SpringMVC源码解析——HTTP请求处理

在SpringMVC源码解析——DispatcherServlet的逻辑处理中，最后介绍到了org.springframework.web.servlet.DispatcherServlet的doDispatch方法中关于处理Web HTTP请求的核心代码是调用AbstractHandlerMethodAdapter类的handle方法，源码如下：

	/*** 此实现期望处理器为 {@link HandlerMethod} 类型。*/@Override@Nullablepublic final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)throws Exception {return handleInternal(request, response, (HandlerMethod) handler);}

上面的处理HTTP请求的逻辑主要是调用handleInternal函数来进行处理，源码如下：

	/*** 处理请求并返回视图模型。** @param request HTTP请求对象* @param response HTTP响应对象* @param handlerMethod 处理方法的对象* @return 视图模型对象* @throws Exception 异常*/@Overrideprotected ModelAndView handleInternal(HttpServletRequest request,HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {ModelAndView mav;checkRequest(request);// 如果需要的话，在同步块中执行invokeHandlerMethod。if (this.synchronizeOnSession) {HttpSession session = request.getSession(false);if (session != null) {Object mutex = WebUtils.getSessionMutex(session);synchronized (mutex) {mav = invokeHandlerMethod(request, response, handlerMethod);}}else {// 没有可用的HttpSession -> 无需互斥锁mav = invokeHandlerMethod(request, response, handlerMethod);}}else {// 一点都没有要求会话同步...mav = invokeHandlerMethod(request, response, handlerMethod);}if (!response.containsHeader(HEADER_CACHE_CONTROL)) {if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);}else {prepareResponse(response);}}return mav;}

这个函数是一个Java方法，它处理HTTP请求并返回一个ModelAndView对象。它首先检查请求，然后根据需要在同步块中执行invokeHandlerMethod方法。如果需要在会话级别进行同步，则使用互斥锁来确保线程安全。接下来，根据响应是否包含缓存控制头，它会根据处理程序的方法来处理会话属性或将响应准备发送给客户端。最后，它返回一个ModelAndView对象。

/*** 调用RequestMapping处理器方法，如果需要解析视图，则准备ModelAndView。* * @since 4.2* @see #createInvocableHandlerMethod(HandlerMethod)*/
@SuppressWarnings("deprecation")
@Nullable
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {// 创建ServletRequestWebRequestServletWebRequest webRequest = new ServletWebRequest(request, response);// 获取WebDataBinderFactoryWebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);// 获取ModelFactoryModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);// 创建ServletInvocableHandlerMethodServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);// 设置处理器方法的参数解析器if (this.argumentResolvers != null) {invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);}// 设置处理器方法的返回值解析器if (this.returnValueHandlers != null) {invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);}// 设置DataBinderFactoryinvocableMethod.setDataBinderFactory(binderFactory);// 设置参数名称发现器invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);// 设置方法验证器invocableMethod.setMethodValidator(this.methodValidator);// 创建ModelAndViewContainerModelAndViewContainer mavContainer = new ModelAndViewContainer();// 添加所有属性到ModelAndViewContainermavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));// 初始化模型modelFactory.initModel(webRequest, mavContainer, invocableMethod);// 设置是否在重定向时忽略默认模型mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);// 创建AsyncWebRequestAsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);// 设置异步请求超时时间asyncWebRequest.setTimeout(this.asyncRequestTimeout);// 获取WebAsyncManagerWebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);// 设置TaskExecutorasyncManager.setTaskExecutor(this.taskExecutor);// 设置AsyncWebRequestasyncManager.setAsyncWebRequest(asyncWebRequest);// 注册Callable拦截器asyncManager.registerCallableInterceptors(this.callableInterceptors);// 注册DeferredResult拦截器asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);// 如果异步结果已并行处理if (asyncManager.hasConcurrentResult()) {// 获取并行处理的结果和ModelAndViewContainerObject result = asyncManager.getConcurrentResult();mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];asyncManager.clearConcurrentResult();// 调试日志LogFormatUtils.traceDebug(logger, traceOn -> {String formatted = LogFormatUtils.formatValue(result, !traceOn);return "Resume with async result [" + formatted + "]";});// 将并行处理的结果包装到ServletInvocableHandlerMethodinvocableMethod = invocableMethod.wrapConcurrentResult(result);}// 调用处理器方法并处理invocableMethod.invokeAndHandle(webRequest, mavContainer);// 如果异步处理已开始if (asyncManager.isConcurrentHandlingStarted()) {// 返回nullreturn null;}// 返回ModelAndViewreturn getModelAndView(mavContainer, modelFactory, webRequest);
}

