你真的会用HttpMessageConverter吗?

用户2032165

发布于 2018-06-13 10:36:20

2.8K0

发布于 2018-06-13 10:36:20

文章被收录于专栏：cmazxiaoma的架构师之路cmazxiaoma的架构师之路

前言

最近看了Spring MVC源码，感觉特别有趣，像发现了新大陆一般，不能自拔。

图片来自互联网.png

图片来自互联网

阐述问题

最近发现一个关于FastJsonHttpMessageConverter特别有趣的一个点，它默认的supportMediaType竟然是MediaType.ALL。

    /**
     * Can serialize/deserialize all types.
     */
    public FastJsonHttpMessageConverter() {

        super(MediaType.ALL);
    }

说明FastJsonHttpMessageConverter不配置supportMediaType，那么默认是MediaType.ALL。

FastJsonHttpMessageConverter应该显式配置其支持的消息格式是MediaType.APPLICATION_JSON_UTF8

    //使用阿里 FastJson 作为JSON MessageConverter
    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        FastJsonHttpMessageConverter converter = new FastJsonHttpMessageConverter();

        log.info("converters:" + converters.toString());
        List<MediaType> supportMediaTypeList = new ArrayList<>();
//        supportMediaTypeList.add(MediaType.TEXT_HTML);
          supportMediaTypeList.add(MediaType.APPLICATION_JSON_UTF8);
//        supportMediaTypeList.add(MediaType.IMAGE_GIF);
//        supportMediaTypeList.add(MediaType.IMAGE_JPEG);
//        supportMediaTypeList.add(MediaType.IMAGE_PNG);

        FastJsonConfig config = new FastJsonConfig();
//        config.setDateFormat("yyyy-MM-dd HH:mm:ss");
        config.setSerializerFeatures(
                SerializerFeature.WriteMapNullValue,//保留空的字段
                SerializerFeature.WriteNullStringAsEmpty,//String null -> ""
                SerializerFeature.WriteNullNumberAsZero,//Number null -> 0
                SerializerFeature.WriteNullListAsEmpty,//List null-> []
                SerializerFeature.WriteNullBooleanAsFalse);//Boolean null -> false
        converter.setFastJsonConfig(config);
        converter.setSupportedMediaTypes(supportMediaTypeList);
        converter.setDefaultCharset(Charset.forName("UTF-8"));
        converters.add(converter);
    }

我有一个业务场景，需要缓存商品详情页，手动渲染Thymeleaft模板，在Controller中返回html片段。配置如下:

    @ResponseBody
    public String list(MiaoshaUser miaoshaUser) throws IOException {
        modelMap.addAttribute("user", miaoshaUser);
        //取缓存
        String htmlCached = redisService.get(GoodsKey.getGoodsList, "", String.class);
        if (!StringUtils.isEmpty(htmlCached)) {
            return htmlCached;
        }
        List<GoodsVo> goodsVoList = goodsService.listGoodsVo();
        modelMap.addAttribute("goodsList", goodsVoList);
        SpringWebContext springWebContext = new SpringWebContext(request, response, request.getServletContext(),
                request.getLocale(), modelMap, applicationContext);
        String html = thymeleafViewResolver.getTemplateEngine().process("goods_list", springWebContext);

        if (!StringUtils.isEmpty(html)) {
            redisService.set(GoodsKey.getGoodsList, "", html);
        }
        return html;
    }

可是最终渲染的界面惨不忍睹，界面出现了大量的\n\t这样的字符

惨不忍睹.png

如果我不用FastJsonHttpMessageConverter呢，界面却显示很正常，这是为什么呢。

正常.png

虽然在Controller中可以通过手动打开输出流，设置ContentType，把Thymeleaf模板输出到Response的body，可以解决问题。

        response.setCharacterEncoding("UTF-8");
        response.setHeader("Content-type", "application/json;charset=UTF-8");
        response.setStatus(200);

        try {
            response.getWriter().write(html);
        } catch (IOException ex) {
            log.error(ex.getMessage());
        }

