SpringMVC源码：参数解析、方法调用与返回值处理

参考资料：

《SpringMVC源码解析系列》

《SpringMVC源码分析》

《Spring MVC源码》

        写在开头：本文为个人学习笔记，内容比较随意，夹杂个人理解，如有错误，欢迎指正。

前文：

《SpringMVC源码：DispatcherServlet初始化流程》

《SpringMVC源码：HandlerMapping加载1》

《SpringMVC源码：HandlerMapping加载2》

《SpringMVC源码：getHandler、getHandlerAdapter过程》

        前文我们已经介绍了，doDispatch方法过程中的handler查找、适配器查找以及拦截器preHandle方法的执行，本文我们会继续介绍方法的调用。

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 {//检查是否为Multipart请求,如果是则HttpServletRequest转换为MultipartHttpServletRequestprocessedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);//1.根据request信息寻找对应的Handler//并返回mappedHeadler,它是一个执行链,包含拦截器和自定义controllermappedHandler = getHandler(processedRequest);if (mappedHandler == null) {//没有找到Handler则通过response反馈错误信息noHandlerFound(processedRequest, response);return;}// 2.根据当前的handler寻找对应的HandlerAdapterHandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// 当前handler处理last-modifiedString 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;}}//3.mappedHandler调用拦截器的preHandler方法if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// 4.HandlerAdapter调用自定义Controller中的方法并返回ModelAndViewmv = ha.handle(processedRequest, response, mappedHandler.getHandler());//5.视图解析,为request请求找到视图applyDefaultViewName(processedRequest, mv);//6.调用拦截器的postHandle方法mappedHandler.applyPostHandle(processedRequest, response, mv);}processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}
}

目录

一、handle

        1、AbstractHandlerMethodAdapter#handle

        2、RequestMappingHandlerAdapter#handleInternal

        3、RequestMappingHandlerAdapter#invokeHandlerMethod

        4、ServletInvocableHandlerMethod#invokeAndHandle

        InvocableHandlerMethod#invokeForRequest

        5、RequestMappingHandlerAdapter#getModelAndView

二、@RequestBody与@ResponseBody

        1、接口转换

         2、参数解析

        2.1、查找解析器

        2.2、解析过程

         3、返回值处理

         3.1、选择处理器

        3.2、处理过程

补充

---

一、handle

        1、AbstractHandlerMethodAdapter#handle

        这里以AbstractHandlerMethodAdapter类为例，可以看到内部实际上是调用了handleInternal方法，而这个方法实际上是由子类RequestMappingHandlerAdapter实现的。（注意下这里的返回值为ModelAndView）

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

        2、RequestMappingHandlerAdapter#handleInternal

        该方法主要进行了对请求request的相关校验、解析、转换、执行相关的handler的invokeHandlerMethod 并返回对应的数据视图对象。

	@Overrideprotected ModelAndView handleInternal(HttpServletRequest request,HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {ModelAndView mav;//如果设置supportMethods、requireSession属性需要对其进行校验//如果处理的请求方式不是supportMethods集合支持的抛出异常//或者requireSession为true但是request不包含session同样抛出异常checkRequest(request);// 针对线程不安全的session 提供两种方式 一种是同步 从而保证该会话的用户串行执行请求// 一种是非同步处理请求 不管同步与否最终调用核心方法invokeHandlerMethod 调用HandlerMethodif (this.synchronizeOnSession) {HttpSession session = request.getSession(false);if (session != null) {Object mutex = WebUtils.getSessionMutex(session);synchronized (mutex) {//执行过程调用(参数解析 过程调用)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);}//为响应资源设置对应的缓存时间，指定时间内可以直接使用本地缓存if (!response.containsHeader(HEADER_CACHE_CONTROL)) {if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);}else {prepareResponse(response);}}return mav;}