这个函数用于调用@RequestMapping注解的处理方法，并准备 ModelAndView对象（如果需要解析视图）。它会执行处理方法，并处理异步结果和模型视图。如果异步处理已经开始，则返回null。上面的函数会调用ServletInvocableHandlerMethod类的函数invokeAndHandle进行处理，invokeAndHandle函数的源码如下：

    /*** 调用方法并处理返回值** @param webRequest    ServletWebRequest 对象，表示当前请求的上下文* @param mavContainer  ModelAndViewContainer 对象，用于处理模型和视图* @param providedArgs  提供的参数值* @throws Exception 抛出异常*/public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer, Object... providedArgs) throws Exception {// 调用方法并获取返回值Object returnValue = this.invokeForRequest(webRequest, mavContainer, providedArgs);// 设置响应状态this.setResponseStatus(webRequest);if (returnValue == null) {// 如果请求未修改、响应状态为 null 或者已处理请求if (this.isRequestNotModified(webRequest) || this.getResponseStatus() != null || mavContainer.isRequestHandled()) {mavContainer.setRequestHandled(true);return;}} else if (StringUtils.hasText(this.getResponseStatusReason())) {// 如果响应状态原因文本不为空mavContainer.setRequestHandled(true);return;}mavContainer.setRequestHandled(false);// 断言返回值处理器不为空Assert.state(this.returnValueHandlers != null, "No return value handlers");try {// 处理返回值this.returnValueHandlers.handleReturnValue(returnValue, this.getReturnValueType(returnValue), mavContainer, webRequest);} catch (Exception var6) {// 如果日志级别为 TRACE，则记录处理返回值时的异常信息if (this.logger.isTraceEnabled()) {this.logger.trace(this.getReturnValueHandlingErrorMessage("Error handling return value", returnValue), var6);}// 抛出异常throw var6;}}

这个函数是一个公共方法，它调用了其他的方法来处理HTTP请求，并根据处理结果进行相应的操作。首先，它调用了invokeForRequest方法来处理请求并获取返回值。然后，它通过调用setResponseStatus方法设置响应状态。接下来，它根据一些条件判断来决定是否设置请求已处理并返回。如果返回值为null，它会判断请求是否未修改、响应状态是否为null或请求是否已处理，如果满足条件，则设置请求已处理并返回。如果返回值不为null且响应状态原因文本不为空，同样会设置请求已处理并返回。如果以上条件都不满足，则将请求标记为未处理，并通过断言来验证返回值处理器不为空。最后，它调用returnValueHandlers的handleReturnValue方法来处理返回值，并将处理结果类型、模型视图容器和HTTP请求作为参数传递给该方法。如果处理过程中出现异常，它会记录日志并抛出异常。

/*** 在给定请求的上下文中解析方法参数值并调用方法。* <p>参数值通常通过 {@link HandlerMethodArgumentResolver} 解析。* 但是，{@code providedArgs} 参数可以提供要直接使用的参数值，即不进行参数解析的情况。* 例如，提供的参数值包括 {@link WebDataBinder}、{@link SessionStatus} 或抛出的异常实例。* 在参数解析器之前检查提供的参数值。* @param request 当前请求* @param mavContainer 本请求的 ModelAndViewContainer* @param providedArgs 与类型匹配的"给定"参数，未解析* @return 被调用方法的原始返回值* @throws Exception 如果找不到合适的参数解析器，或者方法引发了异常，则抛出异常*/
@Nullable
public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,Object... providedArgs) throws Exception {Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);if (logger.isTraceEnabled()) {logger.trace("调用方法 '" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +"' 与参数 " + Arrays.toString(args));}Object returnValue = doInvoke(args);if (logger.isTraceEnabled()) {logger.trace("方法 [" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +"] 返回 [" + returnValue + "]");}return returnValue;
}