但是本着阿Q精神，我Debug消息转换器的流程，发现问题出在FastJsonHttpMessageConverter没有配置MediaType.APPLICATION_JSON_UTF8, 导致其默认的MediaType.ALL影响Thymeleaf模板最后的输出。

Debug之路

RequestResponseBodyProcessor实现了HandlerMethodReturnValueHandler、HandlerMethodAgumentResolver接口，实现了boolean supportsParamter()、void resolveArgument()、boolean supportsReturnType()、void handleReturnValue() 所以具备了处理参数、处理返回值的能力。官方解释.RequestResponseBodyProcessor能够解析用@RequestBody注解的参数和通过使用HttpMessageConverter读取并写入请求体或响应来处理用@ResponseBody注解的方法的返回值。

/**
 * 处理返回值
 */
public interface HandlerMethodReturnValueHandler {

    /**
     * 方法返回值是否被处理程序支持
     */
    boolean supportsReturnType(MethodParameter returnType);

    /**
     * 通过向模型添加属性并设置视图或设置视图来处理给定的返回值
     * {@link ModelAndViewContainer#setRequestHandled}标志到{@code true}，表明响应已被直接处理。
     */
    void handleReturnValue(Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception;
}

/**
 * 解析请求中的参数值
 */
public interface HandlerMethodArgumentResolver {

    /**
     * 是否能支持解析{@linkplain MethodParameter参数}
     */
    boolean supportsParameter(MethodParameter parameter);

    /**
     * 将方法参数解析为来自给定请求的参数值。
     */
    Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
            NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception;

}

public class RequestResponseBodyMethodProcessor extends AbstractMessageConverterMethodProcessor {}

public abstract class AbstractMessageConverterMethodProcessor extends AbstractMessageConverterMethodArgumentResolver
        implements HandlerMethodReturnValueHandler {}

public abstract class AbstractMessageConverterMethodArgumentResolver implements HandlerMethodArgumentResolver {}

handleReturnValue()获取inputMessage,outMessage对象，调用writeWithMessageConverters()方法，让消息转换器对消息进行下一步处理.

public interface HttpMessage {

    /**
     * Return the headers of this message.
     * @return a corresponding HttpHeaders object (never {@code null})
     */
    HttpHeaders getHeaders();

}

public interface HttpOutputMessage extends HttpMessage {

    /**
     * Return the body of the message as an output stream.
     * @return the output stream body (never {@code null})
     * @throws IOException in case of I/O Errors
     */
    OutputStream getBody() throws IOException;

}

public interface HttpInputMessage extends HttpMessage {

    /**
     * Return the body of the message as an input stream.
     * @return the input stream body (never {@code null})
     * @throws IOException in case of I/O Errors
     */
    InputStream getBody() throws IOException;

}

    @Override
    public void handleReturnValue(Object returnValue, MethodParameter returnType,
            ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
            throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

        mavContainer.setRequestHandled(true);
        ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
        ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);

        // Try even with null return value. ResponseBodyAdvice could get involved.
        writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
    }

在AbstarctMessageConverterProcessor的writeWithMessageConverters()中，根据HandlerMethod中的ReturnValueMethodParameter对象获valueType(返回值类型),同时也能得到outputValue(返回值)、decalredType(返回值实际类型)

        Object outputValue;
        Class<?> valueType;
        Type declaredType;

        if (value instanceof CharSequence) {
            outputValue = value.toString();
            valueType = String.class;
            declaredType = String.class;
        }
        else {
            outputValue = value;
            valueType = getReturnValueType(outputValue, returnType);
            declaredType = getGenericType(returnType);
        }

根据request对象中获取请求中Accept的属性值，得requestedMediaTypes

    List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);

    private List<MediaType> getAcceptableMediaTypes(HttpServletRequest request) throws HttpMediaTypeNotAcceptableException {
        List<MediaType> mediaTypes = this.contentNegotiationManager.resolveMediaTypes(new ServletWebRequest(request));
        return (mediaTypes.isEmpty() ? Collections.singletonList(MediaType.ALL) : mediaTypes);
    }


