SpringMVC源码剖析-SpringMVC执行流程

大家好，又见面了，我是你们的朋友全栈君。

前言

上一篇文章写得是SpringMVC组件初始化，这篇文章我们来探讨一下SpringMVC的执行流程

SpringMVC执行流程

SpringMVC执行流程几乎是在面试时面试官对SpringMVC部分的必问之题，下面是SpirngMVC的执行原理图

这个是请求在SpringMVC的执行流程

1. DispatcherServlet：请求打过来由DispatcherServlet处理，它是 SpringMVC 中的前端控制器(中央控制器), 负责接收 Request 并将 Request 转发给对应的处理组件
2. HandlerMapping：HandlerMapping 维护了 url 和 Controller(Handler)的 映 射关系 。 DispatcherServlet 接 收 请求, 然 后 从 HandlerMapping 查找处理请求的Controller(Handler)，标注了@RequestMapping 的每个 method 都可以看成是一个 Handler，HandlerMapping 在请求到达之后， 它的作用便是找到请求相应的处理器 Handler 和 Interceptors。
3. HandlerAdapter：SpringMVC通过HandlerAdapter对Handler进行执行，这是适配器模式的应用，通过扩展适配器可以对更多类型的处理器进行执行。它的作用就是按照特定的规则去执行 Controller (Handler)
4. Handler : Controller (Handler)负责处理请求，Controller 执行后并返回 ModelAndView 对象，其中包括了数据模型和逻辑视图,ModelAndView 是封装结果 视图的组件。Handler把结果返回给HandlerAdapter，HandlerAdapter把结果返回给DispatcherServlet前端控制器。
5. ViewResolver：DispatcherServlet收到ModelAndView,调用视图解析器(ViewResolver)来解析HandlerAdapter传递的ModelAndView。Handler执行完成后返回的是逻辑视图，也就是视图名字，一个String ，还有一个Model就是数据模型，封装成ModelAndView。ViewResolver视图解析的作用就是根据视图名，把本地模板文件(比如：xx.jsp;xx.ftl)解析为View视图对象。View用来渲染视图，也就是负责把Handler返回的数据模型model,填充到模板(jsp;ftl)形成html格式的静态内容。
6. 最后就是把生成的html通过response写给浏览器，浏览器进行html渲染展示。

核心类认识

开始之前先来认识几个对象，否则下面会晕菜

HandlerMethod ：是的controller中方法的封装，其中包括在Controller的Bean对象以及Method方法对象

public class HandlerMethod { 
   

	/** Logger that is available to subclasses */
	protected final Log logger = LogFactory.getLog(HandlerMethod.class);

	private final Object bean;	//controller对应的bean

	private final Method method;	//方法对象

	private final BeanFactory beanFactory;

	private final MethodParameter[] parameters;

	private final Method bridgedMethod;

MappedInterceptor ：对拦截器的封装

public final class MappedInterceptor { 
   

	private final String[] includePatterns;	

	private final String[] excludePatterns; //排除

	private final HandlerInterceptor interceptor; //拦截器

RequestMappingInfo : 对@Controller和@RequestMapping注解的类的方法进行解析，然后把解析的结果封装成RequestMappingInfo ， 对象中包含了针对@RequestMapping注解的6个属性的匹配条件，DispaterServlet需要根据找到请求匹配的Handler就需要满足6个匹配条件。

• PatternsRequestCondition ：模式请求路径过滤器 ，根据request请求路径进行匹配
• RequestMethodsRequestCondition ： 请求方法过滤器，根据request请求方式进行匹配
• ParamsRequestCondition ：请求参数过滤器，根据请求的参数进行匹配
• HeadersRequestCondition ： 请求过滤器 ，根据请求头进行匹配
• ConsumesRequestCondition ：根据 筛选出同请求Accpet匹配的媒体类型表达式列表
• ProducesRequestCondition ：应答媒体类型过滤器，根据请求Content-Type媒体类型进行匹配
• RequestCondition (optional, custom request condition)

public final class RequestMappingInfo implements RequestCondition<RequestMappingInfo> { 
   
	//根据url进行匹配
	private final PatternsRequestCondition patternsCondition;
	//方法匹配条件
	private final RequestMethodsRequestCondition methodsCondition;
	//请求参数过滤器，参数匹配条件
	private final ParamsRequestCondition paramsCondition;
	//头字段过滤器，请求头匹配条件
	private final HeadersRequestCondition headersCondition;
   //Consumes匹配条件
	private final ConsumesRequestCondition consumesCondition;
   // Produces匹配条件
	private final ProducesRequestCondition producesCondition;

DispatchServlet执行流程

DispatchServlet是一个Servlet，请求进来会执行 DispatcherServlet#doService 方法

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception { 
   
		...省略...