这个函数用于在给定请求的上下文中解析方法参数值并调用方法。参数值通常通过HandlerMethodArgumentResolver解析，但是提供的参数值可以直接使用，无需解析。提供的参数值在解析器之前进行检查。如果找不到合适的参数解析器或方法引发了异常，则会抛出异常。函数返回调用方法的原始返回值。

doInvoke方法主要是根据HTTP请求的参数调用控制器的函数来获取最终的结果。doInvoke方法的源码如下：

	/*** 使用给定的参数值调用处理方法。*/protected Object doInvoke(Object... args) throws Exception {ReflectionUtils.makeAccessible(getBridgedMethod());try {return getBridgedMethod().invoke(getBean(), args);}catch (IllegalArgumentException ex) {assertTargetBean(getBridgedMethod(), getBean(), args);String text = (ex.getMessage() != null ? ex.getMessage() : "Illegal argument");throw new IllegalStateException(getInvocationErrorMessage(text, args), ex);}catch (InvocationTargetException ex) {// 解封装...Throwable targetException = ex.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 text = getInvocationErrorMessage("Failed to invoke handler method", args);throw new IllegalStateException(text, targetException);}}}

其中getBridgedMethod()获取的是Controller中相应的函数实例，getBean函数获取对应Controller的实例，运行时的堆栈信息如下：

根据上面的Controller实例，方法信息和输入参数信息，就能够执行相应控制器的方法了，并将执行结果返回给调用者。下面就能得到返回结果了。

上面调用invokeForRequest方法获取到的结果是业务端Controller中的函数返回的结果，但是HTTP通信的协议一般是JSON、Text等形式，所以还需要调用handleReturnValue函数将返回结果进行特殊处理，handleReturnValue函数的源码如下：

	/*** 迭代遍历注册的 {@link HandlerMethodReturnValueHandler} 接口，并调用支持它的处理器。* @throws IllegalStateException 如果找不到合适的 {@link HandlerMethodReturnValueHandler}。*/@Overridepublic void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);if (handler == null) {throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());}handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);}

上面的函数先根据返回值类型调用selectHandler函数根据返回数据和返回数据的类型获取HandlerMethodReturnValueHandler实例，因为我们测试用的HTTP接口返回的结果最终需要转换为JSON数据，所以HandlerMethodReturnValueHandler需要支持注解为ResponseBody的HandlerMethodReturnValueHandler实例，而RequestResponseBodyMethodProcessor就符合该条件。最终处理返回结果的逻辑委托给RequestResponseBodyMethodProcessor接口的handleReturnValue函数进行处理。源码如下：

	/*** 处理返回值** @param returnValue 返回值* @param returnType 返回类型* @param mavContainer ModelAndViewContainer* @param webRequest  NativeWebRequest* @throws IOException* @throws HttpMediaTypeNotAcceptableException* @throws HttpMessageNotWritableException*/@Overridepublic void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,ModelAndViewContainer mavContainer, NativeWebRequest webRequest)throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {mavContainer.setRequestHandled(true);ServletServerHttpRequest inputMessage = createInputMessage(webRequest);ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);if (returnValue instanceof ProblemDetail detail) {outputMessage.setStatusCode(HttpStatusCode.valueOf(detail.getStatus()));if (detail.getInstance() == null) {URI path = URI.create(inputMessage.getServletRequest().getRequestURI());detail.setInstance(path);}}// 尝试即使返回值为null。ResponseBodyAdvice可以参与进来。writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);}

该函数接收一些参数，包括返回值、方法参数类型、ModelAndViewContainer和NativeWebRequest。函数首先将mavContainer的 requestHandled属性设置为true。然后，它通过createInputMessage方法获取输入消息，通过createOutputMessage方法获取输出消息。接下来，如果返回值是ProblemDetail类型的对象，它将输出消息的状态码设置为HttpStatusCode中对应的值。如果 ProblemDetail对象的实例为空，它将从输入消息中获取请求的URI，并将其设置为ProblemDetail对象的实例。最后，函数调用 writeWithMessageConverters 方法，将返回值、方法参数类型、输入消息和输出消息作为参数传递给它。writeWithMessageConverters 方法的源码如下：