    @Override
    public List<MediaType> resolveMediaTypes(NativeWebRequest request) throws HttpMediaTypeNotAcceptableException {
        for (ContentNegotiationStrategy strategy : this.strategies) {
            List<MediaType> mediaTypes = strategy.resolveMediaTypes(request);
            if (mediaTypes.isEmpty() || mediaTypes.equals(MEDIA_TYPE_ALL)) {
                continue;
            }
            return mediaTypes;
        }
        return Collections.emptyList();
    }

根据HandlerMapping中的produces属性获得producibleMediaTypes。如果没有显式设置produces属性，我们只能通过遍历所有的HttpMessageConverter，通过canWrite()方法找到支持解析Java对象的HttpMessageConverter，并且把其所支持的mediaType加入mediaTypes集合里面。

List<MediaType> producibleMediaTypes = getProducibleMediaTypes(request, valueType, declaredType);

    protected List<MediaType> getProducibleMediaTypes(HttpServletRequest request, Class<?> valueClass, Type declaredType) {
        Set<MediaType> mediaTypes = (Set<MediaType>) request.getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
        if (!CollectionUtils.isEmpty(mediaTypes)) {
            return new ArrayList<MediaType>(mediaTypes);
        }
        else if (!this.allSupportedMediaTypes.isEmpty()) {
            List<MediaType> result = new ArrayList<MediaType>();
            for (HttpMessageConverter<?> converter : this.messageConverters) {
                if (converter instanceof GenericHttpMessageConverter && declaredType != null) {
                    if (((GenericHttpMessageConverter<?>) converter).canWrite(declaredType, valueClass, null)) {
                        result.addAll(converter.getSupportedMediaTypes());
                    }
                }
                else if (converter.canWrite(valueClass, null)) {
                    result.addAll(converter.getSupportedMediaTypes());
                }
            }
            return result;
        }
        else {
            return Collections.singletonList(MediaType.ALL);
        }
    }

通过两次for循环，比较requestedMediaTypes和producibleMediaTypes 得到compatiableMediaTypes。如果compatiableMediaTypes为空，会抛出HttpMediaTypeNotAcceptableException异常。白话意思就是producibleMediaTypes没有一个MediaType与requestedMediaTypes匹配，肯定无法执行下一步了。

        if (outputValue != null && producibleMediaTypes.isEmpty()) {
            throw new IllegalArgumentException("No converter found for return value of type: " + valueType);
        }

        Set<MediaType> compatibleMediaTypes = new LinkedHashSet<MediaType>();
        for (MediaType requestedType : requestedMediaTypes) {
            for (MediaType producibleType : producibleMediaTypes) {
                if (requestedType.isCompatibleWith(producibleType)) {
                    compatibleMediaTypes.add(getMostSpecificMediaType(requestedType, producibleType));
                }
            }
        }
        if (compatibleMediaTypes.isEmpty()) {
            if (outputValue != null) {
                throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
            }
            return;
        }

排序、for循环compatiableMediaTypes,通过isConcrete()判断消息格式是否具体(类型和子类型是否为通配符*)，得到selectedMediaType(最终的MediaType)

        List<MediaType> mediaTypes = new ArrayList<MediaType>(compatibleMediaTypes);
        MediaType.sortBySpecificityAndQuality(mediaTypes);

