils.isPreFlightRequest(request)) {CorsConfiguration config = (this.corsConfigurationSource != null ? this.corsConfigurationSource.getCorsConfiguration(request) : null);CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);config = (config != null ? config.combine(handlerConfig) : handlerConfig);executionChain = getCorsHandlerExecutionChain(request, executionChain, config);}return executionChain;
}

该方法中去获取合适的 Handler，获取失败的话，则会使用默认的 Handler。并为该请求添加拦截器。

RequestMappingInfoHandlerMapping # getHandlerInternal

protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {request.removeAttribute(PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);try {return super.getHandlerInternal(request);}finally {ProducesRequestCondition.clearMediaTypesAttribute(request);}
}

AbstractHandlerMethodMapping # getHandlerInternal

protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {// 获取请求路径String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);request.setAttribute(LOOKUP_PATH, lookupPath);// 加读锁this.mappingRegistry.acquireReadLock();try {// 根据请求路径获取对应的 HandlerMethod HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);}finally {// 释放读锁this.mappingRegistry.releaseReadLock();}
}

该方法中根据请求路径去获取对应的 HandlerMethod

AbstractHandlerMethodMapping # lookupHandlerMethod

protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {List<Match> matches = new ArrayList<>();// 根据请求路径获取对应的映射List<T> directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);if (directPathMatches != null) {// 获取匹配的映射// 这里面会做一系列的比较// 如：方法名比较、参数比较、请求头比较等addMatchingMappings(directPathMatches, matches, request);}if (matches.isEmpty()) {// No choice but to go through all mappings...addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, request);}// 匹配到合适的方法，则从中找到最匹配的方法if (!matches.isEmpty()) {Match bestMatch = matches.get(0);if (matches.size() > 1) {Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));matches.sort(comparator);bestMatch = matches.get(0);if (logger.isTraceEnabled()) {logger.trace(matches.size() + " matching mappings: " + matches);}if (CorsUtils.isPreFlightRequest(request)) {return PREFLIGHT_AMBIGUOUS_MATCH;}Match secondBestMatch = matches.get(1);if (comparator.compare(bestMatch, secondBestMatch) == 0) {Method m1 = bestMatch.handlerMethod.getMethod();Method m2 = secondBestMatch.handlerMethod.getMethod();String uri = request.getRequestURI();throw new IllegalStateException("Ambiguous handler methods mapped for '" + uri + "': {" + m1 + ", " + m2 + "}");}}request.setAttribute(BEST_MATCHING_HANDLER_ATTRIBUTE, bestMatch.handlerMethod);handleMatch(bestMatch.mapping, lookupPath, request);return bestMatch.handlerMethod;}// 没有找到合适的方法，返回 nullelse {return handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);}
}

该方法中会去获取与请求最匹配的方法，获取不到则返回 null。

这里就已经获取完 handler 了，也就是一个 HandlerMethod 对象，比如 TestController # test 。我们往下走。

@Controller
public class TestController {@GetMapping("/test")public String test() {return "test";}
}

DispatcherServlet # getHandlerAdapter

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {if (this.handlerAdapters != null) {for (HandlerAdapter adapter : this.handlerAdapters) {if (adapter.supports(handler)) {return adapter;}}}throw new ServletException("No adapter for handler [" + handler +"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}

该方法就是去找一个对应的 HandlerAdapter，一般是 RequestMappingHandlerAdapter。

AbstractHandlerMethodAdapter # handle

public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)throws Exception {return handleInternal(request, response, (HandlerMethod) handler);
}

RequestMappingHandlerAdapter # handleInternal

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 necessarymav = 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;
}

该方法中调用了 invokeHandlerMethod() 方法

RequestMappingHandlerAdapter # invokeHandlerMethod

protected ModelAndView invokeHandlerMethod(HttpServletRequest request,HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {ServletWebRequest webRequest = new ServletWebRequest(request, response);try {// 找到 @InitBinder 标注的方法// @InitBinder 与 WebDatabinder 一起使用，用于注册自定义的属性编辑器WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);// 找到 @ModelAttribute 标注的方法ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);// 注册参数解析器if (this.argumentResolvers != null) {invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);}// 注册返回值解析器if (this.returnValueHandlers != null) {invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);}invocableMethod.setDataBinderFactory(binderFactory);// 注册参数名发现器invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);ModelAndViewContainer mavContainer = new ModelAndViewContainer();mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));// 调用 @ModelAttribute 标注的方法，保证它在其他方法执行前先被执行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);}// 调用该方法invocableMethod.invokeAndHandle(webRequest, mavContainer);if (asyncManager.isConcurrentHandlingStarted()) {return null;}// 返回 ModelAndView 对象return getModelAndView(mavContainer, modelFactory, webRequest);}finally {webRequest.requestCompleted();}
}