	/*** 将给定的返回类型写入给定的输出消息。* @param value 要写入输出消息的值* @param returnType 返回值的类型* @param inputMessage 输入消息。用于检查请求中的'Accept'头。* @param outputMessage 输出消息要写入* @throws IOException 当发生I/O错误时抛出* @throws HttpMediaTypeNotAcceptableException 当请求中的'Accept'头由请求的消息转换器满足不了时抛出* @throws HttpMessageNotWritableException 当给定的消息无法由转换器写入，或者服务器选择的Content-type没有兼容的转换器时抛出*/@SuppressWarnings({"rawtypes", "unchecked"})protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {Object body;Class<?> valueType;Type targetType;if (value instanceof CharSequence) {body = value.toString();valueType = String.class;targetType = String.class;}else {body = value;valueType = getReturnValueType(body, returnType);targetType = GenericTypeResolver.resolveType(getGenericType(returnType), returnType.getContainingClass());}if (isResourceType(value, returnType)) {outputMessage.getHeaders().set(HttpHeaders.ACCEPT_RANGES, "bytes");if (value != null && inputMessage.getHeaders().getFirst(HttpHeaders.RANGE) != null &&outputMessage.getServletResponse().getStatus() == 200) {Resource resource = (Resource) value;try {List<HttpRange> httpRanges = inputMessage.getHeaders().getRange();outputMessage.getServletResponse().setStatus(HttpStatus.PARTIAL_CONTENT.value());body = HttpRange.toResourceRegions(httpRanges, resource);valueType = body.getClass();targetType = RESOURCE_REGION_LIST_TYPE;}catch (IllegalArgumentException ex) {outputMessage.getHeaders().set(HttpHeaders.CONTENT_RANGE, "bytes */" + resource.contentLength());outputMessage.getServletResponse().setStatus(HttpStatus.REQUESTED_RANGE_NOT_SATISFIABLE.value());}}}MediaType selectedMediaType = null;MediaType contentType = outputMessage.getHeaders().getContentType();boolean isContentTypePreset = contentType != null && contentType.isConcrete();if (isContentTypePreset) {if (logger.isDebugEnabled()) {logger.debug("在响应中找到了'Content-Type:" + contentType + "'");}selectedMediaType = contentType;}else {HttpServletRequest request = inputMessage.getServletRequest();List<MediaType> acceptableTypes;try {acceptableTypes = getAcceptableMediaTypes(request);}catch (HttpMediaTypeNotAcceptableException ex) {int series = outputMessage.getServletResponse().getStatus() / 100;if (body == null || series == 4 || series == 5) {if (logger.isDebugEnabled()) {logger.debug("忽略错误响应内容（如果有）。" + ex);}return;}throw ex;}List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);if (body != null && producibleTypes.isEmpty()) {throw new HttpMessageNotWritableException("找不到适用于返回值类型的转换器: " + valueType);}List<MediaType> compatibleMediaTypes = new ArrayList<>();determineCompatibleMediaTypes(acceptableTypes, producibleTypes, compatibleMediaTypes);// 对于ProblemDetail，回退到RFC 7807格式if (compatibleMediaTypes.isEmpty() && ProblemDetail.class.isAssignableFrom(valueType)) {determineCompatibleMediaTypes(this.problemMediaTypes, producibleTypes, compatibleMediaTypes);}if (compatibleMediaTypes.isEmpty()) {if (logger.isDebugEnabled()) {logger.debug("找不到匹配的: " + acceptableTypes + ", 支持: " + producibleTypes);}if (body != null) {throw new HttpMediaTypeNotAcceptableException(producibleTypes);}return;}MimeTypeUtils.sortBySpecificity(compatibleMediaTypes);for (MediaType mediaType : compatibleMediaTypes) {if (mediaType.isConcrete()) {selectedMediaType = mediaType;break;}else if (mediaType.isPresentIn(ALL_APPLICATION_MEDIA_TYPES)) {selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;break;}}if (logger.isDebugEnabled()) {logger.debug("使用'" + selectedMediaType + "',给定: " +acceptableTypes + " 和支持: " + producibleTypes);}}if (selectedMediaType != null) {selectedMediaType = selectedMediaType.removeQualityValue();for (HttpMessageConverter<?> converter : this.messageConverters) {GenericHttpMessageConverter genericConverter =(converter instanceof GenericHttpMessageConverter ghmc ? ghmc : null);if (genericConverter != null ?((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) :converter.canWrite(valueType, selectedMediaType)) {body = getAdvice().beforeBodyWrite(body, returnType, selectedMediaType,(Class<? extends HttpMessageConverter<?>>) converter.getClass(),inputMessage, outputMessage);if (body != null) {Object theBody = body;LogFormatUtils.traceDebug(logger, traceOn ->"正在写入 [" + LogFormatUtils.formatValue(theBody, !traceOn) + "]");addContentDispositionHeader(inputMessage, outputMessage);if (genericConverter != null) {genericConverter.write(body, targetType, selectedMediaType, outputMessage);}else {((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);}}else {if (logger.isDebugEnabled()) {logger.debug("没有内容可写: null body");}}return;}}}if (body != null) {Set<MediaType> producibleMediaTypes =(Set<MediaType>) inputMessage.getServletRequest().getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);if (isContentTypePreset || !CollectionUtils.isEmpty(producibleMediaTypes)) {throw new HttpMessageNotWritableException("无法写入 [" + valueType + "]，Content-Type为: '" + contentType + "'");}throw new HttpMediaTypeNotAcceptableException(getSupportedMediaTypes(body.getClass()));}}

