一、问题

当我们项目中需要引入"jackson-dataformat-xml"后，我们会发现我们原本返回json数据的接口也开始返回xml数据格式了

二、原因

public abstract class AbstractMessageConverterMethodProcessor extends AbstractMessageConverterMethodArgumentResolver
        implements HandlerMethodReturnValueHandler {
        /**
     * Writes the given return type to the given output message.
     * @param value the value to write to the output message
     * @param returnType the type of the value
     * @param inputMessage the input messages. Used to inspect the {@code Accept} header.
     * @param outputMessage the output message to write to
     * @throws IOException thrown in case of I/O errors
     * @throws HttpMediaTypeNotAcceptableException thrown when the conditions indicated
     * by the {@code Accept} header on the request cannot be met by the message converters
     */
    @SuppressWarnings({"rawtypes", "unchecked"})
    protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
            ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
            throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

        ... 省略部分代码
      
        List<MediaType> mediaTypesToUse;

        MediaType contentType = outputMessage.getHeaders().getContentType();
        if (contentType != null && contentType.isConcrete()) {
            mediaTypesToUse = Collections.singletonList(contentType);
        }
        else {
            HttpServletRequest request = inputMessage.getServletRequest();
      // 从request请求头中，获取accept变量的value
            List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);
      // 在RequestMapping中获取 produces的value
            List<MediaType> producibleMediaTypes = getProducibleMediaTypes(request, valueType, declaredType);

            if (outputValue != null && producibleMediaTypes.isEmpty()) {
                throw new HttpMessageNotWritableException(
                        "No converter found for return value of type: " + valueType);
            }
            mediaTypesToUse = new ArrayList<>();
      
      // 两个List进行匹配选择可用的返回类型
            for (MediaType requestedType : requestedMediaTypes) {
                for (MediaType producibleType : producibleMediaTypes) {
                    if (requestedType.isCompatibleWith(producibleType)) {
                        mediaTypesToUse.add(getMostSpecificMediaType(requestedType, producibleType));
                    }
                }
            }
            if (mediaTypesToUse.isEmpty()) {
                if (outputValue != null) {
                    throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
                }
                return;
            }
            MediaType.sortBySpecificityAndQuality(mediaTypesToUse);
        }

    // 选择最终的返回数据类型
        MediaType selectedMediaType = null;
        for (MediaType mediaType : mediaTypesToUse) {
            if (mediaType.isConcrete()) {
                selectedMediaType = mediaType;
                break;
            }
            else if (mediaType.equals(MediaType.ALL) || mediaType.equals(MEDIA_TYPE_APPLICATION)) {
                selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
                break;
            }
        }

        if (selectedMediaType != null) {
            selectedMediaType = selectedMediaType.removeQualityValue();
            for (HttpMessageConverter<?> converter : this.messageConverters) {
                GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
                        (GenericHttpMessageConverter<?>) converter : null);
                if (genericConverter != null ?
                        ((GenericHttpMessageConverter) converter).canWrite(declaredType, valueType, selectedMediaType) :
                        converter.canWrite(valueType, selectedMediaType)) {
                    outputValue = getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
                            (Class<? extends HttpMessageConverter<?>>) converter.getClass(),
                            inputMessage, outputMessage);
                    if (outputValue != null) {
                        addContentDispositionHeader(inputMessage, outputMessage);
                        if (genericConverter != null) {
                            genericConverter.write(outputValue, declaredType, selectedMediaType, outputMessage);
                        }
                        else {
                            ((HttpMessageConverter) converter).write(outputValue, selectedMediaType, outputMessage);
                        }
                        if (logger.isDebugEnabled()) {
                            logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
                                    "\" using [" + converter + "]");
                        }
                    }
                    return;
                }
            }
        }

        if (outputValue != null) {
            throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
        }
    }
}

在这一段代码中，虽然application/json和application/xml是相等的，但由于这个排序是稳定排序（相等值顺序不变），而且application/xml的加载顺序在先，因此总是选中了application/xml

三、解决方案

**产生可能有两个原因**

1：查看项目重复引用了jackson-dataformat-xml，去掉一个也恢复了

2：设置默认contentType, 通过 implements WebMvcconfigurer

@Configuration
public class WebInterceptorAdapter implements WebMvcConfigurer {
  @Override
    public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
        configurer.defaultContentType(MediaType.APPLICATION_JSON, MediaType.TEXT_XML, MediaType.APPLICATION_XML);
    }
}

可能有的人加上这段代码还是失效，这时我们就要看下我们的其它类中是否实现过“WebMvcConfigurer”或者“WebMvcConfigurationSupport”，如果有我们则将这段代码加入即可

@Override
    public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
        configurer.defaultContentType(MediaType.APPLICATION_JSON, MediaType.TEXT_XML, MediaType.APPLICATION_XML);
    }

关于更详尽的解决步骤，大家可以参考spring官网，写的很详细