SpringMVC-HttpMessageConverter

HttpMessageConverter 介绍

org.springframework.http.converter.HttpMessageConverter 是一个策略接口，接口说明如下：

Strategy interface that specifies a converter that can convert from and to HTTP requests and responses. 简单说就是 HTTP request (请求)和response (响应)的转换器。该接口有只有5个方法，简单来说就是获取支持的 MediaType（application/json之类），接收到请求时判断是否能读（canRead），能读则读（read）；返回结果时判断是否能写（canWrite），能写则写（write）。这几个方法先有个印象即可：

public interface HttpMessageConverter<T> {
   boolean canRead(Class<?> clazz, @Nullable MediaType mediaType);
   boolean canWrite(Class<?> clazz, @Nullable MediaType mediaType);
   List<MediaType> getSupportedMediaTypes();
   
   T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
         throws IOException, HttpMessageNotReadableException;
   
   void write(T t, @Nullable MediaType contentType, HttpOutputMessage outputMessage)
         throws IOException, HttpMessageNotWritableException;
}

缺省配置

SpringMVC 启动时会自动配置一些HttpMessageConverter，在 WebMvcConfigurationSupport 类中添加了缺省 MessageConverter：

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

protected final void addDefaultHttpMessageConverters(List<HttpMessageConverter<?>> messageConverters) {
   messageConverters.add(new ByteArrayHttpMessageConverter());
   messageConverters.add(new StringHttpMessageConverter());
   messageConverters.add(new ResourceHttpMessageConverter());
   messageConverters.add(new ResourceRegionHttpMessageConverter());
   try {
      messageConverters.add(new SourceHttpMessageConverter<>());
   }
   catch (Throwable ex) {
      // Ignore when no TransformerFactory implementation is available...
   }
   messageConverters.add(new AllEncompassingFormHttpMessageConverter());

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

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

   if (jackson2Present) {
      Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.json();
      if (this.applicationContext != null) {
         builder.applicationContext(this.applicationContext);
      }
      messageConverters.add(new MappingJackson2HttpMessageConverter(builder.build()));
   }
   else if (gsonPresent) {
      messageConverters.add(new GsonHttpMessageConverter());
   }
   else if (jsonbPresent) {
      messageConverters.add(new JsonbHttpMessageConverter());
   }

   if (jackson2SmilePresent) {
      Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.smile();
      if (this.applicationContext != null) {
         builder.applicationContext(this.applicationContext);
      }
      messageConverters.add(new MappingJackson2SmileHttpMessageConverter(builder.build()));
   }
   if (jackson2CborPresent) {
      Jackson2ObjectMapperBuilder builder = Jackson2ObjectMapperBuilder.cbor();
      if (this.applicationContext != null) {
         builder.applicationContext(this.applicationContext);
      }
      messageConverters.add(new MappingJackson2CborHttpMessageConverter(builder.build()));
   }
}

如果我们引入 jackson 相关包，Spring 就会为我们添加该 MappingJackson2HttpMessageConverter。

数据流转解析

数据的请求和响应都要经过 DispatcherServlet 类的 doDispatch(HttpServletRequest request, HttpServletResponse response) 方法的处理，下面以fastjson的MessageConverter为例显示一下调过程。

请求过程解析

HttpMessageConverter调用链.png

org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod的invokeAndHandle方法是做为目标Controller的调用方法的入参解析和出参处理的分界线

public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
      Object... providedArgs) throws Exception {
   // 入参处理及目标方法的调用
   Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
   setResponseStatus(webRequest);

   if (returnValue == null) {
      if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
         disableContentCachingIfNecessary(webRequest);
         mavContainer.setRequestHandled(true);
         return;
      }
   }
   else if (StringUtils.hasText(getResponseStatusReason())) {
      mavContainer.setRequestHandled(true);
      return;
   }

   mavContainer.setRequestHandled(false);
   Assert.state(this.returnValueHandlers != null, "No return value handlers");
   try {
      // 返回值处理
      this.returnValueHandlers.handleReturnValue(
            returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
   }
   catch (Exception ex) {
      if (logger.isTraceEnabled()) {
         logger.trace(formatErrorForReturnValue(returnValue), ex);
      }
      throw ex;
   }
}