		// Make framework objects available to handlers and view objects.
		//把容器对象设置到request
		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
		//把localeResolver设置给request
		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
		//把themeResolver设置给request
		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));
		}
		//设置flashMapManager
		request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
		request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

		try { 
   
			//执行请求分发
			doDispatch(request, response);
		}
		finally { 
   
			if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { 
   
				return;
			}
			// Restore the original attribute snapshot, in case of an include.
			if (attributesSnapshot != null) { 
   
				restoreAttributesAfterInclude(request, attributesSnapshot);
			}
		}
	}

DispatcherServlet#doService调用了DispatcherServlet#doDispatch 做请求处理

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception { 
   
		HttpServletRequest processedRequest = request;
		HandlerExecutionChain mappedHandler = null;
		boolean multipartRequestParsed = false;
		//管理异步请求
		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

		try { 
   
			ModelAndView mv = null;
			Exception dispatchException = null;

			try { 
   
			    //1.检查是否是Multipart文件上传
				processedRequest = checkMultipart(request);
				multipartRequestParsed = processedRequest != request;

				// Determine handler for the current request.
				//2.遍历所有 handlerMappings 找到当前请求对应的handler，
				//比如：RequestMappingHandlerMapping 中维护了一个LinkedHashMap类型的handlerMethods，
				//是对HandlerMethod的存储。底层根据URL从handlerMethods匹配一个HandlerMethod，
				//然后把HandlerMethod封装到一个HandlerExecutionChain中
				//HandlerMethod是对controller方法method的封装，
				//这里拿到的是HandlerExecutionChain handler执行链，其中封装了Handler和inteceptor
				mappedHandler = getHandler(processedRequest, false);
				if (mappedHandler == null || mappedHandler.getHandler() == null) { 
   
					//如果没找到会走404
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
				//3.从handlerAdapters中循环，找到支持handler的，处理请求适配器HandlerAdapter(如：RequestMappingHandlerAdapter)
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

				// Process last-modified header, if supported by the handler.
				//处理lastModified
				String method = request.getMethod();
				boolean isGet = "GET".equals(method);
				if (isGet || "HEAD".equals(method)) { 
   
					long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
					if (logger.isDebugEnabled()) { 
   
						String requestUri = urlPathHelper.getRequestUri(request);
						logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
					}
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) { 
   
						return;
					}
				}
				//4.调用拦截器的preHandle
				if (!mappedHandler.applyPreHandle(processedRequest, response)) { 
   
					return;
				}

				try { 
   
					// Actually invoke the handler.
					//5.调用 HandlerAdapter 实际执行handler,返回ModelAndView
					mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
				}
				finally { 
   
					if (asyncManager.isConcurrentHandlingStarted()) { 
   
						return;
					}
				}
				//6.处理默认的视图名，如果mv != null && !mv.hasView()
				//就会使用RequestToViewNameTranslator#getViewName解析一个默认的view
				applyDefaultViewName(request, mv);
				//7.调用拦截器的postHandle
				mappedHandler.applyPostHandle(processedRequest, response, mv);
			}
			catch (Exception ex) { 
   
				dispatchException = ex;
			}
			//8.处理结果，内部会使用viewResolver.resolveViewName渲染视图得到View对象
			//然后执行：View#render方法渲染
			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()) { 
   
				// Instead of postHandle and afterCompletion
				mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
				return;
			}
			// Clean up any resources used by a multipart request.
			if (multipartRequestParsed) { 
   
				cleanupMultipart(processedRequest);
			}
		}
	}

这里对核心方法做一个解释

查找HandlerMethod

getHandler 方法：该方法是遍历所有 handlerMappings ，找到合适的HandlerMapping，然后通过HandlerMapping#getHandler查找当前请求对应的HandlerMethod。(HandlerMethod是对controller中方法的封装),这里拿到的是HandlerExecutionChain 执行链而不是HandlerMethod，HandlerExecutionChain中封装了HandlerMethod和inteceptor拦截器；见：org.springframework.web.servlet.DispatcherServlet#getHandler(javax.servlet.http.HttpServletRequest)