        MediaType selectedMediaType = null;
        for (MediaType mediaType : mediaTypes) {
            if (mediaType.isConcrete()) {
                selectedMediaType = mediaType;
                break;
            }
            else if (mediaType.equals(MediaType.ALL) || mediaType.equals(MEDIA_TYPE_APPLICATION)) {
                selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
                break;
            }
        }

for循环已配置所有的HttpMessageConverter，调用canWrite()方法，根据valueType(返回值类型)和selectedMediaType来判断消息是否可以转换。

        if (selectedMediaType != null) {
            selectedMediaType = selectedMediaType.removeQualityValue();
            for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
                if (messageConverter instanceof GenericHttpMessageConverter) {
                    if (((GenericHttpMessageConverter) messageConverter).canWrite(
                            declaredType, valueType, selectedMediaType)) {
                        outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
                                (Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
                                inputMessage, outputMessage);
                        if (outputValue != null) {
                            addContentDispositionHeader(inputMessage, outputMessage);
                            ((GenericHttpMessageConverter) messageConverter).write(
                                    outputValue, declaredType, selectedMediaType, outputMessage);
                            if (logger.isDebugEnabled()) {
                                logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
                                        "\" using [" + messageConverter + "]");
                            }
                        }
                        return;
                    }
                }
                else if (messageConverter.canWrite(valueType, selectedMediaType)) {
                    outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
                            (Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
                            inputMessage, outputMessage);
                    if (outputValue != null) {
                        addContentDispositionHeader(inputMessage, outputMessage);
                        ((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);
                        if (logger.isDebugEnabled()) {
                            logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
                                    "\" using [" + messageConverter + "]");
                        }
                    }
                    return;
                }
            }
        }

        if (outputValue != null) {
            throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
        }

如果没有配置MediaType.APPLICATION_JSON_UTF8,默认值是MediaType.ALL，FastJsonHttpMessageConverter会去处理消息格式为"text/html;charset=UTF-8"，会执行这一段代码

                if (messageConverter instanceof GenericHttpMessageConverter) {
                    if (((GenericHttpMessageConverter) messageConverter).canWrite(
                            declaredType, valueType, selectedMediaType)) {
                        outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
                                (Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
                                inputMessage, outputMessage);
                        if (outputValue != null) {
                            addContentDispositionHeader(inputMessage, outputMessage);
                            ((GenericHttpMessageConverter) messageConverter).write(
                                    outputValue, declaredType, selectedMediaType, outputMessage);
                            if (logger.isDebugEnabled()) {
                                logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
                                        "\" using [" + messageConverter + "]");
                            }
                        }
                        return;
                    }
                }

接着调用FastJsonHttpMessageConverter的write()方法写入消息

    /*
     * @see org.springframework.http.converter.GenericHttpMessageConverter.write
     */
    public void write(Object o, Type type, MediaType contentType, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
        super.write(o, contentType, outputMessage);// support StreamingHttpOutputMessage in spring4.0+
        //writeInternal(o, outputMessage);
    }

接着调用AbstaractHttpMessageConverter中的write()写入响应头和消息内容

    /**
     * This implementation sets the default headers by calling {@link #addDefaultHeaders},
     * and then calls {@link #writeInternal}.
     */
    @Override
    public final void write(final T t, MediaType contentType, HttpOutputMessage outputMessage)
            throws IOException, HttpMessageNotWritableException {

        final HttpHeaders headers = outputMessage.getHeaders();
        addDefaultHeaders(headers, t, contentType);

        if (outputMessage instanceof StreamingHttpOutputMessage) {
            StreamingHttpOutputMessage streamingOutputMessage =
                    (StreamingHttpOutputMessage) outputMessage;
            streamingOutputMessage.setBody(new StreamingHttpOutputMessage.Body() {
                @Override
                public void writeTo(final OutputStream outputStream) throws IOException {
                    writeInternal(t, new HttpOutputMessage() {
                        @Override
                        public OutputStream getBody() throws IOException {
                            return outputStream;
                        }
                        @Override
                        public HttpHeaders getHeaders() {
                            return headers;
                        }
                    });
                }
            });
        }
        else {
            writeInternal(t, outputMessage);
            outputMessage.getBody().flush();
        }
    }

接着调用FastJsonHttpMessageConverter的writeInternal()进行消息转换

    @Override
    protected void writeInternal(Object object, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {

