我的架构梦：（九）SpringMVC源码分析

一、前端控制器 DispatcherServlet 继承结构

我们在上一篇知道了用户发送请求至前端控制器DispatcherServlet。所以我们先来分析下DispatcherServlet的继承结构关系。如下图：

二、 重要时机点分析

1、handle方法中打断点

@Controller
@RequestMapping("/riemann")
public class RiemannController  {

	@RequestMapping("/handle")
	public String handle(String name, Map<String,Object> model) {
		System.out.println("handler业务逻辑处理中....");
		Date date = new Date();
		model.put("date", date);
		return "success";
	}

}

2、观察调用栈

doDispathch方法中的1064行代码完成handler方法的调用

3、⻚面渲染时机(打断点并观察调用栈)

三、SpringMVC处理请求的流程

org.springframework.web.servlet.DispatcherServlet#doDispatch方法的执行过程，其中步骤 2、3、4、5是核心步骤

1、调用getHandler()获取到能够处理当前请求的执行链 HandlerExecutionChain(Handler+拦截器)
2、调用getHandlerAdapter();获取能够执行1中Handler的适配器
3、适配器调用Handler执行ha.handle(总会返回一个ModelAndView对象)
4、调用processDispatchResult()方法完成视图渲染跳转

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
	HttpServletRequest processedRequest = request;
	HandlerExecutionChain mappedHandler = null;
	boolean multipartRequestParsed = false;

	WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

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

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

			// Determine handler for the current request.
			/**
			 * 2.取得处理当前请求的 Controller,这里也称为 Handler,即处理器
			 *   这里并不是直接返回 Controller，而是返回 HandlerExecutionChain 请求处理链对象
			 *   该对象封装了 Handler 和 Interceptor
			 */
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null) {
				// 如果 handler 为空，则返回404
				noHandlerFound(processedRequest, response);
				return;
			}

			// Determine handler adapter for the current request.
			// 3.获取处理请求的处理器适配器 HandlerAdapter
			HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

			// Process last-modified header, if supported by the handler.
			// 处理 last-modified 请求头
			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;
			}

			// Actually invoke the handler.
			// 4.实际处理器处理请求，返回结果视图对象。
			mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

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

			// 结果视图对象的处理
			applyDefaultViewName(processedRequest, mv);
			mappedHandler.applyPostHandle(processedRequest, response, mv);
		}
		catch (Exception ex) {
			dispatchException = ex;
		}
		catch (Throwable err) {
			// As of 4.3, we're processing Errors thrown from handler methods as well,
			// making them available for @ExceptionHandler methods and other scenarios.
			dispatchException = new NestedServletException("Handler dispatch failed", err);
		}
		// 该方法完成视图渲染跳转
		processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
	}
	catch (Exception ex) {
		// 最终会调用 HandlerInterceptor 的 afterCompletion 方法
		triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
	}
	catch (Throwable err) {
		triggerAfterCompletion(processedRequest, response, mappedHandler,
				new NestedServletException("Handler processing failed", err));
	}
	finally {
		if (asyncManager.isConcurrentHandlingStarted()) {
			// Instead of postHandle and afterCompletion
			if (mappedHandler != null) {
				mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
			}
		}
		else {
			// Clean up any resources used by a multipart request.
			if (multipartRequestParsed) {
				cleanupMultipart(processedRequest);
			}
		}
	}
}

四、核心步骤getHandler方法剖析

遍历两个HandlerMapping，试图获取能够处理当前请求的执行链

@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
	if (this.handlerMappings != null) {
		for (HandlerMapping mapping : this.handlerMappings) {
			HandlerExecutionChain handler = mapping.getHandler(request);
			if (handler != null) {
				return handler;
			}
		}
	}
	return null;
}

五、核心步骤getHandlerAdapter方法剖析

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");
}

六、核心步骤ha.handle方法剖析

继续跟到 AbstractHandlerMethodAdapter#handle

@Override
@Nullable
public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
		throws Exception {

	return handleInternal(request, response, (HandlerMethod) handler);
}

@Override
protected ModelAndView handleInternal(HttpServletRequest request,
		HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