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { 
   
		for (HandlerMapping hm : this.handlerMappings) { 
   
			if (logger.isTraceEnabled()) { 
   
				logger.trace(
						"Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
			}
			//使用HandlerMapping 获取处理请求的Handler,封装到HandlerExecutionChain执行链对象中
			//HandlerExecutionChain包括了HandlerMethod和interceptor拦截器
			HandlerExecutionChain handler = hm.getHandler(request);
			if (handler != null) { 
   
				return handler;
			}
		}
		return null;
	}
	
  public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception { 
   
  		//找到Handler
		Object handler = getHandlerInternal(request);
		if (handler == null) { 
   
			handler = getDefaultHandler();
		}
		if (handler == null) { 
   
			return null;
		}
		// Bean name or resolved handler?
		if (handler instanceof String) { 
   
			String handlerName = (String) handler;
			handler = getApplicationContext().getBean(handlerName);
		}
		//把handler封装成HandlerExecutionChain ，其中还包括了拦截器的封装
		return getHandlerExecutionChain(handler, request);
	}

	protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) { 
   
		//创建一个Handler执行链对象
		HandlerExecutionChain chain =
			(handler instanceof HandlerExecutionChain) ?
				(HandlerExecutionChain) handler : new HandlerExecutionChain(handler);
		//添加拦截器到HandlerExecutionChain 
		chain.addInterceptors(getAdaptedInterceptors());

		String lookupPath = urlPathHelper.getLookupPathForRequest(request);
		for (MappedInterceptor mappedInterceptor : mappedInterceptors) { 
   
			if (mappedInterceptor.matches(lookupPath, pathMatcher)) { 
   
				chain.addInterceptor(mappedInterceptor.getInterceptor());
			}
		}

		return chain;
	}

getHandler方法最终会走到org.springframework.web.servlet.handler.AbstractHandlerMethodMapping#getHandlerInternal ，该方法就是根据request查找一个HandlerMethod

protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception { 
   
		//拿到请求的路径
		String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
		if (logger.isDebugEnabled()) { 
   
			logger.debug("Looking up handler method for path " + lookupPath);
		}
		//根据请求路径，找到匹配的HandlerMethod
		HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);

		if (logger.isDebugEnabled()) { 
   
			if (handlerMethod != null) { 
   
				logger.debug("Returning handler method [" + handlerMethod + "]");
			}
			else { 
   
				logger.debug("Did not find handler method for [" + lookupPath + "]");
			}
		}

		return (handlerMethod != null) ? handlerMethod.createWithResolvedBean() : null;
	}

这里在拿到请求的路径，根据请求路径，找到匹配的HandlerMethod，再往里面走

protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception { 
   
		//matches是requestMppingInfo和HandlerMethod的匹配器集合
		List<Match> matches = new ArrayList<Match>();
		//根据请求的路径，从 this.urlMap中拿到RequestMappingInfo,
		//urlMap 是一个LinkedHashMap，存储了Url路径和RequestMappingInfo的映射关系
		List<T> directPathMatches = this.urlMap.get(lookupPath);
		if (directPathMatches != null) { 
   
			addMatchingMappings(directPathMatches, matches, request);
		}
		
		if (matches.isEmpty()) { 
   
			// No choice but to go through all mappings
			//如果从urlMap中没有找到RequestMappingInfo，那么matches就是空的
			//根据Request对象封装一个requestMppingInfo，然后handlerMethods的keySet中拿到对应的HandlerMethod，(会根据Request中的URL,请求方式，参数params，请求头headers，consumes，produces去匹配handlerMethods.keySet中的requestMppingInfo)
			//然后从handlerMethods根据requestMppingInfo拿到HandlerMethod
			//把requestMppingInfo和HandlerMethod封装成Match添加到matches中
			addMatchingMappings(this.handlerMethods.keySet(), matches, request);
		}

		if (!matches.isEmpty()) { 
   
			Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));
			Collections.sort(matches, comparator);

			if (logger.isTraceEnabled()) { 
   
				logger.trace("Found " + matches.size() + " matching mapping(s) for [" + lookupPath + "] : " + matches);
			}

			Match bestMatch = matches.get(0);
			if (matches.size() > 1) { 
   
				Match secondBestMatch = matches.get(1);
				if (comparator.compare(bestMatch, secondBestMatch) == 0) { 
   