这个Java函数用于将给定的返回值写入到输出消息中。它根据请求中的"Accept"头检查是否可以满足请求，并使用适当的消息转换器将返回值转换为适当的内容类型。如果找不到适合的内容类型，则会引发异常。最后，它将内容写入到输出响应中。

在上面代码中的消息转换器messageConverters中会有很多种类型，由于常用的HTTP接口响应类型主要是application/json，其对应的消息转换器抽象类是AbstractJsonHttpMessageConverter，接下来会调用RequestResponseBodyAdviceChain的beforeBodyWrite函数进行响应结果写入Response前的操作。beforeBodyWrite函数的源码如下：

	@Override@Nullablepublic Object beforeBodyWrite(@Nullable Object body, MethodParameter returnType, MediaType contentType,Class<? extends HttpMessageConverter<?>> converterType,ServerHttpRequest request, ServerHttpResponse response) {return processBody(body, returnType, contentType, converterType, request, response);}

beforeBodyWrite函数会直接调用processBody函数处理响应的结果，源码如下：

@SuppressWarnings("unchecked")@Nullableprivate <T> Object processBody(@Nullable Object body, MethodParameter returnType, MediaType contentType,Class<? extends HttpMessageConverter<?>> converterType,ServerHttpRequest request, ServerHttpResponse response) {for (ResponseBodyAdvice<?> advice : getMatchingAdvice(returnType, ResponseBodyAdvice.class)) {if (advice.supports(returnType, converterType)) {body = ((ResponseBodyAdvice<T>) advice).beforeBodyWrite((T) body, returnType,contentType, converterType, request, response);}}return body;}

在上面的writeWithMessageConverters 方法中，执行完RequestResponseBodyAdviceChain的beforeBodyWrite函数后，开始调用消息转换器的写入操作了，我们先看一下AbstractGenericHttpMessageConverter的write函数的源码如下：

