Could not extract response: no suitable HttpMessageConverter found for content type [text/html]

报错信息

在使用 RestTemplate请求调用的时候，程序报错

报错信息如下：

org.springframework.web.client.RestClientException: Could not extract response: no suitable HttpMessageConverter found for response type [class afei.common.utils.R] and content type [text/html;charset=UTF-8]at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:121) ~[spring-web-5.1.9.RELEASE.jar:5.1.9.RELEASE]at org.springframework.cloud.openfeign.support.SpringDecoder.decode(SpringDecoder.java:59) ~[spring-cloud-openfeign-core-2.1.3.RELEASE.jar:2.1.3.RELEASE]at org.springframework.cloud.openfeign.support.ResponseEntityDecoder.decode(ResponseEntityDecoder.java:62) ~[spring-cloud-openfeign-core-2.1.3.RELEASE.jar:2.1.3.RELEASE]at feign.optionals.OptionalDecoder.decode(OptionalDecoder.java:36) ~[feign-core-10.2.3.jar:na]

错误信息是未知的ContentType，这个ContentType就是第三方接口返回时候在HTTP头中的Content-Type，如果通过其他工具查看这个接口返回的HTTP头，会发现他的值是 text/html，通常我们见的都是 application/json 类型。（微信接口返回的是text/plain），由于内部没有 HttpMessageConverter 能处理text/html的数据，没有一个实现类的 canRead() 返回 true，所以最后报错

源码分析

public T extractData(ClientHttpResponse response) throws IOException {MessageBodyClientHttpResponseWrapper responseWrapper = new MessageBodyClientHttpResponseWrapper(response);if (responseWrapper.hasMessageBody() && !responseWrapper.hasEmptyMessageBody()) {MediaType contentType = this.getContentType(responseWrapper);try {//拿到messageConverters的迭代器Iterator var4 = this.messageConverters.iterator();while(var4.hasNext()) {//下一个HttpMessageConverterHttpMessageConverter<?> messageConverter = (HttpMessageConverter)var4.next();//如果是GenericHttpMessageConverter接口的实例，继承AbstractHttpMessageConverter会走这个if。if (messageConverter instanceof GenericHttpMessageConverter) {GenericHttpMessageConverter<?> genericMessageConverter = (GenericHttpMessageConverter)messageConverter;//判断这个转换器能不能转换这个contentType类型if (genericMessageConverter.canRead(this.responseType, (Class)null, contentType)) {if (this.logger.isDebugEnabled()) {ResolvableType resolvableType = ResolvableType.forType(this.responseType);this.logger.debug("Reading to [" + resolvableType + "]");}//走到这代表当前的HttpMessageConverter能进行转换，则调用read并返回return genericMessageConverter.read(this.responseType, (Class)null, responseWrapper);}}//还是判断这个转换器能不能进行contentType转换if (this.responseClass != null && messageConverter.canRead(this.responseClass, contentType)) {if (this.logger.isDebugEnabled()) {String className = this.responseClass.getName();this.logger.debug("Reading to [" + className + "] as \"" + contentType + "\"");}//走到这代表当前的HttpMessageConverter能进行转换，则调用read并返回return messageConverter.read(this.responseClass, responseWrapper);}}} catch (HttpMessageNotReadableException | IOException var8) {throw new RestClientException("Error while extracting response for type [" + this.responseType + "] and content type [" + contentType + "]", var8);}//走到这抛出异常，所有的消息转换器都不能进行处理。throw new UnknownContentTypeException(this.responseType, contentType, response.getRawStatusCode(), response.getStatusText(), response.getHeaders(), getResponseBody(response));} else {return null;}
}

messageConverters 集合中就保存着
在 RestTemplate 构造方法中添加的 HttpMessageConverter 实现类

解决方法

修改 mappingJackson2HttpMessageConverter 配置

重新设置 MappingJackson2HttpMessageConverter 能处理的 MediaType

@Bean
public RestTemplate restTemplate(){RestTemplate restTemplate = new RestTemplate();MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter = new MappingJackson2HttpMessageConverter();mappingJackson2HttpMessageConverter.setSupportedMediaTypes(Arrays.asList(MediaType.TEXT_HTML));restTemplate.getMessageConverters().add(mappingJackson2HttpMessageConverter);return  restTemplate;
}

MappingJackson2HttpMessageConverter 也是一个 HttpMessageConverter 转换类，但是他不能处理 text/html 的数据，原因是他的父类 AbstractHttpMessageConverter 中的 supportedMediaTypes 集合中没有 text/html 类型，如果有的话就能处理了，通过 setSupportedMediaTypes 可以给他指定一个新的 MediaType 集合