        3、RequestMappingHandlerAdapter#invokeHandlerMethod

        这一步中将HandlerMethod被封装ServletInvocableHandlerMethod，包裹上方法执行需要的信息并调用invokeAndHandle方法执行，最后通过getModelAndView获取视图。

	protected ModelAndView invokeHandlerMethod(HttpServletRequest request,HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {//先将请求响应对象包装成ServletWebRequestServletWebRequest webRequest = new ServletWebRequest(request, response);try {//获取该方法的所有使用@InitBinder注解修饰的方法//包装成WebDataBinderFactory 在handlerMethod执行前调用（包含类作用范围的和全局作用范围的）WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);//获取该方法的所有使用@ModelAttribute注解修饰的方法 包装成ModelFactory（包含类作用范围的和全局作用范围的）ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);//将HandlerMethod方法包装成ServletInvocableHandlerMethod（该对象是InvocableHandlerMethod子类）//设置相关的组件 比如设置参数解析组件argumentResolvers和返回结果处理组件returnValueHandlers//绑定组件binderFactory和parameterNameDiscovererServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);invocableMethod.setDataBinderFactory(binderFactory);invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);//创建数据模型和视图的容器对象 见名知义其包含我们所常见的数据和视图对象ModelAndViewContainer mavContainer = new ModelAndViewContainer();//对于重定向相关的参数保存需要依赖flashMap对象，如果一个请求是重定向请求 则input_flash_map保存重定向入参//如果一个请求需要进行重定向 则参数会存放到output_flash_map中mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));//调用@ModelAttribute注解修饰的方法将对应的属性存放到mavContainer 的model中modelFactory.initModel(webRequest, mavContainer, invocableMethod);mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);//异步请求处理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();if (logger.isDebugEnabled()) {logger.debug("Found concurrent result value [" + result + "]");}invocableMethod = invocableMethod.wrapConcurrentResult(result);}//通过反射执行相关的handlerMethod 涉及参数解析，返回值的处理、//invocableMethod已经包含了进行参数解析和返回值处理的组件对象invocableMethod.invokeAndHandle(webRequest, mavContainer);if (asyncManager.isConcurrentHandlingStarted()) {return null;}//主要处理sessionAttribute 以及对model中的属性进行属性编辑器的转换处理@InitBinder//对于view对象进行逻辑视图到物理视图的转换以及重定向参数的设置return getModelAndView(mavContainer, modelFactory, webRequest);}finally {webRequest.requestCompleted();}}

        4、ServletInvocableHandlerMethod#invokeAndHandle

        首先调用invokeForRequest进行参数解析与方法调用，得到返回值后将其封装入mavContainer中。

	public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {// 请求处理Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);// 返回值处理this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);}

        InvocableHandlerMethod#invokeForRequest

        invokeForRequest方法首先解析参数，然后反射执行方法。

	public Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {//调用getMethodArgumentValues进行参数解析Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);//调用doInvoke方法反射执行handler处理器Object returnValue = doInvoke(args);return returnValue;}

	protected Object doInvoke(Object... args) throws Exception {ReflectionUtils.makeAccessible(getBridgedMethod());return getBridgedMethod().invoke(getBean(), args);}

        5、RequestMappingHandlerAdapter#getModelAndView

        从mavContainer取出结果包装成ModelAndView

	private ModelAndView getModelAndView(ModelAndViewContainer mavContainer,ModelFactory modelFactory, NativeWebRequest webRequest) throws Exception {modelFactory.updateModel(webRequest, mavContainer);if (mavContainer.isRequestHandled()) {return null;}//从mavContainer中取出model ModelMap model = mavContainer.getModel();//根据取出的model创建视图对象ModelAndView mav = new ModelAndView(mavContainer.getViewName(), model, mavContainer.getStatus());if (!mavContainer.isViewReference()) {mav.setView((View) mavContainer.getView());}//处理redirect请求if (model instanceof RedirectAttributes) {Map<String, ?> flashAttributes = ((RedirectAttributes) model).getFlashAttributes();HttpServletRequest request = webRequest.getNativeRequest(HttpServletRequest.class);RequestContextUtils.getOutputFlashMap(request).putAll(flashAttributes);}return mav;}