					Method m1 = bestMatch.handlerMethod.getMethod();
					Method m2 = secondBestMatch.handlerMethod.getMethod();
					throw new IllegalStateException(
							"Ambiguous handler methods mapped for HTTP path '" + request.getRequestURL() + "': {" +
							m1 + ", " + m2 + "}");
				}
			}

			handleMatch(bestMatch.mapping, lookupPath, request);
			//从bestMatch拿到Handler
			return bestMatch.handlerMethod;
		}
		else { 
   
			return handleNoMatch(handlerMethods.keySet(), lookupPath, request);
		}
	}

这里出现了两个集合，urlMap 和 handlerMethods ，urlMap 存储的是url和RequestMappingInfo的映射关系，handlerMethods 存储的是RequestMappingInfo和HandlerMethod的关系。

这里的代码比较复杂，先是从 urlMap中根据url拿到RequestMappingInfo集合然后，如果没获取到即：matches.isEmpty()就会调用addMatchingMappings去处理，有兴趣自己去断点一下，该方法的大致流程如下

1. 根据request中的url，method，参数params，请求头headers，consumes，produces去创建一个requestMppingInfo
2. 然后根据requestMppingInfo为key从handlerMethods拿到HandlerMethod封装成Match 对象添加到matches集合中
3. 从Match 中拿到当前请求对应的HandlerMethod

那么这里的handlerMethods和urlMap 是哪儿来的？

以：RequestMappingHandlerMapping为例在其父类AbstractHandlerMethodMapping中维护了一个 Map<T, HandlerMethod> handlerMethods = new LinkedHashMap<T, HandlerMethod>()该集合中装的就是RequestMappingInfo和HandlerMethod的映射关系，RequestMappingInfo是的RequstMapping的封装，HandlerMethod是对Conroller中方法的封装。初始化流程如下

1. AbstractHandlerMethodMapping它实现了InitializingBean接口，在RequestMappingHandlerMapping初始化的使用会触发AbstractHandlerMethodMapping#afterPropertiesSet方法
2. 该方法中会调用AbstractHandlerMethodMapping#initHandlerMethods 初始化HandlerMethod，
   其实就是从容器中拿到所有标识了@Controller或者@RequestMapping注解的Bean，也就是controller,然后拿到其中的所有method封装成HandlerMethod对象,然后再拿到method上RequestMapping信息封装成RquestMappingInfo对象，再把RquestMappingInfo和HandlerMethod以前者为key,后置为value注册到handlerMethods集合中。
3. 把Url路径和RequestMappingInfo的映射关系存储到AbstractHandlerMethodMapping类中的 MultiValueMap<String, T> urlMap = new LinkedMultiValueMap<String, T>()集合中

这个是在MappingHandlerMapping初始化的时候就把HandlerMethod封装好存储在了handlerMethods 集合中，当请求过来就根据Request去匹配一个RequestMappingInfo，然后再找到对应的HandlerMethod返回。

查找HandlerAdapter

查找HandlerAdapter在org.springframework.web.servlet.DispatcherServlet#getHandlerAdapter中,

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

// org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter#supports
	public final boolean supports(Object handler) { 
   
		return handler instanceof HandlerMethod && supportsInternal((HandlerMethod) handler);
	}

从handlerAdapters循环所有的HandlerAdapter，调用supports方法判断是否支持该Handler,如果支持就返回HandlerAdapter

调用拦截器的preHandle

代码在org.springframework.web.servlet.HandlerExecutionChain#applyPreHandle方法中,在HandlerExecutionChain维护了一个HandlerInterceptor[] interceptors，存储了所有的拦截器。源码如下

public class HandlerExecutionChain { 
   

	private static final Log logger = LogFactory.getLog(HandlerExecutionChain.class);

	private final Object handler;
	//拦截器数组
	private HandlerInterceptor[] interceptors;
	...省略...
	
	boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception { 
   
		if (getInterceptors() != null) { 
   
			for (int i = 0; i < getInterceptors().length; i++) { 
   
			//拿到所有的拦截器，调用preHandle方法
				HandlerInterceptor interceptor = getInterceptors()[i];
				if (!interceptor.preHandle(request, response, this.handler)) { 
   
				//如果拦截器返回false，即不放行就执行拦截器的afterCompletion方法
					triggerAfterCompletion(request, response, null);
					return false;
				}
				this.interceptorIndex = i;
			}
		}
		return true;
	}

