RestTemplate请求Could not extract response: no suitable HttpMessageConverter found for response type..

迈不过友情╰ 2021-06-24 13:58 500阅读 0赞

使用 Spring Boot 写项目，需要用到微信接口获取用户信息。

在 Jessey 和 Spring RestTemplate 两个 Rest 客户端中，想到尽量不引入更多的东西，然后就选择了 Spring RestTemplate 作为 网络请求的 Client，然后就被微信接口摆了一道，然后踩了一个 RestTemplate 的坑。

二、第一个坑：被微信摆了一道  
报错信息是：

**org.springframework.web.client.RestClientException: Could not extract response: no suitable HttpMessageConverter found for response type \[class com.solar.app.model.weixin.WxBaseUserInfo\] and content type \[text/plain\]**  
之所以被微信摆了一道，是因为微信接口文档虽说返回的是 Json 数据，但是同时返回的 Header 里面的 Content-Type 值确是 text/plain 的！！

最终结果就是导致 RestTemplate 把数据从 HttpResponse 转换成 Object 的时候，找不到合适的 HttpMessageConverter 来转换！

我使用 RestTemplate 时配置 Bean 时使用默认的构造函数：

@Bean
    RestTemplate restTemplate(){
        return new RestTemplate();
    }

继续看 RestTemplate() 默认构造函数都干了啥：

/**
     * Create a new instance of the {@link RestTemplate} using default settings.
     * Default {@link HttpMessageConverter}s are initialized.
     */
    public RestTemplate() {
        this.messageConverters.add(new ByteArrayHttpMessageConverter());
        this.messageConverters.add(new StringHttpMessageConverter());
        this.messageConverters.add(new ResourceHttpMessageConverter());
        this.messageConverters.add(new SourceHttpMessageConverter<Source>());
        this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());
    
        if (romePresent) {
            this.messageConverters.add(new AtomFeedHttpMessageConverter());
            this.messageConverters.add(new RssChannelHttpMessageConverter());
        }
    
        if (jackson2XmlPresent) {
            this.messageConverters.add(new MappingJackson2XmlHttpMessageConverter());
        }
        else if (jaxb2Present) {
            this.messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
        }
    
        if (jackson2Present) {
            this.messageConverters.add(new MappingJackson2HttpMessageConverter());// tag1
        }
        else if (gsonPresent) {
            this.messageConverters.add(new GsonHttpMessageConverter());
        }
    }

可以看到，RestTemplate() 默认构造函数设置了一系列 HttpMessageConverter。

我的项目里引入了 com.fasterxml.jackson，所以 RestTemplate() 会构造一个 MappingJackson2HttpMessageConverter 加到它的 messageConverters 中，即上面的代码：【tag1】

继续看 MappingJackson2HttpMessageConverter() 默认构造函数：

/**
     * Construct a new {@link MappingJackson2HttpMessageConverter} using default configuration
     * provided by {@link Jackson2ObjectMapperBuilder}.
     */
    public MappingJackson2HttpMessageConverter() {
        this(Jackson2ObjectMapperBuilder.json().build());
    }
    
    /**
     * Construct a new {@link MappingJackson2HttpMessageConverter} with a custom {@link ObjectMapper}.
     * You can use {@link Jackson2ObjectMapperBuilder} to build it easily.
     * @see Jackson2ObjectMapperBuilder#json()
     */
    public MappingJackson2HttpMessageConverter(ObjectMapper objectMapper) {
        super(objectMapper, MediaType.APPLICATION_JSON, new MediaType("application", "*+json"));
    }

可以看到，默认构造的 MappingJackson2HttpMessageConverter 中的 supportedMediaTypes 只支持：application/json 的 MediaType。

再看 RestTemplate 请求的流程，会执行到这里：

/**
     * Execute the given method on the provided URI.
     * <p>The {@link ClientHttpRequest} is processed using the {@link RequestCallback};
     * the response with the {@link ResponseExtractor}.
     * @param url the fully-expanded URL to connect to
     * @param method the HTTP method to execute (GET, POST, etc.)
     * @param requestCallback object that prepares the request (can be {@code null})
     * @param responseExtractor object that extracts the return value from the response (can be {@code null})
     * @return an arbitrary object, as returned by the {@link ResponseExtractor}
     */
    protected <T> T doExecute(URI url, HttpMethod method, RequestCallback requestCallback,
            ResponseExtractor<T> responseExtractor) throws RestClientException {
    
