SpringMVC 源码解析

前言

年初面试时接触到一道面试题，在聊到SpringMVC时提到了SpringMVC的开发者为何要设计父子容器呢，又或者说是父子容器的设计有什么更实际的作用呢？
         首先要理解对于一个web应用，当其部署在web容器上时，容器会为其提供一个全局上下文环境ServletContext，这个上下文环境将为后续的Spring提供宿主环境。

SpringMVC工作流程

DispatcherServlet上下文继承关系

SpringMVC设计的父子容器

父子容器配置文件

--在web.xml中配置，两个重要的xml:applicationContext.xml和SpringMVC-conf.xml
<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath*:applictionContext.xml</param-value>
</context-param>
<listener>
   <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
  
<servlet>
    <servlet-name>dispatcher-servlet</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
      <param-name>contextConfigLocation</param-name>
      <param-value>classpath*:springMVC-conf.xml</param-value>
    </init-param>
</servlet>
  
<servlet-mapping>
  <servlet-name>dispatcher-servlet</servlet-name>
  <url-pattern>/*</url-pattern>
</servlet-mapping>

父子容器的设计目的

根据SpringMVC的官方解释，父（根）容器主要包括一些基础脚手架的bean，比如Pool、DataSource、Dao、Service。目的是在不同的Servlet实例之间共享。这些不同的bean可以在子容器中重写。
     而子容器主要包括一些Controller、View等一些web相关的bean。

DispatcherServlet源码分析

既然SpringMVC中同时包含Spring容器和SpringMVC容器，那么这两个容器都是在什么时候初始化呢？

根容器初始化

首先，根容器是通过ServletContext监听器进行创建，默认的监听器为ContextLoaderListener，当web应用启动时，会调用监听器的contextInitialized方法。
      那么根容器的初始化就从ContextLoaderListener类说起吧，，Spring官方对该类的描述是启动监听器去启动和关闭Spring的root WebApplicationContext（翻译的实在有点蹩脚）。
     

public class ContextLoaderListener extends ContextLoader implements ServletContextListener {
	public ContextLoaderListener() {
	}

	public ContextLoaderListener(WebApplicationContext context) {
		super(context);
	}

    //===初始化root WebApplicationContext===
	@Override
	public void contextInitialized(ServletContextEvent event) {
		initWebApplicationContext(event.getServletContext());
	}

	@Override
	public void contextDestroyed(ServletContextEvent event) {
		closeWebApplicationContext(event.getServletContext());
		ContextCleanupListener.cleanupAttributes(event.getServletContext());
	}
}

//ContextLoader.java
public WebApplicationContext initWebApplicationContext(ServletContext servletContext) {
	//初始化Spring容器时如果发现servlet 容器中已存在根Spring容根器则抛出异常，证明rootWebApplicationContext只能有一个。
	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!");
	}

	try {
		//创建webApplicationContext实例
		if (this.context == null) {
		    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WebApplicationContext
				configureAndRefreshWebApplicationContext(cwac, servletContext);
			}
		}
		
		/**
		   把生成的webApplicationContext设置成root WebApplicationContext。保存在ServletContext上下文中。
		   下一步初始化MVC ApplicationContext时需要从ServletContext取出根上下文作为其父上下文。
		**/ 
		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);
		}
		
		return this.context;
	}
	catch (RuntimeException | Error ex) {
		servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
		throw ex;
	}
}

以上代码主要完成两个功能：创建实例WebApplicationContext实例、把所创建的WebApplicationContext设置为根上下文，也就是设置成为ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE的值。

MVC容器初始化

大家知道Servlet生命周期都是从init方法开始，desctory方法结束，由jvm负责垃圾回收。而DispatcherServlet也是一个普通的Servlet，先看一下DispatcherServlet的继承关系图，对整个继承关系有个了解。

   既然说起Servlet，那就从Servlet的初始化（init）方法入手

//HttpServletBean.java
@Override
public final void init() throws ServletException {

	PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
	if (!pvs.isEmpty()) {
		try {
			BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
			ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
			bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
			initBeanWrapper(bw);
			bw.setPropertyValues(pvs, true);
		}
		catch (BeansException ex) {
			throw ex;
		}
	}