二、@RequestBody与@ResponseBody

        以下内容参考自《Spring MVC源码(三) ----- @RequestBody和@ResponseBody原理解析》

        在上文中我们介绍了invokeAndHandle方法，其中的两个步骤分别为参数解析与返回值处理，这一节我们结合两个常用注解@RequestBody与@ResponseBody来进行这部分内容的讲解。

        首先来看个样例，第一个requestBody请求，使用@RequestBody将HTTP请求体转换成String类型，第二个responseBody请求，将Map对象转换成json格式输出到HTTP响应中。

@RequestMapping("/requestBody")
public void requestBody(@RequestBody String body, Writer writer) throws IOException{writer.write(body);
}@RequestMapping(value="/responseBody", produces="application/json")
@ResponseBody
public Map<String, Object> responseBody(){Map<String, Object> retMap = new HashMap<>();retMap.put("param1", "abc");return retMap;
}

         这两个注解都实现了HTTP报文信息同Controller方法中对象的转换，下面我们就来进行原理分析。

        1、接口转换

        SpringMVC中为了实现HTTP消息体和参数及返回值进行转换，所有转换类都实现了HttpMessageConverter接口，该接口定义了5个方法，用于将HTTP请求报文转换为java对象，以及将java对象转换为HTTP响应报文。

public interface HttpMessageConverter<T> {// 当前转换器是否能将HTTP报文转换为对象类型boolean canRead(Class<?> clazz, MediaType mediaType);// 当前转换器是否能将对象类型转换为HTTP报文boolean canWrite(Class<?> clazz, MediaType mediaType);// 转换器能支持的HTTP媒体类型List<MediaType> getSupportedMediaTypes();// 转换HTTP报文为特定类型T read(Class<? extends T> clazz, HttpInputMessage inputMessage)throws IOException, HttpMessageNotReadableException;// 将特定类型对象转换为HTTP报文void write(T t, MediaType contentType, HttpOutputMessage outputMessage)throws IOException, HttpMessageNotWritableException;}

        对应到SpringMVC的Controller方法，read方法即是读取HTTP请求转换为参数对象，write方法即是将返回值对象转换为HTTP响应报文。SpringMVC定义了两个接口来操作这两个过程：参数解析器HandlerMethodArgumentResolver和返回值处理器HandlerMethodReturnValueHandler。

// 参数解析器接口
public interface HandlerMethodArgumentResolver {// 解析器是否支持方法参数boolean supportsParameter(MethodParameter parameter);// 解析HTTP报文中对应的方法参数Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception;}// 返回值处理器接口
public interface HandlerMethodReturnValueHandler {// 处理器是否支持返回值类型boolean supportsReturnType(MethodParameter returnType);// 将返回值解析为HTTP响应报文void handleReturnValue(Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;}

        参数解析器和返回值处理器在底层处理时，都是通过HttpMessageConverter进行转换。流程如下：

        RequestResponseBodyMethodProcessor类实现了HandlerMethodArgumentResolver和HandlerMethodReturnValueHandler两个接口，@RequestBody和@ResponseBody这两个注解的解析均由该类来实现。

         2、参数解析

        在上文中我们介绍了getMethodArgumentValues方法实现了参数解析但并未深入解析，这里我们进行下源码分析

	private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {MethodParameter[] parameters = getMethodParameters();Object[] args = new Object[parameters.length];// 遍历所有参数，逐个解析for (int i = 0; i < parameters.length; i++) {MethodParameter parameter = parameters[i];parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);args[i] = resolveProvidedArgument(parameter, providedArgs);if (args[i] != null) {continue;}    // 通过supportsParameter方法寻找参数合适的解析器if (this.argumentResolvers.supportsParameter(parameter)) {try {args[i] = this.argumentResolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);continue;}catch (Exception ex) {if (logger.isDebugEnabled()) {logger.debug(getArgumentResolutionErrorMessage("Failed to resolve", i), ex);}throw ex;}}if (args[i] == null) {throw new IllegalStateException("Could not resolve method parameter at index " +parameter.getParameterIndex() + " in " + parameter.getMethod().toGenericString() +": " + getArgumentResolutionErrorMessage("No suitable resolver for", i));}}return args;}

