錯誤如下：

傳送請求開始
-----
[ChannelFeign#formRecog] ---> END HTTP (304117-byte body)
傳送請求結束
返回開始
[ChannelFeign#formRecog] <--- HTTP/1.1 200 OK (4948ms)
[ChannelFeign#formRecog] content-length: 5207
[ChannelFeign#formRecog] content-type: text/json;charset=UTF-8
[ChannelFeign#formRecog] date: Mon, 08 Oct 2018 10:47:03 GMT
[ChannelFeign#formRecog] x-vcap-request-id: c323f65a-12e6-4604-7393-a4bf0ca403d5
[ChannelFeign#formRecog] 
[ChannelFeign#formRecog] {json格式的資料}
[ChannelFeign#formRecog] <--- END HTTP (5207-byte body)
返回結束
ERROR org.apache.catalina.core.ContainerBase.[Tomcat].[localhost].[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is feign.codec.DecodeException: Could not extract response: no suitable HttpMessageConverter found for response type [channel.domain.ChannelResponse<TableData>] and content type [text/json;charset=UTF-8]] with root cause
org.springframework.web.client.RestClientException: Could not extract response: no suitable HttpMessageConverter found for response type [channel.domain.ChannelResponse<TableData>] and content type [text/json;charset=UTF-8]
	at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:110) ~[spring-web-4.3.13.RELEASE.jar:4.3.13.RELEASE]
	at org.springframework.cloud.netflix.feign.support.SpringDecoder.decode(SpringDecoder.java:59) ~[spring-cloud-netflix-core-1.3.6.RELEASE.jar:1.3.6.RELEASE]
	at org.springframework.cloud.netflix.feign.support.ResponseEntityDecoder.decode(ResponseEntityDecoder.java:47) ~[spring-cloud-netflix-core-1.3.6.RELEASE.jar:1.3.6.RELEASE]
	at feign.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:165) ~[feign-core-9.5.0.jar:?]
	at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:133) ~[feign-core-9.5.0.jar:?]
	at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:76) ~[feign-core-9.5.0.jar:?]
	at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:103) ~[feign-core-9.5.0.jar:?]
 

org.springframework.web.client.RestClientException: Could not extract response: no suitable HttpMessageConverter found for response type [channel.domain.ChannelResponse<TableData>] and content type [text/json;charset=UTF-8]

可以看到返回的型別為[ChannelFeign#formRecog] content-type: text/json;charset=UTF-8

錯誤原因

介面返回為JSON格式資料但卻將資料表示為了[text/json]導致Feign沒有采用JSON解析器來解析，從而無法將響應資料轉化為對應的POJO物件；

原始碼分析

feign客戶端傳送請求入口函式invoke()

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      if ("equals".equals(method.getName())) {
        try {
          Object
              otherHandler =
              args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;
          return equals(otherHandler);
        } catch (IllegalArgumentException e) {
          return false;
        }
      } else if ("hashCode".equals(method.getName())) {
        return hashCode();
      } else if ("toString".equals(method.getName())) {
        return toString();
      }
      // 分發請求
      return dispatch.get(method).invoke(args);
    }
 

decode()返回請求的解碼函式

  Object decode(Response response) throws Throwable {
    try {
      return decoder.decode(response, metadata.returnType());
    } catch (FeignException e) {
      throw e;
    } catch (RuntimeException e) {
      throw new DecodeException(e.getMessage(), e);
    }
  }

進入decode.decode()，提取資料

@Override
	@SuppressWarnings({"unchecked", "rawtypes", "resource"})
	public T extractData(ClientHttpResponse response) throws IOException {
		MessageBodyClientHttpResponseWrapper responseWrapper = new MessageBodyClientHttpResponseWrapper(response);
		if (!responseWrapper.hasMessageBody() || responseWrapper.hasEmptyMessageBody()) {
			return null;
		}
		MediaType contentType = getContentType(responseWrapper);

		for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
			if (messageConverter instanceof GenericHttpMessageConverter) {
				GenericHttpMessageConverter<?> genericMessageConverter =
						(GenericHttpMessageConverter<?>) messageConverter;
				if (genericMessageConverter.canRead(this.responseType, null, contentType)) {
					if (logger.isDebugEnabled()) {
						logger.debug("Reading [" + this.responseType + "] as \"" +
								contentType + "\" using [" + messageConverter + "]");
					}
					return (T) genericMessageConverter.read(this.responseType, null, responseWrapper);
				}
			}
			if (this.responseClass != null) {
				if (messageConverter.canRead(this.responseClass, contentType)) {
					if (logger.isDebugEnabled()) {
						logger.debug("Reading [" + this.responseClass.getName() + "] as \"" +
								contentType + "\" using [" + messageConverter + "]");
					}
					return (T) messageConverter.read((Class) this.responseClass, responseWrapper);
				}
			}
		}

		throw new RestClientException("Could not extract response: no suitable HttpMessageConverter found " +
				"for response type [" + this.responseType + "] and content type [" + contentType + "]");
	}

進入genericMessageConverter.canRead(this.responseType, null, contentType) 

	protected boolean canRead(MediaType mediaType) {
		if (mediaType == null) {
			return true;
		}
		for (MediaType supportedMediaType : getSupportedMediaTypes()) {
			if (supportedMediaType.includes(mediaType)) {
				return true;
			}
		}
		return false;
	}

通過斷點發現mediaType為介面返回的content-type：text/json型別。而supportedMediaType為application/json，所以返回false，找不到合適的轉換器。 

解決方案一

替代Feign的解碼器，使json解析器同時解析[text/plain]的資料

// 建立一個新的轉換器 解析微信的 [text/plain] 
public class WxMessageConverter extends MappingJackson2HttpMessageConverter {
    public WxMessageConverter(){
        List<MediaType> mediaTypes = new ArrayList<>();
        mediaTypes.add(MediaType.TEXT_PLAIN);
        setSupportedMediaTypes(mediaTypes);
    }
}

注入新的Decoder Feign將自動 替換
// 解決微信返回引數為[text/plain] 無法轉化為json
@Bean
public Decoder feignDecoder(){
    WxMessageConverter wxConverter = new WxMessageConverter();
    ObjectFactory<HttpMessageConverters> objectFactory = () -> new HttpMessageConverters(wxConverter);
    return new SpringDecoder(objectFactory);
}

參考：Buynow_Zhao部落格 https://blog.csdn.net/a610786189/article/details/80508353?utm_source=copy

解決方案二

對返回的json字串使用fastjosn轉換

        String result = channelFeign.formRecogTest(channelRequest);
        ChannelResponse<TableData> hello = JSONObject.parseObject(result,
                new TypeReference<ChannelResponse<TableData>>() {
                });

錯誤2

傳送請求時物件轉換json會自動將屬性的首字母小寫

解決方法：

//@Data
public class ChannelRequest {
    //@JSONField(name="Header")
    @JsonProperty
    private ChannelReqHead Header;
    //@JSONField(name="Body")
    @JsonProperty
    private ChannelReqBody Body;
    
    // 如果get方法上不加JsonIgnore，jason化時小寫header也會出現
    @JsonIgnore
    public ChannelReqHead getHeader() {
        return Header;
    }
    @JsonIgnore
    public void setHeader(ChannelReqHead header) {
        Header = header;
    }
    @JsonIgnore
    public ChannelReqBody getBody() {
        return Body;
    }
    @JsonIgnore
    public void setBody(ChannelReqBody body) {
        Body = body;
    }
    
}

使用jsonField不起作用，

不使用jsonIgnore會生成大寫和小寫

如：{“Header”:xxx,"header":xxx}