1. 現(xiàn)象

如題

2. 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 {

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

        MediaType contentType = outputMessage.getHeaders().getContentType();
        if (contentType != null && contentType.isConcrete()) {
            mediaTypesToUse = Collections.singletonList(contentType);
        }
        else {
            HttpServletRequest request = inputMessage.getServletRequest();
      // 從request請求頭中，獲取accept變量的value
            List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);
      // 在RequestMapping中獲取 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<>();
      
      // 兩個List進行匹配選擇可用的返回類型
            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);
        }

    // 選擇最終的返回數(shù)據(jù)類型
        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);
        }
    }
}

返回xml原因：requestedMediaTypes 在Request的accept中沒有值。而producibleMediaTypes中返回類型排序是 application/xml 排第一位， 在 application/json 之前。因此最終匹配的時候，返回了 application/xml

3. 解決辦法

• 方法一：設(shè)置默認contentType, 通過 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);
      }
  }
  

• 方法二：由于我的項目重復引用了fastxml，去掉一個也恢復了

4. 關(guān)聯(lián)問題

問題

? 在上面的問題中，第一次只設(shè)定了 configurer.defaultContentType(MediaType.APPLICATION_JSON), 出現(xiàn)了問題，如下：

Could not find acceptable representation

Spring 不知道接受的類型是什么，因此你需要告訴Spring。

解決方案

• 通過RequestMapping中參數(shù)consumer告訴Spring。

• 設(shè)置默認contentType，設(shè)置很全。

  configurer.defaultContentType(MediaType.APPLICATION_JSON, MediaType.TEXT_XML, MediaType.APPLICATION_XML);

相關(guān)閱讀*

*https://spring.io/blog/2013/05/11/content-negotiation-using-spring-mvc