	//交给子类重写
	initServletBean();
}

//FrameworkServlet.java
@Override
protected final void initServletBean() throws ServletException {
	try {
		this.webApplicationContext = initWebApplicationContext();
		initFrameworkServlet();
	}
	catch (ServletException | RuntimeException ex) {
		throw ex;
	}
}

//FrameworkServlet.java
//初始化MVC容器
protected WebApplicationContext initWebApplicationContext() {
    //从ServletContext取出根上下文
	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);
				}
				configureAndRefreshWebApplicationContext(cwac);
			}
		}
	}
	if (wac == null) {
		wac = findWebApplicationContext();
	}
	
	//如果还没有webApplicatioinContext，创建webApplicationContext
	if (wac == null) {
		wac = createWebApplicationContext(rootContext);
	}

	//子类自定义对servlet子上下文后续操作，在DispatcherServlet中实现
	if (!this.refreshEventReceived) {
		synchronized (this.onRefreshMonitor) {
			//执行子类扩展方法onRefresh，在DispatcherServlet内初始化所有web相关组件
			onRefresh(wac);
		}
	}

	//发布servlet子上下文到ServletContext
	if (this.publishContext) {
		String attrName = getServletContextAttributeName();
		//将servlet子上下文以org.springframework.web.servlet.FrameworkServlet.CONTEXT. + servletName的属性名称注册到ServletContext中
		getServletContext().setAttribute(attrName, wac);
	}

	return wac;
}

protected WebApplicationContext createWebApplicationContext(@Nullable WebApplicationContext parent) {
	return createWebApplicationContext((ApplicationContext) parent);
}

protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
    //获取WebApplicationContext实现类，此处其实就是XmlWebApplicationContext
	Class<?> contextClass = getContextClass();
	if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
		throw new ApplicationContextException("Fatal initialization error in servlet with name '" + getServletName() +
				"': custom WebApplicationContext class [" + contextClass.getName() +
				"] is not of type ConfigurableWebApplicationContext");
	}
	
	//生成XmlWebApplicationContext实例
	ConfigurableWebApplicationContext wac =
			(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

	wac.setEnvironment(getEnvironment());
	//设置根容器为父容器 
	wac.setParent(parent);
	String configLocation = getContextConfigLocation();
	if (configLocation != null) {
	    //设置配置文件
		wac.setConfigLocation(configLocation);
	}
	
	//配置webApplicationContext
	configureAndRefreshWebApplicationContext(wac);

	return wac;
}

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
	if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
		if (this.contextId != null) {
			wac.setId(this.contextId);
		}
		else {
			wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + ObjectUtils.getDisplayString(getServletContext().getContextPath()) + '/' + getServletName());
		}
	}

	wac.setServletContext(getServletContext());
	wac.setServletConfig(getServletConfig());
	wac.setNamespace(getNamespace());
	wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));

	ConfigurableEnvironment env = wac.getEnvironment();
	if (env instanceof ConfigurableWebEnvironment) {
		((ConfigurableWebEnvironment) env).initPropertySources(getServletContext(), getServletConfig());
	}

	postProcessWebApplicationContext(wac);
	applyInitializers(wac);
	
	//开始处理bean
	wac.refresh();
}

上面的关键代码都在FrameworkServlet类中，有几个关键点：取除根上下文，创建子上下文并设置父上下文，完成刷新，把子上下文发布到ServletContext中。 到这里可以说子容器（子上下文）已经创建完成。 并把其他初始化web组件的相关工作交给onRefresh方法完成，由DispatcherServlet来重写onRefresh方法，这就又回到了我们熟悉的initStrategies方法。

web组件初始化

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

protected void initStrategies(ApplicationContext context) {
	//文件上传解析器
	initMultipartResolver(context);
	
	//本地化解析器
	initLocaleResolver(context);
	
	//主题解析器
	initThemeResolver(context);
	
	//处理器映射器(url和Controller方法的映射)
	initHandlerMappings(context);
	
	//处理器适配器(实际执行Controller方法)
	initHandlerAdapters(context);
	
	//处理器异常解析器
	initHandlerExceptionResolvers(context);
	
	//RequestToViewName解析器
	initRequestToViewNameTranslator(context);
	
	//视图解析器(视图的匹配和渲染)
	initViewResolvers(context);
	
	//FlashMap管理者
	initFlashMapManager(context);
}

这里我们主要关注一下三个重要组件：HandlerMapping、HandlerAdapter、ViewResolver。分析这3个组件之前，我们先看一下我们的springMVC-conf.xml配置文件，mvc的配置文件中，我们配置了两行代码：