	ModelAndView mav;
	checkRequest(request);

	// Execute invokeHandlerMethod in synchronized block if required.
	// 判断当前是否需要支持在同一个session中只能线性的处理请求
	if (this.synchronizeOnSession) {
		// 获取当前请求的session对象
		HttpSession session = request.getSession(false);
		if (session != null) {
			// 为当前session生成一个唯一的可以用于锁定的key
			Object mutex = WebUtils.getSessionMutex(session);
			synchronized (mutex) {
				// 对HandlerMethod进行参数等的适配处理，并调用目标handler
				mav = invokeHandlerMethod(request, response, handlerMethod);
			}
		}
		else {
			// No HttpSession available -> no mutex necessary
			// 如果当前不存在session,则直接对HandlerMethod进行适配
			mav = invokeHandlerMethod(request, response, handlerMethod);
		}
	}
	else {
		// No synchronization on session demanded at all...
		// 如果当前不需要对session进行同步处理，则直接对HandlerMethod进行适配
		mav = invokeHandlerMethod(request, response, handlerMethod);
	}

	if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
		if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
			applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
		}
		else {
			prepareResponse(response);
		}
	}

	return mav;
}

@Nullable
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
			HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {

	ServletWebRequest webRequest = new ServletWebRequest(request, response);
	try {
		// 获取容器中全局配置的InitBinder和当前HandlerMethod所对应的Controller中配置的InitBinder，用于进行参数的绑定。
		WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
		// 获取容器中全局配置的ModelAttribute和当前HandlerMethod所对应的Controller中配置的ModelAttribute，这些配置的方式将会在目标方法调用之前进行
		ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);

		// 将handlerMethod封装为一个ServletInvocableHandlerMethod对象
		ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
		if (this.argumentResolvers != null) {
			// 设置当前容器中配置的所有ArgumentResolver
			invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
		}
		if (this.returnValueHandlers != null) {
			// 设置当前容器中配置的所有ReturnValueHandler
			invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
		}
		// 将前面创建的WebDataBinderFactory设置到ServletInvocableHandlerMethod中
		invocableMethod.setDataBinderFactory(binderFactory);
		invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);

		ModelAndViewContainer mavContainer = new ModelAndViewContainer();
		mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
		/**
		 * 这里initModel方法主要作用是调用前面获取到的@ModelAttribute标注的方法
		 * 从而达到ModelAttribute标注的方法能够在目标Handler调用之前调用的目的
		 */
		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);
		}

		// 对请求参数进行处理，调用目标HandleMethod,并且将返回值封装为一个ModelAndViewd对象
		invocableMethod.invokeAndHandle(webRequest, mavContainer);
		if (asyncManager.isConcurrentHandlingStarted()) {
			return null;
		}

		/**
		 * 对封装的ModelAndView进行处理，主要是判断当前的请求是否进行了重定向，
		 * 如果进行了重定向，还会判断是否需要将FlashAttributes封装到新的请求中。
		 */
		return getModelAndView(mavContainer, modelFactory, webRequest);
	}
	finally {
		webRequest.requestCompleted();
	}
}

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {

	// 对目标handler的参数进行处理，并且调用目标handler
	Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
	// 设置相关的返回状态
	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 {
		this.returnValueHandlers.handleReturnValue(
				returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
	}
	catch (Exception ex) {
		if (logger.isTraceEnabled()) {
			logger.trace(formatErrorForReturnValue(returnValue), ex);
		}
		throw ex;
	}
}

@Nullable
public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
		Object... providedArgs) throws Exception {

	// 将request中的参数转换为当前handler的参数形式
	Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
	if (logger.isTraceEnabled()) {
		logger.trace("Arguments: " + Arrays.toString(args));
	}
	// 主要是结合处理后的参数，使用反射对目标方法进行调用
	return doInvoke(args);
}

protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
			Object... providedArgs) throws Exception {