该方法中会找到并注册 @InitBinder 标注的方法，找到并调用 @ModelAttribute 标注的的方法。调用 invokeAndHandleI() 方法，最终返回 ModelAndView 对象

ServletInvocableHandlerMethod # invokeAndHandle

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {// 调用方法并获取返回值Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);setResponseStatus(webRequest);if (returnValue == null) {if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {disableContentCachingIfNecessary(webRequest);mavContainer.setRequestHandled(true);return;}}else if (StringUtils.hasText(getResponseStatusReason())) {mavContainer.setRequestHandled(true);return;}mavContainer.setRequestHandled(false);Assert.state(this.returnValueHandlers != null, "No return value handlers");try {// 使用对应的 HandlerMethodReturnValueHandler 去处理返回值this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);}catch (Exception ex) {if (logger.isTraceEnabled()) {logger.trace(formatErrorForReturnValue(returnValue), ex);}throw ex;}
}

InvocableHandlerMethod # invokeForRequest

public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {// 获取形参值Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);if (logger.isTraceEnabled()) {logger.trace("Arguments: " + Arrays.toString(args));}// 调用方法return doInvoke(args);
}

InvocableHandlerMethod # getMethodArgumentValues

protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {// 获取 MethodParameter 数组MethodParameter[] parameters = getMethodParameters();if (ObjectUtils.isEmpty(parameters)) {return EMPTY_ARGS;}Object[] args = new Object[parameters.length];for (int i = 0; i < parameters.length; i++) {MethodParameter parameter = parameters[i];parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);args[i] = findProvidedArgument(parameter, providedArgs);if (args[i] != null) {continue;}if (!this.resolvers.supportsParameter(parameter)) {throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));}try {// 解析参数args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);}catch (Exception ex) {// Leave stack trace for later, exception may actually be resolved and handled...if (logger.isDebugEnabled()) {String exMsg = ex.getMessage();if (exMsg != null && !exMsg.contains(parameter.getExecutable().toGenericString())) {logger.debug(formatArgumentError(parameter, exMsg));}}throw ex;}}return args;
}

该方法中调用了 resolveArgument() 去解析参数

HandlerMethodArgumentResolverComposite # resolveArgument

public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {// 获取 HandlerMethodArgumentResolver -> RequestParamMethodArgumentResolverHandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);if (resolver == null) {throw new IllegalArgumentException("Unsupported parameter type [" +parameter.getParameterType().getName() + "]. supportsParameter should be called first.");}// 解析参数return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
}

AbstractNamedValueMethodArgumentResolver # resolveArgument

public final Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {NamedValueInfo namedValueInfo = getNamedValueInfo(parameter);MethodParameter nestedParameter = parameter.nestedIfOptional();Object resolvedName = resolveEmbeddedValuesAndExpressions(namedValueInfo.name);if (resolvedName == null) {throw new IllegalArgumentException("Specified name must not resolve to null: [" + namedValueInfo.name + "]");}Object arg = resolveName(resolvedName.toString(), nestedParameter, webRequest);if (arg == null) {if (namedValueInfo.defaultValue != null) {arg = resolveEmbeddedValuesAndExpressions(namedValueInfo.defaultValue);}else if (namedValueInfo.required && !nestedParameter.isOptional()) {handleMissingValue(namedValueInfo.name, nestedParameter, webRequest);}arg = handleNullValue(namedValueInfo.name, arg, nestedParameter.getNestedParameterType());}else if ("".equals(arg) && namedValueInfo.defaultValue != null) {arg = resolveEmbeddedValuesAndExpressions(namedValueInfo.defaultValue);}if (binderFactory != null) {WebDataBinder binder = binderFactory.createBinder(webRequest, null, namedValueInfo.name);try {arg = binder.convertIfNecessary(arg, parameter.getParameterType(), parameter);}catch (ConversionNotSupportedException ex) {throw new MethodArgumentConversionNotSupportedException(arg, ex.getRequiredType(),namedValueInfo.name, parameter, ex.getCause());}catch (TypeMismatchException ex) {throw new MethodArgumentTypeMismatchException(arg, ex.getRequiredType(),namedValueInfo.name, parameter, ex.getCause());}}handleResolvedValue(arg, namedValueInfo.name, parameter, mavContainer, webRequest);return arg;
}