<context:component-scan base-package="com.zhangfei"/>
<mvc:annotation-driven>

第二行代码主要是添加了默认的HandleMapping,ViewResolver,HandleAdapter。我们看看annotation-driven的源码定义，根据spring自定义schema定义，我们找到如下代码，如图所示：

该文件就一行代码：

http://www.springframework.org/schema/mvc=org.springframework.web.servlet.config.MvcNamespaceHandler

//MVC所有的标签解析器都定义在此
public class MvcNamespaceHandler extends NamespaceHandlerSupport {
	@Override
	public void init() {
		registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());
		registerBeanDefinitionParser("default-servlet-handler", new DefaultServletHandlerBeanDefinitionParser());
		registerBeanDefinitionParser("interceptors", new InterceptorsBeanDefinitionParser());
		registerBeanDefinitionParser("resources", new ResourcesBeanDefinitionParser());
		registerBeanDefinitionParser("view-controller", new ViewControllerBeanDefinitionParser());
		registerBeanDefinitionParser("redirect-view-controller", new ViewControllerBeanDefinitionParser());
		registerBeanDefinitionParser("status-controller", new ViewControllerBeanDefinitionParser());
		registerBeanDefinitionParser("view-resolvers", new ViewResolversBeanDefinitionParser());
		registerBeanDefinitionParser("tiles-configurer", new TilesConfigurerBeanDefinitionParser());
		registerBeanDefinitionParser("freemarker-configurer", new FreeMarkerConfigurerBeanDefinitionParser());
		registerBeanDefinitionParser("groovy-configurer", new GroovyMarkupConfigurerBeanDefinitionParser());
		registerBeanDefinitionParser("script-template-configurer", new ScriptTemplateConfigurerBeanDefinitionParser());
		registerBeanDefinitionParser("cors", new CorsBeanDefinitionParser());
	}
}

那么通过分析AnnotationDrivenBeanDefinitionParser类，主要完成以下三大组件的装配工作：

初始化处理器映射器

private void initHandlerMappings(ApplicationContext context) {
	this.handlerMappings = null;

    //这里detectAllHandlerMappings默认值为true，可以通过配置文件设置为false
	if (this.detectAllHandlerMappings) {
	    //从上下文(包含父上下文)中查找所有HandlerMapping实现类
		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 {
		    //这里只取固定的bean
			HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
			this.handlerMappings = Collections.singletonList(hm);
		}
		catch (NoSuchBeanDefinitionException ex) {
			
		}
	}
	
	/***
	  确保至少有一个HandlerMapping，如果没能找到，注册一个默认的
	  默认规则在DispatcherServlet.properties中，这里也就是取BeanNameUrlHandlerMapping、RequestMappingHandlerMapping
	***/
	if (this.handlerMappings == null) {
		this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);		
	}
}

初始化处理器适配器

private void initHandlerAdapters(ApplicationContext context) {
	this.handlerAdapters = null;

	if (this.detectAllHandlerAdapters) {
	    //从上下文（包括父上下文）中查找所有HandlerAdapter实现类
		Map<String, HandlerAdapter> matchingBeans =BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
		if (!matchingBeans.isEmpty()) {
			this.handlerAdapters = new ArrayList<>(matchingBeans.values());
			AnnotationAwareOrderComparator.sort(this.handlerAdapters);
		}
	}
	else {
		try {
		    //这里取bean名字为handlerAdapter，类型为HandlerAdapter的处理器适配器
			HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
			this.handlerAdapters = Collections.singletonList(ha);
		}
		catch (NoSuchBeanDefinitionException ex) {
			
		}
	}
	
	/**
	如果没找到，则从默认规则里取出指定的三个实现类：HttpRequestHandlerAdapter、SimpleControllerHandlerAdapter、RequestMappingHandlerAdapter
	**/
	if (this.handlerAdapters == null) {
		this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);		
	}
}

