spring提供了15种默认返回值处理器：

一、处理返回值：

this.returnValueHandlers.handleReturnValue(
      returnValue, getReturnValueType(returnValue), mavContainer, webRequest)

二、获取对应的返回值处理器：

private HandlerMethodReturnValueHandler selectHandler(@Nullable Object value, MethodParameter returnType) {
   boolean isAsyncValue = isAsyncReturnValue(value, returnType);
   //通过循环一个一个判断当前返回值处理器是否支持处理当前返回值
   for (HandlerMethodReturnValueHandler handler : this.returnValueHandlers) {
      if (isAsyncValue && !(handler instanceof AsyncHandlerMethodReturnValueHandler)) {
         continue;
      }
      //是否支持处理当前返回值
      if (handler.supportsReturnType(returnType)) {
         return handler;
      }
   }
   return null;
}

三、根据返回的返回值处理器处理返回值：

if (handler == null) {
   throw new IllegalArgumentException("Unknown return value type: " + returnType.getParameterType().getName());
}
//执行处理返回值
handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);

扩展：返回值处理器接口:

HandlerMethodReturnValueHandler

public interface HandlerMethodReturnValueHandler {

   /**
    * 当前返回值处理器是否支持处理返回值
    */
   boolean supportsReturnType(MethodParameter returnType);

   /**
    * 处理当前返回值
    */
   void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
         ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;

}

二、内容协商 

springmvc可以根据游览器可接受的内容和响应头接受的类型进行返回值协商：

①判断响应头是否有返回值要求：

MediaType selectedMediaType = null;
MediaType contentType = outputMessage.getHeaders().getContentType();
boolean isContentTypePreset = contentType != null && contentType.isConcrete();
if (isContentTypePreset) {
   if (logger.isDebugEnabled()) {
      logger.debug("Found 'Content-Type:" + contentType + "' in response");
   }
   selectedMediaType = contentType;
}

②判断游览器支持的返回值类型：

   2.1：获取游览器可接受的返回值类型。

   2.2：服务器可生产的返回值类型。

HttpServletRequest request = inputMessage.getServletRequest();
List<MediaType> acceptableTypes;
try {
   //获取游览器支持的响应类型
   acceptableTypes = getAcceptableMediaTypes(request);
}
catch (HttpMediaTypeNotAcceptableException ex) {
   int series = outputMessage.getServletResponse().getStatus() / 100;
   if (body == null || series == 4 || series == 5) {
      if (logger.isDebugEnabled()) {
         logger.debug("Ignoring error response content (if any). " + ex);
      }
      return;
   }
   throw ex;
}
//服务器可生产的返回值类型
List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);

if (body != null && producibleTypes.isEmpty()) {
   throw new HttpMessageNotWritableException(
         "No converter found for return value of type: " + valueType);
}
List<MediaType> mediaTypesToUse = new ArrayList<>();
//通过游览器可接受的类型和服务器可生产的类型进行匹配
for (MediaType requestedType : acceptableTypes) {
   for (MediaType producibleType : producibleTypes) {
      if (requestedType.isCompatibleWith(producibleType)) {
         mediaTypesToUse.add(getMostSpecificMediaType(requestedType, producibleType));
      }
   }
}
if (mediaTypesToUse.isEmpty()) {
   if (logger.isDebugEnabled()) {
      logger.debug("No match for " + acceptableTypes + ", supported: " + producibleTypes);
   }
   if (body != null) {
      throw new HttpMediaTypeNotAcceptableException(producibleTypes);
   }
   return;
}

MediaType.sortBySpecificityAndQuality(mediaTypesToUse);
//找到服务器可生产的类型以及游览器可接受的类型，双向同意
for (MediaType mediaType : mediaTypesToUse) {
   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("Using '" + selectedMediaType + "', given " +
         acceptableTypes + " and supported " + producibleTypes);
}

③进行内容转换（消息转换）：

			selectedMediaType = selectedMediaType.removeQualityValue();
            //循环所有的消息转换器
			for (HttpMessageConverter<?> converter : this.messageConverters) {
				GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
						(GenericHttpMessageConverter<?>) converter : null);
				if (genericConverter != null ?
						((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) :
                        //是否能写
						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 ->
								"Writing [" + 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("Nothing to write: null body");
						}
					}
					return;
				}
			}

内容协商策略：

public List<MediaType> resolveMediaTypes(NativeWebRequest request) throws HttpMediaTypeNotAcceptableException {
   for (ContentNegotiationStrategy strategy : this.strategies) {
      List<MediaType> mediaTypes = strategy.resolveMediaTypes(request);
      if (mediaTypes.equals(MEDIA_TYPE_ALL_LIST)) {
         continue;
      }
      return mediaTypes;
   }
   return MEDIA_TYPE_ALL_LIST;
}

默认是基于请求头的策略：

 

开启基于参数的内容协商策略：

spring:
  mvc:
    contentnegotiation:
      favor-parameter: true

通过url？format=xml....基于参与内容协商