关于@ResponseBody 默认输出的误区的解答

@Controller
public class DemoController {
 @ResponseBody
 @GetMapping(value = "/demo")
 public DemoVO demo() {
  return new DemoVO("lengleng", "123456");
 }
}

public class RequestResponseBodyMethodProcessor {
// 处理 ResponseBody 标注的方法
@Override
public boolean supportsReturnType(MethodParameter returnType) {
  return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) ||
    returnType.hasMethodAnnotation(ResponseBody.class));
 }
// 处理返回值
@Override
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
               ModelAndViewContainer mavContainer, NativeWebRequest webRequest) {
  mavContainer.setRequestHandled(true);
  ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
  ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
  // 处理返回值
  writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
 }
}

protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
            ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage) {
 HttpServletRequest request = inputMessage.getServletRequest();
 // 获取请求头中的目标资源类型
 List<MediaType> acceptableTypes = getAcceptableMediaTypes(request);
 // 获取接口指定支持的资源类型
 List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);
 // 获取能够输出资源类型
 List<MediaType> mediaTypesToUse = new ArrayList<>();
 for (MediaType requestedType : acceptableTypes) {
  for (MediaType producibleType : producibleTypes) {
   if (requestedType.isCompatibleWith(producibleType)) {
    mediaTypesToUse.add(getMostSpecificMediaType(requestedType, producibleType));
   }
  }
 }
 /// 排序
 MediaType.sortBySpecificityAndQuality(mediaTypesToUse);

 for (MediaType mediaType : mediaTypesToUse) {
  // 判断资源类型是否是具体的类型，而不是带通配符 * 这种
  if (mediaType.isConcrete()) {
   selectedMediaType = mediaType;
   break;
  }
  else if (mediaType.isPresentIn(ALL_APPLICATION_MEDIA_TYPES)) {
   selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
   break;
  }
 }

 selectedMediaType = selectedMediaType.removeQualityValue();
 // 查找支持选中资源类型的 HttpMessageConverter，输出body
 for (HttpMessageConverter<?> converter : this.messageConverters) {
  GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
   (GenericHttpMessageConverter<?>) converter : 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);
   return;
  }
 }
}

public MappingJackson2XmlHttpMessageConverter(ObjectMapper objectMapper) {
 super(objectMapper, new MediaType("application", "xml", StandardCharsets.UTF_8),
   new MediaType("text", "xml", StandardCharsets.UTF_8),
   new MediaType("application", "*+xml", StandardCharsets.UTF_8));
 Assert.isInstanceOf(XmlMapper.class, objectMapper, "XmlMapper required");
}