初始化试图解析器

private void initViewResolvers(ApplicationContext context) {
	this.viewResolvers = null;

	if (this.detectAllViewResolvers) {
	    //从上下文（包括父上下文）中查找所有ViewResolver实现类
		Map<String, ViewResolver> matchingBeans =BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.class, true, false);
		if (!matchingBeans.isEmpty()) {
			this.viewResolvers = new ArrayList<>(matchingBeans.values());
			AnnotationAwareOrderComparator.sort(this.viewResolvers);
		}
	}
	else {
		try {
			ViewResolver vr = context.getBean(VIEW_RESOLVER_BEAN_NAME, ViewResolver.class);
			this.viewResolvers = Collections.singletonList(vr);
		}
		catch (NoSuchBeanDefinitionException ex) {
			
		}
	}
	
	/**
	如果没找到，则从默认规则里取出指定的实现类：InternalResourceViewResolver
	**/
	if (this.viewResolvers == null) {
		this.viewResolvers = getDefaultStrategies(context, ViewResolver.class);	
	}
}

三大组件的初始化最后判断为NULL时都会调用getDefaultStrategies方法，也就是从DispatcherServlet.properties中取出指定默认值。

protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
	String key = strategyInterface.getName();
	String value = defaultStrategies.getProperty(key);
	if (value != null) {
		String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
		List<T> strategies = new ArrayList<>(classNames.length);
		for (String className : classNames) {
			try {
				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<>();
	}
}

DispatcherServlet请求处理过程

提到请求处理过程，我们再来回顾一下Servlet生命周期，处理请求都放在service方法中处理，那么也从DispatcherServlet的service方法入手。DispatcherServlet继承FrameworkServlet，在FrameworkServlet中重写了service、doGet、doPost、doPut、doDelete方法。

//FrameworkServlet.java
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
		throws ServletException, IOException {
	HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
	if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
		processRequest(request, response);
	}
	else {
		super.service(request, response);
	}
}

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

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

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

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


protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

	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());
	
	//把新构造的LocaleContext对象和ServletRequestAttributes对象和当前请求线程绑定（后面要解除绑定）
	initContextHolders(request, localeContext, requestAttributes);

	try {
	    //抽象方法，交给DispatcherServlet方法实现
		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LocaleContext和RequestAttributes对象，也就是解除LocaleContext对象和ServletRequestAttributes对象和当前请求线程的绑定
		resetContextHolders(request, previousLocaleContext, previousAttributes);
		if (requestAttributes != null) {
			requestAttributes.requestCompleted();
		}
		//发布ServletRequestHandledEvent事件
		publishRequestHandledEvent(request, response, startTime, failureCause);
	}
}

//DispatcherServlet.java
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
	
	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));
			}
		}
	}
	
	//在当前request对象中填充4个属性
	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());

	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 {
	    //主要处理分发请求
		doDispatch(request, response);
	}
	finally {
		if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
			if (attributesSnapshot != null) {
				restoreAttributesAfterInclude(request, attributesSnapshot);
			}
		}
	}
}


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 {
			processedRequest = checkMultipart(request);
			multipartRequestParsed = (processedRequest != request);
            
            //调用handlerMapping获取handlerChain
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null) {
				noHandlerFound(processedRequest, response);
				return;
			}

            //获取支持该handler解析的HandlerAdapter
			HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

			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;
				}
			}

			if (!mappedHandler.applyPreHandle(processedRequest, response)) {
				return;
			}

            //使用HandlerAdapter完成handler处理
			mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

			if (asyncManager.isConcurrentHandlingStarted()) {
				return;
			}

            //视图处理(页面渲染)
			applyDefaultViewName(processedRequest, mv);
			mappedHandler.applyPostHandle(processedRequest, response, mv);
		}
		catch (Exception ex) {
			dispatchException = ex;
		}
		catch (Throwable err) {
			dispatchException = new NestedServletException("Handler dispatch failed", err);
		}
		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);
			}
		}
	}
}

DispatcherServlet的doDispatch方法概括起来大致就是以下几点：首先根据当前请求路径找到对应的HandlerMethod，一个HandlerMethod和若干个拦截器构造一个HandlerExecutionChain.通过HandlerExecutionChain得到HandlerAdapter对象，通过执行HandlerAdapter的handle方法得到ModelAndView对象，调用ModelAndView解析视图，渲染视图，Response结束。

参考

https://juejin.im/post/5cb89dae6fb9a0686b47306d
https://juejin.im/post/5cbc10b46fb9a0689f4c2c22
https://www.cnblogs.com/fangjian0423/p/springMVC-dispatcherServlet.html