AbstractNamedValueMethodArgumentResolver # getNamedValueInfo

private NamedValueInfo getNamedValueInfo(MethodParameter parameter) {// 先从缓存中获取NamedValueInfo namedValueInfo = this.namedValueInfoCache.get(parameter);// 缓存中获取不到if (namedValueInfo == null) {// 获取 @RequestParam 中的 value 属性值，如果有，则将参数名设置为该值namedValueInfo = createNamedValueInfo(parameter);// 如果没有使用 @RequestParam 注解或使用了但没有设置 value 属性// 则使用 ASM 框架去获取字节码来获取属性名namedValueInfo = updateNamedValueInfo(parameter, namedValueInfo);this.namedValueInfoCache.put(parameter, namedValueInfo);}return namedValueInfo;
}

RequestParamMethodArgumentResolver # createNamedValueInfo

protected NamedValueInfo createNamedValueInfo(MethodParameter parameter) {// 根据 @RequestParam 注解去创建 NamedValueInfo // 如果 @RequestParam 中的 value 属性有值，则设置参数名为该值RequestParam ann = parameter.getParameterAnnotation(RequestParam.class);return (ann != null ? new RequestParamNamedValueInfo(ann) : new RequestParamNamedValueInfo());
}

AbstractNamedValueMethodArgumentResolver # updateNamedValueInfo

private NamedValueInfo updateNamedValueInfo(MethodParameter parameter, NamedValueInfo info) {String name = info.name;if (info.name.isEmpty()) {name = parameter.getParameterName();if (name == null) {throw new IllegalArgumentException("Name for argument of type [" + parameter.getNestedParameterType().getName() +"] not specified, and parameter name information not found in class file either.");}}String defaultValue = (ValueConstants.DEFAULT_NONE.equals(info.defaultValue) ? null : info.defaultValue);return new NamedValueInfo(name, info.required, defaultValue);
}

MethodParameter # getParameterName

public String getParameterName() {if (this.parameterIndex < 0) {return null;}ParameterNameDiscoverer discoverer = this.parameterNameDiscoverer;if (discoverer != null) {String[] parameterNames = null;if (this.executable instanceof Method) {parameterNames = discoverer.getParameterNames((Method) this.executable);}else if (this.executable instanceof Constructor) {parameterNames = discoverer.getParameterNames((Constructor<?>) this.executable);}if (parameterNames != null) {this.parameterName = parameterNames[this.parameterIndex];}this.parameterNameDiscoverer = null;}return this.parameterName;
}

LocalVariableTableParameterNameDiscoverer # getParameterNames

public String[] getParameterNames(Method method) {Method originalMethod = BridgeMethodResolver.findBridgedMethod(method);return doGetParameterNames(originalMethod);
}

LocalVariableTableParameterNameDiscoverer # doGetParameterNames

private String[] doGetParameterNames(Executable executable) {Class<?> declaringClass = executable.getDeclaringClass();Map<Executable, String[]> map = this.parameterNamesCache.computeIfAbsent(declaringClass, this::inspectClass);return (map != NO_DEBUG_INFO_MAP ? map.get(executable) : null);
}