        2.1、查找解析器

        HandlerMethodArgumentResolverComposite类实现了supportsParameter方法，后续调用argumentResolverCache即参数解析器

	private final List<HandlerMethodArgumentResolver> argumentResolvers =new LinkedList<HandlerMethodArgumentResolver>();@Overridepublic boolean supportsParameter(MethodParameter parameter) {return getArgumentResolver(parameter) != null;}private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);if (result == null) {for (HandlerMethodArgumentResolver resolver : this.argumentResolvers) {if (resolver.supportsParameter(parameter)) {result = resolver;this.argumentResolverCache.put(parameter, result);break;}}}return result;}

         我们直接进入HandlerMethodArgumentResolver接口的实现类，也正是我们上问提到的RequestResponseBodyMethodProcessor来看看

	@Overridepublic boolean supportsParameter(MethodParameter parameter) {return parameter.hasParameterAnnotation(RequestBody.class);}

         可以看到这里实际上就是判断方法上是否有@RequestBody，这就印证了我们所说的@RequestBody由RequestResponseBodyMethodProcessor来实现解析。

        2.2、解析过程

        已经通过supportsParameter确定了由RequestResponseBodyMethodProcessor类来进行解析，下面再看如何解析，于是我们跟着源码进入resolveArgument方法。

	@Overridepublic Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,NativeWebRequest webRequest, 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);}

         其核心内容为通过readWithMessageConverters读取HTTP的请求体内容，而该方法由AbstractMessageConverterMethodArgumentResolver负责实现。

    // RequestResponseBodyMethodProcessor.java@Overridepublic Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {parameter = parameter.nestedIfOptional();// 读取HTTP报文Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());String name = Conventions.getVariableNameForParameter(parameter);WebDataBinder binder = binderFactory.createBinder(webRequest, arg, name);if (arg != null) {validateIfApplicable(binder, parameter);if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) {throw new MethodArgumentNotValidException(parameter, binder.getBindingResult());}}mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());return adaptArgumentIfNecessary(arg, parameter);}@Overrideprotected <T> Object readWithMessageConverters(NativeWebRequest webRequest, MethodParameter parameter,Type paramType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);ServletServerHttpRequest inputMessage = new ServletServerHttpRequest(servletRequest);//调用AbstractMessageConverterMethodArgumentResolver中的实现Object arg = readWithMessageConverters(inputMessage, parameter, paramType);if (arg == null) {if (checkRequired(parameter)) {throw new HttpMessageNotReadableException("Required request body is missing: " +parameter.getMethod().toGenericString());}}return arg;}

         这里的关键部分即是遍历所有的HttpMessageConverter，通过canRead方法判断转换器是否支持对参数的转换，然后执行read方法完成转换。

protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {....try {inputMessage = new EmptyBodyCheckingHttpInputMessage(inputMessage);for (HttpMessageConverter<?> converter : this.messageConverters) {Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();....// 判断转换器是否支持参数类型if (converter.canRead(targetClass, contentType)) {if (inputMessage.getBody() != null) {inputMessage = getAdvice().beforeBodyRead(inputMessage, parameter, targetType, converterType);// read方法执行HTTP报文到参数的转换body = ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage);body = getAdvice().afterBodyRead(body, inputMessage, parameter, targetType, converterType);}else {body = getAdvice().handleEmptyBody(null, inputMessage, parameter, targetType, converterType);}break;}...}}catch (IOException ex) {throw new HttpMessageNotReadableException("I/O error while reading input message", ex);}....return body;
}

         3、返回值处理

        完成Controller方法的调用后，在ServletInvocableHandlerMethod的invokeAndHandle中，使用返回值处理器对返回值进行转换。