调用HandlerAdatapter

找到HanlderMethod之后执行拦截器的preHandler方法，然后执行HandlerAdapter#handle,最终会调用RequestMappingHandlerAdapter#invokeHandleMethod

private ModelAndView invokeHandleMethod(HttpServletRequest request,
			HttpServletResponse response, HandlerMethod handlerMethod) throws Exception { 
   
		//把request和response包装到ServletWebRequest对象
		ServletWebRequest webRequest = new ServletWebRequest(request, response);
		
		WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
		ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
		ServletInvocableHandlerMethod requestMappingMethod = createRequestMappingMethod(handlerMethod, binderFactory);
		//ModelAndvView的容器对象，方法调用之后的view和model就会在该容器中
		ModelAndViewContainer mavContainer = new ModelAndViewContainer();
		mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
		modelFactory.initModel(webRequest, mavContainer, requestMappingMethod);
		mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);

		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);
		}
		
		//调用HandlerMethod方法,底层回从HandlerMethod拿到Method,,然后使用反射进行调用
		//把方法返回的viewName和model添加到mavContainer
		requestMappingMethod.invokeAndHandle(webRequest, mavContainer);

		if (asyncManager.isConcurrentHandlingStarted()) { 
   
			return null;
		}
		//获取ModelAndView,从mavContainer拿到viewName和model生成ModelAndView
		return getModelAndView(mavContainer, modelFactory, webRequest);
	}






	

invokeAndHandle底层调用该方法nvocableHandlerMethod#invokeForRequest

public final Object invokeForRequest(NativeWebRequest request,
									 ModelAndViewContainer mavContainer,
									 Object... providedArgs) throws Exception { 
   
	//获取方法的参数
	Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);

	if (logger.isTraceEnabled()) { 
   
		StringBuilder builder = new StringBuilder("Invoking [");
		builder.append(this.getMethod().getName()).append("] method with arguments ");
		builder.append(Arrays.asList(args));
		logger.trace(builder.toString());
	}
	//传入参数，调用方法
	Object returnValue = invoke(args);

	if (logger.isTraceEnabled()) { 
   
		logger.trace("Method [" + this.getMethod().getName() + "] returned [" + returnValue + "]");
	}

	return returnValue;
}

private Object invoke(Object... args) throws Exception { 
   
		ReflectionUtils.makeAccessible(this.getBridgedMethod());
		try { 
   
			//反射调用方法
			return getBridgedMethod().invoke(getBean(), args);
		}
		catch (IllegalArgumentException e) { 
   
			String msg = getInvocationErrorMessage(e.getMessage(), args);
			throw new IllegalArgumentException(msg, e);
		}
		catch (InvocationTargetException e) { 
   
			// Unwrap for HandlerExceptionResolvers ...
			Throwable targetException = e.getTargetException();
			if (targetException instanceof RuntimeException) { 
   
				throw (RuntimeException) targetException;
			}
			else if (targetException instanceof Error) { 
   
				throw (Error) targetException;
			}
			else if (targetException instanceof Exception) { 
   
				throw (Exception) targetException;
			}
			else { 
   
				String msg = getInvocationErrorMessage("Failed to invoke controller method", args);
				throw new IllegalStateException(msg, targetException);
			}
		}
	}

viewResolver渲染视图

handler调用完成之后，代码来到DispatcherServlet#processDispatchResult负责处理结果

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
			HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception { 
   

		boolean errorView = false;

		if (exception != null) { 
   
			if (exception instanceof ModelAndViewDefiningException) { 
   
				logger.debug("ModelAndViewDefiningException encountered", exception);
				mv = ((ModelAndViewDefiningException) exception).getModelAndView();
			}
			else { 
   
			//这里在调用handlerExceptionResolvers处理异常，会循环所有的handlerExceptionResolvers
			//然后根据exception生成一个ModelAndView
				Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
				mv = processHandlerException(request, response, handler, exception);
				errorView = (mv != null);
			}
		}

		// Did the handler return a view to render?
		if (mv != null && !mv.wasCleared()) { 
   
		//【重要】渲染视图
			render(mv, request, response);
			if (errorView) { 
   
				WebUtils.clearErrorRequestAttributes(request);
			}
		}
		else { 
   
			if (logger.isDebugEnabled()) { 
   
				logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
						"': assuming HandlerAdapter completed request handling");
			}
		}

		if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { 
   
			// Concurrent handling started during a forward
			return;
		}

		if (mappedHandler != null) { 
   
			//[重要]执行拦截器的interceptor.afterCompletion
			mappedHandler.triggerAfterCompletion(request, response, null);
		}
	}

