使用feign发送http请求解析报错

错误如下：

发送请求开始
-----
[ChannelFeign#formRecog] ---> END HTTP (304117-byte body)
发送请求结束
返回开始
[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格式的数据}
[ChannelFeign#formRecog] <--- END HTTP (5207-byte body)
返回结束
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]

可以看到返回的类型为[ChannelFeign#formRecog] content-type: text/json;charset=UTF-8

错误原因

接口返回为JSON格式数据但却将数据表示为了[text/json]导致Feign没有采用JSON解析器来解析，从而无法将响应数据转化为对应的POJO对象；

源码分析

feign客户端发送请求入口函数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();
      }
      // 分发请求
      return dispatch.get(method).invoke(args);
    }

decode()返回请求的解码函数

  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);
    }
  }

进入decode.decode()，提取数据

@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 + "]");
	}

进入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;
	}

通过断点发现mediaType为接口返回的content-type：text/json类型。而supportedMediaType为application/json，所以返回false，找不到合适的转换器。 

解决方案一

替代Feign的解码器，使json解析器同时解析[text/plain]的数据

// 创建一个新的转换器 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将自动 替换
// 解决微信返回参数为[text/plain] 无法转化为json
@Bean
public Decoder feignDecoder(){
    WxMessageConverter wxConverter = new WxMessageConverter();
    ObjectFactory<HttpMessageConverters> objectFactory = () -> new HttpMessageConverters(wxConverter);
    return new SpringDecoder(objectFactory);
}

参考：Buynow_Zhao博客 https://blog.csdn.net/a610786189/article/details/80508353?utm_source=copy

解决方案二

对返回的json字符串使用fastjosn转换

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

错误2

发送请求时对象转换json会自动将属性的首字母小写

解决方法：

//@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化时小写header也会出现
    @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会生成大写和小写

如：{“Header”:xxx,"header":xxx}