	// 获取当前handler所声明的所有参数，主要包括参数名、参数类型、参数位置、所标注的注解等等属性
	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);
		/**
		 * providedArgs是调用方提供的参数，这里主要是判断这些参数中是否有当前类型，如果有，则直接使用调用方提供的参数，对于请求处理而言，默认情况下，
		 * 调用方提供的参数都是长度为0的数组
		 */
		args[i] = findProvidedArgument(parameter, providedArgs);
		if (args[i] != null) {
			continue;
		}
		/**
		 * 如果在调用方提供的参数中不能找到当前类型的参数值，则遍历Spring容器中所有的ArgumentResolver,
		 * 判断哪种类型的Resolver支持对当前参数的解析，这里的判断方式比较简单，比如
		 * RequestMethodArgumentResolver就是判断当前参数是否使用@RequestParam注解进行了标注。
		 */
		if (!this.resolvers.supportsParameter(parameter)) {
			throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
		}
		try {
			/**
			 * 如果能够找到对当前参数进行处理的ArgumentResolver，则调用其resolveArgument方法
			 * 从request中获取对应的参数值，并且进行转换。
			 */
			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;
}

@Override
@Nullable
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
		NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {

	HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
	if (resolver == null) {
		throw new IllegalArgumentException("Unsupported parameter type [" +
				parameter.getParameterType().getName() + "]. supportsParameter should be called first.");
	}
	return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
}

@Override
@Nullable
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 = resolveStringValue(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 = resolveStringValue(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 = resolveStringValue(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;
}

@Override
@Nullable
protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest request) throws Exception {
	HttpServletRequest servletRequest = request.getNativeRequest(HttpServletRequest.class);

	if (servletRequest != null) {
		Object mpArg = MultipartResolutionDelegate.resolveMultipartArgument(name, parameter, servletRequest);
		if (mpArg != MultipartResolutionDelegate.UNRESOLVABLE) {
			return mpArg;
		}
	}

	Object arg = null;
	MultipartRequest multipartRequest = request.getNativeRequest(MultipartRequest.class);
	if (multipartRequest != null) {
		List<MultipartFile> files = multipartRequest.getFiles(name);
		if (!files.isEmpty()) {
			arg = (files.size() == 1 ? files.get(0) : files);
		}
	}
	if (arg == null) {
		String[] paramValues = request.getParameterValues(name);
		if (paramValues != null) {
			arg = (paramValues.length == 1 ? paramValues[0] : paramValues);
		}
	}
	return arg;
}

七、 核心步骤processDispatchResult方法剖析

1、render方法完成渲染

2、视图解析器解析出View视图对象

3、在解析出View视图对象的过程中会判断是否重定向、是否转发等，不同的情况封装的是不同的 View实现

4、解析出View视图对象的过程中，要将逻辑视图名解析为物理视图名

protected AbstractUrlBasedView buildView(String viewName) throws Exception {
	Class<?> viewClass = getViewClass();
	Assert.state(viewClass != null, "No view class");

	AbstractUrlBasedView view = (AbstractUrlBasedView) BeanUtils.instantiateClass(viewClass);
	// 逻辑视图名转换为物理视图名
	view.setUrl(getPrefix() + viewName + getSuffix());

	String contentType = getContentType();
	if (contentType != null) {
		view.setContentType(contentType);
	}

	view.setRequestContextAttribute(getRequestContextAttribute());
	view.setAttributesMap(getAttributesMap());

	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;
}

5、封装View视图对象之后，调用了view对象的render方法

6、渲染数据

7、把modelMap中的数据暴露到request域中，这也是为什么后台model.add之后在jsp中可以从请求域取出来的根本原因。

继续跟进去

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

	// Expose the model object as request attributes.
	exposeModelAsRequestAttributes(model, request);

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

	// Determine the path for the request dispatcher.
	String dispatcherPath = prepareForRendering(request, response);