上面的写法会导致 MappingJackson2HttpMessageConverter 只能处理 text/html 类型的数据

继承 mappingJackson2HttpMessageConverter

使用MappingJackson2HttpMessageConverter，只需要给他能处理的MediaType

public class QQHttpMessageConverter extends MappingJackson2HttpMessageConverter {public QQHttpMessageConverter() {setSupportedMediaTypes(Arrays.asList(MediaType.TEXT_HTML));}
}

然后将这个消息转换器追加到RestTemplate中的 messageConverters

@Bean
public RestTemplate restTemplate() {RestTemplate restTemplate = new RestTemplate();restTemplate.getMessageConverters().add(new QQHttpMessageConverter());  // 兼容 text/plainreturn restTemplate;
}

实现 HttpMessageConverter

直接继承HttpMessageConverter（当然更推荐的是继承 Abstract HttpMessageConverter）来实现

public interface HttpMessageConverter<T> {/*** 根据mediaType判断clazz是否可读*/boolean canRead(Class<?> clazz, @Nullable MediaType mediaType);/*** 根据mediaType判断clazz是否可写*/boolean canWrite(Class<?> clazz, @Nullable MediaType mediaType);/*** 获取支持的mediaType*/List<MediaType> getSupportedMediaTypes();/*** 将HttpInputMessage流中的数据绑定到clazz中*/T read(Class<? extends T> clazz, HttpInputMessage inputMessage)throws IOException, HttpMessageNotReadableException;/*** 将t对象写入到HttpOutputMessage流中*/void write(T t, @Nullable MediaType contentType, HttpOutputMessage outputMessage)throws IOException, HttpMessageNotWritableException;
}

canWrite，write方式是不需要处理的，只管canRead和read就行

在 canRead 方法中判断了是不是 text/html 类型，是的话就会返回true，Spring 就会调用 read，用来将字节流中的数据转换成具体实体，aClass就是我们最终想要得到的实例对象的Class

StreamUtils 这个工具类是SpringBoot自带的一个，用来读取InputStream中的数据并返回String字符串，SpringBoott内部很多地方都用到了这个工具类

import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.springframework.http.HttpInputMessage;
import org.springframework.http.HttpOutputMessage;
import org.springframework.http.MediaType;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.HttpMessageNotReadableException;
import org.springframework.http.converter.HttpMessageNotWritableException;
import org.springframework.util.StreamUtils;import java.io.IOException;
import java.nio.charset.Charset;
import java.util.Arrays;
import java.util.List;public class QQHttpMessageConverter implements HttpMessageConverter<Object> {@Overridepublic boolean canRead(Class<?> aClass, MediaType mediaType) {if (mediaType != null) {return mediaType.isCompatibleWith(MediaType.TEXT_HTML);}return false;}@Overridepublic boolean canWrite(Class<?> aClass, MediaType mediaType) {return false;}@Overridepublic List<MediaType> getSupportedMediaTypes() {return Arrays.asList(MediaType.TEXT_HTML);}@Overridepublic Object read(Class<?> aClass, HttpInputMessage httpInputMessage) throws IOException, HttpMessageNotReadableException {String json = StreamUtils.copyToString(httpInputMessage.getBody(), Charset.forName("UTF-8"));ObjectMapper objectMapper = new ObjectMapper();objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);return objectMapper.readValue(json, aClass);}@Overridepublic void write(Object o, MediaType mediaType, HttpOutputMessage httpOutputMessage) throws IOException, HttpMessageNotWritableException {}
}

最后需要要进行配置，getMessageConverters() 会返回现有的 HttpMessageConverter 集合，我们在这个基础上加入我们自定义的 HttpMessageConverter 即可

@Bean
public RestTemplate restTemplate(){RestTemplate restTemplate = new RestTemplate();restTemplate.getMessageConverters().add(new QQHttpMessageConverter());return  restTemplate;
}

继承 AbstractHttpMessageConverter

public class QQHttpMessageConverter extends AbstractHttpMessageConverter<Object> {public QQHttpMessageConverter() {super(MediaType.TEXT_HTML);}@Overrideprotected boolean supports(Class<?> aClass) {return true;}@Overrideprotected Object readInternal(Class<?> aClass, HttpInputMessage httpInputMessage) throws IOException, HttpMessageNotReadableException {String json = StreamUtils.copyToString(httpInputMessage.getBody(), Charset.forName("UTF-8"));ObjectMapper objectMapper = new ObjectMapper();objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);return objectMapper.readValue(json, aClass);}@Overrideprotected void writeInternal(Object o, HttpOutputMessage httpOutputMessage) throws IOException, HttpMessageNotWritableException {}
}