LocalVariableTableParameterNameDiscoverer # inspectClass

private Map<Executable, String[]> inspectClass(Class<?> clazz) {// 获取字节码文件InputStream is = clazz.getResourceAsStream(ClassUtils.getClassFileName(clazz));if (is == null) {if (logger.isDebugEnabled()) {logger.debug("Cannot find '.class' file for class [" + clazz +"] - unable to determine constructor/method parameter names");}return NO_DEBUG_INFO_MAP;}try {ClassReader classReader = new ClassReader(is);Map<Executable, String[]> map = new ConcurrentHashMap<>(32);// ASM 框架提升字节码文件classReader.accept(new ParameterNameDiscoveringVisitor(clazz, map), 0);return map;}catch (IOException ex) {if (logger.isDebugEnabled()) {logger.debug("Exception thrown while reading '.class' file for class [" + clazz +"] - unable to determine constructor/method parameter names", ex);}}catch (IllegalArgumentException ex) {if (logger.isDebugEnabled()) {logger.debug("ASM ClassReader failed to parse class file [" + clazz +"], probably due to a new Java class file version that isn't supported yet " +"- unable to determine constructor/method parameter names", ex);}}finally {try {is.close();}catch (IOException ex) {}}return NO_DEBUG_INFO_MAP;
}

走到这一步便是通过 ASM 框架来提升字节码文件获取参数名称了。所以推荐使用 @ReuquestParam 并设置 value 属性值，这样可以直接获取到参数名，避免了 ASM 框架编辑字节码所带来的性能消耗。

DispatcherServlet # 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) {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);}}if (mv != null && !mv.wasCleared()) {// 渲染 ModelAndViewrender(mv, request, response);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 forwardreturn;}if (mappedHandler != null) {// Exception (if any) is already handled..mappedHandler.triggerAfterCompletion(request, response, null);}
}

本方法中调用了 render() 方法去渲染视图

DispatcherServlet # render

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {Locale locale =(this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());response.setLocale(locale);View view;String viewName = mv.getViewName();if (viewName != null) {// 根据 viewName 解析出 Viewview = 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;}
}

本方法中调用了 resolveViewName() 去获取 View

DispatcherServlet # resolveViewName

protected View resolveViewName(String viewName, @Nullable Map<String, Object> model,Locale locale, HttpServletRequest request) throws Exception {if (this.viewResolvers != null) {for (ViewResolver viewResolver : this.viewResolvers) {// 使用 ViewResolver 去解析出 ViewView view = viewResolver.resolveViewName(viewName, locale);if (view != null) {return view;}}}return null;
}

本方法中找到合适的 ViewResolver 去解析出 View

public View resolveViewName(String viewName, Locale locale) throws Exception {// 是否启用了缓存功能，默认启用了if (!isCache()) {return createView(viewName, locale);}else {// 先尝试从缓存中取 ViewObject cacheKey = getCacheKey(viewName, locale);View view = this.viewAccessCache.get(cacheKey);if (view == null) {synchronized (this.viewCreationCache) {view = this.viewCreationCache.get(cacheKey);if (view == null) {// 缓存中没有去到，交给子类去创建 Viewview = createView(viewName, locale);if (view == null && this.cacheUnresolved) {view = UNRESOLVED_VIEW;}if (view != null && this.cacheFilter.filter(view, viewName, locale)) {this.viewAccessCache.put(cacheKey, view);this.viewCreationCache.put(cacheKey, view);}}}}else {if (logger.isTraceEnabled()) {logger.trace(formatKey(cacheKey) + "served from cache");}}return (view != UNRESOLVED_VIEW ? view : null);}
}

本方法中判断是否开启了视图缓存功能，默认是开启了的。先尝试从缓存中取 View，取不到的话则交给子类去创建 View。

UrlBasedViewResolver # createView

@Override
protected View createView(String viewName, Locale locale) throws Exception {// 判断该视图解析器是否能处理这个视图// 如果不能的话，交给其他视图解析器去处理if (!canHandle(viewName, locale)) {return null;}// 检查前缀是否为 "redirect:"// 有的话则是请求重定向if (viewName.startsWith(REDIRECT_URL_PREFIX)) {// 截取 "redirect:" 后的字符串String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());// 创建重定视图RedirectView view = new RedirectView(redirectUrl,isRedirectContextRelative(), isRedirectHttp10Compatible());String[] hosts = getRedirectHosts();if (hosts != null) {view.setHosts(hosts);}return applyLifecycleMethods(REDIRECT_URL_PREFIX, view);}// 检查前缀是否为 "forward:"// 有的话则是请求转发if (viewName.startsWith(FORWARD_URL_PREFIX)) {String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());InternalResourceView view = new InternalResourceView(forwardUrl);return applyLifecycleMethods(FORWARD_URL_PREFIX, view);}// 如果没有加这两个前缀的话，则回调给父类处理// 其实最终的处理结果是跟请求转发一样的return super.createView(viewName, locale);
}