代码来到org.springframework.web.servlet.DispatcherServlet#render，该方法负责渲染视图

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.resolveLocale(request);
		response.setLocale(locale);

		View view;
		if (mv.isReference()) { 
   
			// We need to resolve the view name.
			//调用视图解析器，解析viewName，得到View对象
			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 { 
   
			// 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.isDebugEnabled()) { 
   
			logger.debug("Rendering view [" + view + "] in DispatcherServlet with name '" + getServletName() + "'");
		}
		//【重要】调用view.render渲染视图
		view.render(mv.getModelInternal(), request, response);
	}
	
	protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale,
			HttpServletRequest request) throws Exception { 
   
		//调用视图解析器，根据viewName生产一个View
		for (ViewResolver viewResolver : this.viewResolvers) { 
   
			View view = viewResolver.resolveViewName(viewName, locale);
			if (view != null) { 
   
				return view;
			}
		}
		return null;
	}

resolveViewName方法循环所有的viewResolvers ，调用viewResolver.resolveViewName根据viewName解析一个View对象，底层调用AbstractCachingViewResolver#resolveViewName创建View，如下

protected AbstractUrlBasedView buildView(String viewName) throws Exception { 
   
		AbstractUrlBasedView view = (AbstractUrlBasedView) BeanUtils.instantiateClass(getViewClass());
		//前缀+viewName+后缀
		view.setUrl(getPrefix() + viewName + getSuffix());
		String contentType = getContentType();
		if (contentType != null) { 
   
			view.setContentType(contentType);
		}
		view.setRequestContextAttribute(getRequestContextAttribute());
		view.setAttributesMap(getAttributesMap());
		if (this.exposePathVariables != null) { 
   
			view.setExposePathVariables(exposePathVariables);
		}
		return view;
	}

找到View之后，会调用View的render方法渲染视图，见：org.springframework.web.servlet.view.AbstractView#render

public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception { 
   
		if (logger.isTraceEnabled()) { 
   
			logger.trace("Rendering view with name '" + this.beanName + "' with model " + model +
				" and static attributes " + this.staticAttributes);
		}
		//拿到model数据模型
		Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);

		prepareResponse(request, response);
		//渲染
		renderMergedOutputModel(mergedModel, request, response);
	}

继续跟踪org.springframework.web.servlet.view.InternalResourceView#renderMergedOutputModel方法

protected void renderMergedOutputModel(
			Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception { 
   

		// Determine which request handle to expose to the RequestDispatcher.
		HttpServletRequest requestToExpose = getRequestToExpose(request);

		// Expose the model object as request attributes.
		//暴露model对象，把model中的数据设置为request的属性
		exposeModelAsRequestAttributes(model, requestToExpose);

		// Expose helpers as request attributes, if any.
		exposeHelpers(requestToExpose);

		// Determine the path for the request dispatcher.
		//视图中的模板地址 如：WEB-INF/XX.JSP
		String dispatcherPath = prepareForRendering(requestToExpose, response);

		// Obtain a RequestDispatcher for the target resource (typically a JSP).
		//获取目标资源的RequestDispatcher（通常是JSP）。
		RequestDispatcher rd = getRequestDispatcher(requestToExpose, dispatcherPath);
		if (rd == null) { 
   
			throw new ServletException("Could not get RequestDispatcher for [" + getUrl() +
					"]: Check that the corresponding file exists within your web application archive!");
		}

		// If already included or response already committed, perform include, else forward.
		//如果已included或已提交响应，请执行include，否则forward。
		if (useInclude(requestToExpose, response)) { 
   
			response.setContentType(getContentType());
			if (logger.isDebugEnabled()) { 
   
				logger.debug("Including resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'");
			}
			rd.include(requestToExpose, response);
		}

		else { 
   
			// Note: The forwarded resource is supposed to determine the content type itself.
			exposeForwardRequestAttributes(requestToExpose);
			if (logger.isDebugEnabled()) { 
   
				logger.debug("Forwarding to resource [" + getUrl() + "] in InternalResourceView '" + getBeanName() + "'");
			}
			//执行forward
			rd.forward(requestToExpose, response);
		}
	}

最后总结一个流程图

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