        这里的returnValueHandlers也是HandlerMethodReturnValueHandler的集合体HandlerMethodReturnValueHandlerComposite

	public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {// 请求处理Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);// 返回值处理this.returnValueHandlers.handleReturnValue(returnValue, getReturnValueType(returnValue), mavContainer, webRequest);}

         3.1、选择处理器

        handleReturnValue首先会通过selectHandler找出支持处理当前返回值的处理器，然后再调用该处理器的handleReturnValue方法进行真正的处理。

    //HandlerMethodReturnValueHandlerComposite.java@Overridepublic void handleReturnValue(Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {// 选择合适的HandlerMethodReturnValueHandler，如果没有用@ResposeBody注解和用了注解其返回值处理器肯定不同HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);if (handler == null) {throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());}// 执行返回值处理handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);}

        而selectHandler方法遍历所有HandlerMethodReturnValueHandler，调用其supportsReturnType方法选择合适的HandlerMethodReturnValueHandler，然后调用其handleReturnValue方法完成处理。 

    //HandlerMethodReturnValueHandlerComposite.javaprivate HandlerMethodReturnValueHandler selectHandler(Object value, MethodParameter returnType) {boolean isAsyncValue = isAsyncReturnValue(value, returnType);for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {continue;}// 判断当前的HandlerMethodReturnValueHandler是否支持处理返回值if (handler.supportsReturnType(returnType)) {return handler;}}return null;}

         RequestResponseBodyMethodProcessor要求方法上有@ResponseBody注解或者方法所在的Controller类上有@ResponseBody的注解。这就是常常用@RestController注解代替@Controller注解的原因，因为@RestController注解自带@ResponseBody。

	@Overridepublic boolean supportsReturnType(MethodParameter returnType) {return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) ||returnType.hasMethodAnnotation(ResponseBody.class));}

        3.2、处理过程

        handleReturnValue方法实际也是调用HttpMessageConverter来完成转换处理

    //RequestResponseBodyMethodProcessor.java@Overridepublic void handleReturnValue(Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest)throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {mavContainer.setRequestHandled(true);ServletServerHttpRequest inputMessage = createInputMessage(webRequest);ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);// 调用HttpMessageConverter执行writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);}

         AbstractMessageConverterMethodProcessor使用canWrite方法选择合适的HttpMessageConverter，然后调用write方法完成转换。

//AbstractMessageConverterMethodProcessor.java
protected <T> void writeWithMessageConverters(T value, MethodParameter returnType,ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {....if (selectedMediaType != null) {selectedMediaType = selectedMediaType.removeQualityValue();for (HttpMessageConverter<?> messageConverter : this.messageConverters) {// 判断是否支持返回值类型，返回值类型很有可能不同，如String，Map，List等if (messageConverter.canWrite(valueType, selectedMediaType)) {outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,(Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),inputMessage, outputMessage);if (outputValue != null) {addContentDispositionHeader(inputMessage, outputMessage);// 执行返回值转换((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);...}return;}}}....
}

补充

        上文中提到的解析器和转换器都是在适配器初始化的时候添加的。

    //RequestMappingHandlerAdapter.javapublic RequestMappingHandlerAdapter() {StringHttpMessageConverter stringHttpMessageConverter = new StringHttpMessageConverter();stringHttpMessageConverter.setWriteAcceptCharset(false);  // see SPR-7316this.messageConverters = new ArrayList<HttpMessageConverter<?>>(4);this.messageConverters.add(new ByteArrayHttpMessageConverter());this.messageConverters.add(stringHttpMessageConverter);this.messageConverters.add(new SourceHttpMessageConverter<Source>());this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());}@Overridepublic void afterPropertiesSet() {// Do this first, it may add ResponseBody advice beansinitControllerAdviceCache();if (this.argumentResolvers == null) {List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);}if (this.initBinderArgumentResolvers == null) {List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);}if (this.returnValueHandlers == null) {List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);}}