What Determines @ResponseBody’s Default Return Format in Spring MVC?

Background

@ResponseBody default return format when backend does not specify produces MediaType?

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

Search results show that @ResponseBody converts a Java object to JSON.

Correct Answer

We first publish the correct answer.

The output format of @ResponseBody, by default, is determined by the client’s

Accept

request header.

Source Code Analysis

RequestResponseBodyMethodProcessor

<code>public class RequestResponseBodyMethodProcessor {
  // handle @ResponseBody annotated methods
  @Override
  public boolean supportsReturnType(MethodParameter returnType) {
    return (AnnotatedElementUtils.hasAnnotation(returnType.getContainingClass(), ResponseBody.class) ||
            returnType.hasMethodAnnotation(ResponseBody.class));
  }
  // handle return value
  @Override
  public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
                                ModelAndViewContainer mavContainer, NativeWebRequest webRequest) {
    mavContainer.setRequestHandled(true);
    ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
    ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
    // write with converters
    writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
  }
}
</code>

writeWithMessageConverters

<code>protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
                        ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage) {
  HttpServletRequest request = inputMessage.getServletRequest();
  // Get Accept header media types
  List<MediaType> acceptableTypes = getAcceptableMediaTypes(request);
  // Get producible media types declared by the controller
  List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);
  // Determine media types to use
  List<MediaType> mediaTypesToUse = new ArrayList<>();
  for (MediaType requestedType : acceptableTypes) {
    for (MediaType producibleType : producibleTypes) {
      if (requestedType.isCompatibleWith(producibleType)) {
        mediaTypesToUse.add(getMostSpecificMediaType(requestedType, producibleType));
      }
    }
  }
  // Sort by specificity and quality
  MediaType.sortBySpecificityAndQuality(mediaTypesToUse);

  for (MediaType mediaType : mediaTypesToUse) {
    if (mediaType.isConcrete()) {
      selectedMediaType = mediaType;
      break;
    } else if (mediaType.isPresentIn(ALL_APPLICATION_MEDIA_TYPES)) {
      selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
      break;
    }
  }

  selectedMediaType = selectedMediaType.removeQualityValue();
  // Find a suitable HttpMessageConverter and write the body
  for (HttpMessageConverter<?> converter : this.messageConverters) {
    GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
        (GenericHttpMessageConverter<?>) converter : null);
    if (genericConverter != null ?
        genericConverter.canWrite(targetType, valueType, selectedMediaType) :
        converter.canWrite(valueType, selectedMediaType)) {
      body = getAdvice().beforeBodyWrite(body, returnType, selectedMediaType,
        (Class<? extends HttpMessageConverter<?>>) converter.getClass(),
        inputMessage, outputMessage);
      return;
    }
  }
}
</code>

Why Study This Issue

When upgrading to

spring cloud alibaba 2.2.1

, the sentinel module brings in additional dependencies.

If the

dataformat

jar appears, RestTemplate adds the following media types to the default Accept header:

application/xml | text/xml | application/*+xml

<code>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");
}
</code>

When using RestTemplate without specifying Accept, the response may be XML, preventing a smooth upgrade.