该方法中会获取字符串前缀。前缀为 “redirect:” 则是请求重定向，为 “forward:” 则是请求转发。如果没有这两个前缀的话，又回调到父类 AbstractCachingViewResolver # createView 方法中。

AbstractCachingViewResolver # createView

protected View createView(String viewName, Locale locale) throws Exception {return loadView(viewName, locale);
}

回调到父类 AbstractCachingViewResolver # createView 方法，并调用了 loadView() 方法

InternalResourceViewResolver # buildView

protected AbstractUrlBasedView buildView(String viewName) throws Exception {// 可以看到，这里创建的 View 类型跟请求转发的相同InternalResourceView view = (InternalResourceView) super.buildView(viewName);if (this.alwaysInclude != null) {view.setAlwaysInclude(this.alwaysInclude);}view.setPreventDispatchLoop(true);return view;
}

本方法中调用了 buildView() 方法创建 View，可以看到，这里创建的 View 类型跟请求转发的相同

UrlBasedViewResolver # buildView

protected AbstractUrlBasedView buildView(String viewName) throws Exception {Class<?> viewClass = getViewClass();Assert.state(viewClass != null, "No view class");AbstractUrlBasedView view = (AbstractUrlBasedView) BeanUtils.instantiateClass(viewClass);// 拼接上前缀和后缀// 可以从 web.xml 文件中获取view.setUrl(getPrefix() + viewName + getSuffix());view.setAttributesMap(getAttributesMap());String contentType = getContentType();if (contentType != null) {view.setContentType(contentType);}String requestContextAttribute = getRequestContextAttribute();if (requestContextAttribute != null) {view.setRequestContextAttribute(requestContextAttribute);}Boolean exposePathVariables = getExposePathVariables();if (exposePathVariables != null) {view.setExposePathVariables(exposePathVariables);}Boolean exposeContextBeansAsAttributes = getExposeContextBeansAsAttributes();if (exposeContextBeansAsAttributes != null) {view.setExposeContextBeansAsAttributes(exposeContextBeansAsAttributes);}String[] exposedContextBeanNames = getExposedContextBeanNames();if (exposedContextBeanNames != null) {view.setExposedContextBeanNames(exposedContextBeanNames);}return view;
}

本方法中将 viewName 拼接了前缀和后缀，获取到的便是最终的完整路径。这里并将 View 获取成功了。

AbstractView # render

public void render(@Nullable Map<String, ?> model, HttpServletRequest request,HttpServletResponse response) throws Exception {if (logger.isDebugEnabled()) {logger.debug("View " + formatViewName() +", model " + (model != null ? model : Collections.emptyMap()) +(this.staticAttributes.isEmpty() ? "" : ", static attributes " + this.staticAttributes));}// 创建并合并属性到 Map 中// 如：保存到 ModelAndView 或 Model 中的属性Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);// 准备好 HttpServletResponse 对象prepareResponse(request, response);// 解析出视图renderMergedOutputModel(mergedModel, getRequestToExpose(request), response);
}

该方法中主要是将我们自己保存到 ModelAndView 或 Model 中的属性转换为 Map 对象。最后调用 renderMergedOutputModel() 方法。

InternalResourceView # renderMergedOutputModel

protected void renderMergedOutputModel(Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {// 将属性保存到 request 域中exposeModelAsRequestAttributes(model, request);// Expose helpers as request attributes, if any.exposeHelpers(request);// 确定请求的 urlString dispatcherPath = prepareForRendering(request, response);// 获取一个请求转发器RequestDispatcher rd = getRequestDispatcher(request, dispatcherPath);if (rd == null) {throw new ServletException("Could not get RequestDispatcher for [" + getUrl() +"]: Check that the corresponding file exists within your web application archive!");}// 是一个 include 请求，jsp 中经常用到，实现 jsp 页面的复用if (useInclude(request, response)) {response.setContentType(getContentType());if (logger.isDebugEnabled()) {logger.debug("Including [" + getUrl() + "]");}rd.include(request, response);}else {if (logger.isDebugEnabled()) {logger.debug("Forwarding to [" + getUrl() + "]");}// 请求转发rd.forward(request, response);}
}