        ByteArrayOutputStream outnew = new ByteArrayOutputStream();
        try {
            HttpHeaders headers = outputMessage.getHeaders();

            //获取全局配置的filter
            SerializeFilter[] globalFilters = fastJsonConfig.getSerializeFilters();
            List<SerializeFilter> allFilters = new ArrayList<SerializeFilter>(Arrays.asList(globalFilters));

            boolean isJsonp = false;

            //不知道为什么会有这行代码， 但是为了保持和原来的行为一致，还是保留下来
            Object value = strangeCodeForJackson(object);

            if (value instanceof FastJsonContainer) {
                FastJsonContainer fastJsonContainer = (FastJsonContainer) value;
                PropertyPreFilters filters = fastJsonContainer.getFilters();
                allFilters.addAll(filters.getFilters());
                value = fastJsonContainer.getValue();
            }

            //revise 2017-10-23 ,
            // 保持原有的MappingFastJsonValue对象的contentType不做修改 保持旧版兼容。
            // 但是新的JSONPObject将返回标准的contentType：application/javascript ，不对是否有function进行判断
            if (value instanceof MappingFastJsonValue) {
                if(!StringUtils.isEmpty(((MappingFastJsonValue) value).getJsonpFunction())){
                    isJsonp = true;
                }
            } else if (value instanceof JSONPObject) {
                isJsonp = true;
            }


            int len = JSON.writeJSONString(outnew, //
                    fastJsonConfig.getCharset(), //
                    value, //
                    fastJsonConfig.getSerializeConfig(), //
                    //fastJsonConfig.getSerializeFilters(), //
                    allFilters.toArray(new SerializeFilter[allFilters.size()]),
                    fastJsonConfig.getDateFormat(), //
                    JSON.DEFAULT_GENERATE_FEATURE, //
                    fastJsonConfig.getSerializerFeatures());

            if (isJsonp) {
                headers.setContentType(APPLICATION_JAVASCRIPT);
            }

            if (fastJsonConfig.isWriteContentLength()) {
                headers.setContentLength(len);
            }

            outnew.writeTo(outputMessage.getBody());

        } catch (JSONException ex) {
            throw new HttpMessageNotWritableException("Could not write JSON: " + ex.getMessage(), ex);
        } finally {
            outnew.close();
        }
    }

我们可以观察到这个方法中有一个JSON.writeJSONString()方法，我点进去看一看

    public static final int writeJSONString(OutputStream os, // 
                                             Charset charset, // 
                                             Object object, // 
                                             SerializeConfig config, //
                                             SerializeFilter[] filters, //
                                             String dateFormat, //
                                             int defaultFeatures, //
                                             SerializerFeature... features) throws IOException {
        SerializeWriter writer = new SerializeWriter(null, defaultFeatures, features);

        try {
            JSONSerializer serializer = new JSONSerializer(writer, config);
            
            if (dateFormat != null && dateFormat.length() != 0) {
                serializer.setDateFormat(dateFormat);
                serializer.config(SerializerFeature.WriteDateUseDateFormat, true);
            }

            if (filters != null) {
                for (SerializeFilter filter : filters) {
                    serializer.addFilter(filter);
                }
            }
            
            serializer.write(object);
            
            int len = writer.writeToEx(os, charset);
            return len;
        } finally {
            writer.close();
        }
    }

一看吓一跳。卧槽，把html序列化了，GG。终于找到问题的始作俑者了。

如果配置了MediaType.APPLICATION_JSON_UTF8,FastJsonHttpMessageConverter 只能处理"application/json;charset=UTF-8"的消息，"text/html;charset=UTF-8"格式的消息被StringHttpMessageConverter得到了处理,会执行这一段代码

                else if (messageConverter.canWrite(valueType, selectedMediaType)) {
                    outputValue = (T) getAdvice().beforeBodyWrite(outputValue, returnType, selectedMediaType,
                            (Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
                            inputMessage, outputMessage);
                    if (outputValue != null) {
                        addContentDispositionHeader(inputMessage, outputMessage);
                        ((HttpMessageConverter) messageConverter).write(outputValue, selectedMediaType, outputMessage);
                        if (logger.isDebugEnabled()) {
                            logger.debug("Written [" + outputValue + "] as \"" + selectedMediaType +
                                    "\" using [" + messageConverter + "]");
                        }
                    }
                    return;
                }