        Assert.notNull(url, "'url' must not be null");
        Assert.notNull(method, "'method' must not be null");
        ClientHttpResponse response = null;
        try {
            ClientHttpRequest request = createRequest(url, method);
            if (requestCallback != null) {
                requestCallback.doWithRequest(request);
            }
            response = request.execute();
            handleResponse(url, method, response);
            if (responseExtractor != null) {
                return responseExtractor.extractData(response);// tag2
            }
            else {
                return null;
            }
        }
        catch (IOException ex) {
            String resource = url.toString();
            String query = url.getRawQuery();
            resource = (query != null ? resource.substring(0, resource.indexOf(query) - 1) : resource);
            throw new ResourceAccessException("I/O error on " + method.name() +
                    " request for \"" + resource + "\": " + ex.getMessage(), ex);
        }
        finally {
            if (response != null) {
                response.close();
            }
        }
    }

从 HttpResponse 中获取数据实际是执行 【tag2】。这个操作由 HttpMessageConverterExtractor 类来完成：

@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);// tag3, 微信返回的是 text/plain
    
        for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
            if (messageConverter instanceof GenericHttpMessageConverter) {
                GenericHttpMessageConverter<?> genericMessageConverter = (GenericHttpMessageConverter<?>) messageConverter;
                if (genericMessageConverter.canRead(this.responseType, null, contentType)) {// tag4
                    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 + "]");
    }

【tag4】处的代码用于判断 MappingJackson2HttpMessageConverter 是否支持 【tag3】 类型的 MediaType。

AbstractJackson2HttpMessageConverter：

@Override
    public boolean canRead(Type type, Class<?> contextClass, MediaType mediaType) {
        if (!canRead(mediaType)) {// tag5
            return false;
        }
        JavaType javaType = getJavaType(type, contextClass);
        if (!logger.isWarnEnabled()) {
            return this.objectMapper.canDeserialize(javaType);
        }
        AtomicReference<Throwable> causeRef = new AtomicReference<Throwable>();
        if (this.objectMapper.canDeserialize(javaType, causeRef)) {
            return true;
        }
        logWarningIfNecessary(javaType, causeRef.get());
        return false;
    }

AbstractHttpMessageConverter：

/**
     * Returns {@code true} if any of the {@linkplain #setSupportedMediaTypes(List)
     * supported} media types {@link MediaType#includes(MediaType) include} the
     * given media type.
     * @param mediaType the media type to read, can be {@code null} if not specified.
     * Typically the value of a {@code Content-Type} header.
     * @return {@code true} if the supported media types include the media type,
     * or if the media type is {@code null}
     */
    protected boolean canRead(MediaType mediaType) {
        if (mediaType == null) {
            return true;
        }
        for (MediaType supportedMediaType : getSupportedMediaTypes()) {
            if (supportedMediaType.includes(mediaType)) {
                return true;
            }
        }
        return false;
    }

一路追踪下来，可以确定，只要让 MappingJackson2HttpMessageConverter 能处理头部 Content-Type 为 text/plain 类型的 Json 返回值的话，我们就能让其帮我们把 Json 反序列化成我们要的对象。

我们继承 MappingJackson2HttpMessageConverter 并在构造过程中设置其支持的 MediaType 类型即可：

public class WxMappingJackson2HttpMessageConverter extends MappingJackson2HttpMessageConverter {
        public WxMappingJackson2HttpMessageConverter(){
            List<MediaType> mediaTypes = new ArrayList<>();
            mediaTypes.add(MediaType.TEXT_PLAIN);
            setSupportedMediaTypes(mediaTypes);// tag6
        }
    }

【tag6】的代码，会覆盖其默认的 MediaType 设置。

然后把这个 WxMappingJackson2HttpMessageConverter 追加到 RestTemplate 的 messageConverters 消息转换链中去：

@Bean
    RestTemplate restTemplate(){
        RestTemplate restTemplate = new RestTemplate();
        restTemplate.getMessageConverters().add(new WxMappingJackson2HttpMessageConverter());
        return restTemplate;
    }

**我既不推荐把 WxMappingJackson2HttpMessageConverter 实例当作构造 RestTemplate 时的参数来构造 RestTemplate，也不推荐 使用新的 WxMappingJackson2HttpMessageConverter 替换 RestTemplate 默认构造中创建的 MappingJackson2HttpMessageConverter 实例，因为这两种方式都会导致 Content-Type 为 application/json 的 Json 响应没有转换器来反序列化，所以最佳的方式还是“追加”。**