	/*** 此实现通过调用{@link #addDefaultHeaders}设置默认头部，然后调用{@link #writeInternal}。*/@Overridepublic final void write(final T t, @Nullable final Type type, @Nullable MediaType contentType,HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {final HttpHeaders headers = outputMessage.getHeaders();addDefaultHeaders(headers, t, contentType);if (outputMessage instanceof StreamingHttpOutputMessage streamingOutputMessage) {streamingOutputMessage.setBody(new StreamingHttpOutputMessage.Body() {@Overridepublic void writeTo(OutputStream outputStream) throws IOException {writeInternal(t, type, new HttpOutputMessage() {@Overridepublic OutputStream getBody() {return outputStream;}@Overridepublic HttpHeaders getHeaders() {return headers;}});}@Overridepublic boolean repeatable() {return supportsRepeatableWrites(t);}});}else {writeInternal(t, type, outputMessage);outputMessage.getBody().flush();}}

该函数用于写入HTTP输出消息。它首先通过调用addDefaultHeaders方法设置默认头部，然后调用writeInternal方法写入内容。如果输出消息是StreamingHttpOutputMessage类型，则将内容写入StreamingHttpOutputMessage的Body中，并设置可重复写入标志。否则，直接调用writeInternal方法并将内容写入输出消息的Body中，并刷新Body。

上面最终会调用函数writeInternal将控制器中HTTP接口的返回结果写入到body中，源码如下：

/*** 内部写入方法，将对象写入到指定的HttpOutputMessage中。** @param object 要写入的对象* @param type 要写入的对象的类型* @param outputMessage 要写入的HttpOutputMessage对象* @throws IOException 如果发生IO错误* @throws HttpMessageNotWritableException 如果发生消息写入错误*/
@Override
protected void writeInternal(Object object, @Nullable Type type, HttpOutputMessage outputMessage)throws IOException, HttpMessageNotWritableException {// 获取输出消息的媒体类型MediaType contentType = outputMessage.getHeaders().getContentType();// 获取JSON编码方式JsonEncoding encoding = getJsonEncoding(contentType);// 获取对象的Class类型Class<?> clazz = (object instanceof MappingJacksonValue mappingJacksonValue ?mappingJacksonValue.getValue().getClass() : object.getClass());// 选择合适的ObjectMapperObjectMapper objectMapper = selectObjectMapper(clazz, contentType);Assert.state(objectMapper != null, () -> "No ObjectMapper for " + clazz.getName());// 获取输出流，并关闭与之关联的输出流OutputStream outputStream = StreamUtils.nonClosing(outputMessage.getBody());try (JsonGenerator generator = objectMapper.getFactory().createGenerator(outputStream, encoding)) {// 写入前缀信息writePrefix(generator, object);Object value = object;Class<?> serializationView = null;FilterProvider filters = null;JavaType javaType = null;if (object instanceof MappingJacksonValue mappingJacksonValue) {// 处理MappingJacksonValue类型对象value = mappingJacksonValue.getValue();serializationView = mappingJacksonValue.getSerializationView();filters = mappingJacksonValue.getFilters();}if (type != null && TypeUtils.isAssignable(type, value.getClass())) {// 处理指定的类型javaType = getJavaType(type, null);}// 创建ObjectWriter对象ObjectWriter objectWriter = (serializationView != null ?objectMapper.writerWithView(serializationView) : objectMapper.writer());if (filters != null) {// 设置过滤器objectWriter = objectWriter.with(filters);}if (javaType != null && (javaType.isContainerType() || javaType.isTypeOrSubTypeOf(Optional.class))) {// 设置JavaTypeobjectWriter = objectWriter.forType(javaType);}// 获取配置信息SerializationConfig config = objectWriter.getConfig();if (contentType != null && contentType.isCompatibleWith(MediaType.TEXT_EVENT_STREAM) &&config.isEnabled(SerializationFeature.INDENT_OUTPUT)) {// 设置缩进输出objectWriter = objectWriter.with(this.ssePrettyPrinter);}// 自定义ObjectWriterobjectWriter = customizeWriter(objectWriter, javaType, contentType);// 写入对象的值objectWriter.writeValue(generator, value);// 写入后缀信息writeSuffix(generator, object);// 刷新输出流generator.flush();}catch (InvalidDefinitionException ex) {// 处理无效定义异常throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);}catch (JsonProcessingException ex) {// 处理JSON处理异常throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getOriginalMessage(), ex);}
}

上面的代码主要是将控制器中HTTP接口的返回结果序列化成JSON数据后写入到Body中。