接着调用AbstaractHttpMessageConverter中的write()写入响应头和消息内容

    @Override
    public final void write(final T t, MediaType contentType, HttpOutputMessage outputMessage)
            throws IOException, HttpMessageNotWritableException {

        final HttpHeaders headers = outputMessage.getHeaders();
        addDefaultHeaders(headers, t, contentType);

        if (outputMessage instanceof StreamingHttpOutputMessage) {
            StreamingHttpOutputMessage streamingOutputMessage =
                    (StreamingHttpOutputMessage) outputMessage;
            streamingOutputMessage.setBody(new StreamingHttpOutputMessage.Body() {
                @Override
                public void writeTo(final OutputStream outputStream) throws IOException {
                    writeInternal(t, new HttpOutputMessage() {
                        @Override
                        public OutputStream getBody() throws IOException {
                            return outputStream;
                        }
                        @Override
                        public HttpHeaders getHeaders() {
                            return headers;
                        }
                    });
                }
            });
        }
        else {
            writeInternal(t, outputMessage);
            outputMessage.getBody().flush();
        }
    }

最终调用StringHttpMessageConverter的writeInternal写入消息到outputMessage.getBody()中，输出html片段。

    @Override
    protected void writeInternal(String str, HttpOutputMessage outputMessage) throws IOException {
        if (this.writeAcceptCharset) {
            outputMessage.getHeaders().setAcceptCharset(getAcceptedCharsets());
        }
        Charset charset = getContentTypeCharset(outputMessage.getHeaders().getContentType());
        StreamUtils.copy(str, charset, outputMessage.getBody());
    }

关于HttpMessageConverter加载顺序，可以在WebMvcConfigurationSupport看到端倪。

    protected final List<HttpMessageConverter<?>> getMessageConverters() {
        if (this.messageConverters == null) {
            this.messageConverters = new ArrayList<HttpMessageConverter<?>>();
            configureMessageConverters(this.messageConverters);
            if (this.messageConverters.isEmpty()) {
                addDefaultHttpMessageConverters(this.messageConverters);
            }
            extendMessageConverters(this.messageConverters);
        }
        return this.messageConverters;
    }
    
        protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
        StringHttpMessageConverter stringConverter = new StringHttpMessageConverter();
        stringConverter.setWriteAcceptCharset(false);

        messageConverters.add(new ByteArrayHttpMessageConverter());
        messageConverters.add(stringConverter);
        messageConverters.add(new ResourceHttpMessageConverter());
        messageConverters.add(new SourceHttpMessageConverter<Source>());
        messageConverters.add(new AllEncompassingFormHttpMessageConverter());

        if (romePresent) {
            messageConverters.add(new AtomFeedHttpMessageConverter());
            messageConverters.add(new RssChannelHttpMessageConverter());
        }

        if (jackson2XmlPresent) {
            messageConverters.add(new MappingJackson2XmlHttpMessageConverter(
                    Jackson2ObjectMapperBuilder.xml().applicationContext(this.applicationContext).build()));
        }
        else if (jaxb2Present) {
            messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
        }

        if (jackson2Present) {
            messageConverters.add(new MappingJackson2HttpMessageConverter(
                    Jackson2ObjectMapperBuilder.json().applicationContext(this.applicationContext).build()));
        }
        else if (gsonPresent) {
            messageConverters.add(new GsonHttpMessageConverter());
        }
    }

我们其实是重写configureMessageConverters()方法去配置FastJsonHttpMessageConverter,所以它是第一个。