	// Obtain a RequestDispatcher for the target resource (typically a JSP).
	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!");
	}

	// If already included or response already committed, perform include, else forward.
	if (useInclude(request, response)) {
		response.setContentType(getContentType());
		if (logger.isDebugEnabled()) {
			logger.debug("Including [" + getUrl() + "]");
		}
		rd.include(request, response);
	}

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

8、将数据设置到请求域中

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

	model.forEach((name, value) -> {
		if (value != null) {
			request.setAttribute(name, value);
		}
		else {
			request.removeAttribute(name);
		}
	});
}

八、SpringMVC九大组件初始化

1、在DispatcherServlet中定义了九个属性，每一个属性都对应一种组件

/** MultipartResolver used by this servlet. */
/** 多部件解析器 */
@Nullable
private MultipartResolver multipartResolver;

/** LocaleResolver used by this servlet. */
/** 区域化、国际化解析器 */
@Nullable
private LocaleResolver localeResolver;

/** ThemeResolver used by this servlet. */
/** 主题解析器 */
@Nullable
private ThemeResolver themeResolver;

/** List of HandlerMappings used by this servlet. */
/** 处理器映射器组件 */
@Nullable
private List<HandlerMapping> handlerMappings;

/** List of HandlerAdapters used by this servlet. */
/** 处理器适配器组件 */
@Nullable
private List<HandlerAdapter> handlerAdapters;

/** List of HandlerExceptionResolvers used by this servlet. */
/** 异常解析器 */
@Nullable
private List<HandlerExceptionResolver> handlerExceptionResolvers;

/** RequestToViewNameTranslator used by this servlet. */
/** 默认视图名转换器组件 */
@Nullable
private RequestToViewNameTranslator viewNameTranslator;

/** FlashMapManager used by this servlet. */
/** flash属性管理组件 */
@Nullable
private FlashMapManager flashMapManager;

/** List of ViewResolvers used by this servlet. */
/** 视图解析器 */
@Nullable
private List<ViewResolver> viewResolvers;

九大组件都是定义了接口，接口其实就是定义了该组件的规范，比如ViewResolver、HandlerAdapter 等都是接口。

2、九大组件的初始化时机

DispatcherServlet中的onRefresh()，该方法中初始化了九大组件：

initStrategies方法

3、观察其中的一个组件initHandlerMappings(context)

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

	if (this.detectAllHandlerMappings) {
		// Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
		// 找到所有的 HandlerMapping
		Map<String, HandlerMapping> matchingBeans =
				BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
		if (!matchingBeans.isEmpty()) {
			this.handlerMappings = new ArrayList<>(matchingBeans.values());
			// We keep HandlerMappings in sorted order.
			AnnotationAwareOrderComparator.sort(this.handlerMappings);
		}
	}
	else {
		try {
			// 否则在ioc中按照固定名称去找
			HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
			this.handlerMappings = Collections.singletonList(hm);
		}
		catch (NoSuchBeanDefinitionException ex) {
			// Ignore, we'll add a default HandlerMapping later.
		}
	}

	// Ensure we have at least one HandlerMapping, by registering
	// a default HandlerMapping if no other mappings are found.
	if (this.handlerMappings == null) {
		// 最后还为空的话则按照默认策略生成
		this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
		if (logger.isTraceEnabled()) {
			logger.trace("No HandlerMappings declared for servlet '" + getServletName() +
					"': using default strategies from DispatcherServlet.properties");
		}
	}
}

4、如果按照类型和按照固定id从ioc容器中找不到对应组件，则会按照默认策略进行注册初始化，默 认策略在DispatcherServlet.properties文件中配置

protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
	String key = strategyInterface.getName();
	String value = defaultStrategies.getProperty(key); // DispatcherServlet.properties
	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.properties

5、注意:多部件解析器的初始化必须按照id注册对象(multipartResolver)

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OK,SpringMVC源码分析告一段落了。后面会对SpringBoot源码进行分析，敬请期待。