返回过程解析

org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor#writeWithMessageConverters

protected <T> void writeWithMessageConverters(@Nullable T value, MethodParameter returnType,
      ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
      throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {

   Object body;
   Class<?> valueType;
   Type targetType;

   if (value instanceof CharSequence) {
      body = value.toString();
      valueType = String.class;
      targetType = String.class;
   }
   else {
      body = value;
      valueType = getReturnValueType(body, returnType);
      targetType = GenericTypeResolver.resolveType(getGenericType(returnType), returnType.getContainingClass());
   }

   if (isResourceType(value, returnType)) {
      outputMessage.getHeaders().set(HttpHeaders.ACCEPT_RANGES, "bytes");
      if (value != null && inputMessage.getHeaders().getFirst(HttpHeaders.RANGE) != null &&
            outputMessage.getServletResponse().getStatus() == 200) {
         Resource resource = (Resource) value;
         try {
            List<HttpRange> httpRanges = inputMessage.getHeaders().getRange();
            outputMessage.getServletResponse().setStatus(HttpStatus.PARTIAL_CONTENT.value());
            body = HttpRange.toResourceRegions(httpRanges, resource);
            valueType = body.getClass();
            targetType = RESOURCE_REGION_LIST_TYPE;
         }
         catch (IllegalArgumentException ex) {
            outputMessage.getHeaders().set(HttpHeaders.CONTENT_RANGE, "bytes */" + resource.contentLength());
            outputMessage.getServletResponse().setStatus(HttpStatus.REQUESTED_RANGE_NOT_SATISFIABLE.value());
         }
      }
   }

   MediaType selectedMediaType = null;
   MediaType contentType = outputMessage.getHeaders().getContentType();
   boolean isContentTypePreset = contentType != null && contentType.isConcrete();
   if (isContentTypePreset) {
      if (logger.isDebugEnabled()) {
         logger.debug("Found 'Content-Type:" + contentType + "' in response");
      }
      selectedMediaType = contentType;
   }
   else {
      HttpServletRequest request = inputMessage.getServletRequest();
      List<MediaType> acceptableTypes = getAcceptableMediaTypes(request);
      List<MediaType> producibleTypes = getProducibleMediaTypes(request, valueType, targetType);

      if (body != null && producibleTypes.isEmpty()) {
         throw new HttpMessageNotWritableException(
               "No converter found for return value of type: " + valueType);
      }
      List<MediaType> mediaTypesToUse = new ArrayList<>();
      for (MediaType requestedType : acceptableTypes) {
         for (MediaType producibleType : producibleTypes) {
            if (requestedType.isCompatibleWith(producibleType)) {
               mediaTypesToUse.add(getMostSpecificMediaType(requestedType, producibleType));
            }
         }
      }
      if (mediaTypesToUse.isEmpty()) {
         if (body != null) {
            throw new HttpMediaTypeNotAcceptableException(producibleTypes);
         }
         if (logger.isDebugEnabled()) {
            logger.debug("No match for " + acceptableTypes + ", supported: " + producibleTypes);
         }
         return;
      }

      MediaType.sortBySpecificityAndQuality(mediaTypesToUse);

      for (MediaType mediaType : mediaTypesToUse) {
         if (mediaType.isConcrete()) {
            selectedMediaType = mediaType;
            break;
         }
         else if (mediaType.isPresentIn(ALL_APPLICATION_MEDIA_TYPES)) {
            selectedMediaType = MediaType.APPLICATION_OCTET_STREAM;
            break;
         }
      }

      if (logger.isDebugEnabled()) {
         logger.debug("Using '" + selectedMediaType + "', given " +
               acceptableTypes + " and supported " + producibleTypes);
      }
   }

   if (selectedMediaType != null) {
      selectedMediaType = selectedMediaType.removeQualityValue();
      for (HttpMessageConverter<?> converter : this.messageConverters) {
         GenericHttpMessageConverter genericConverter = (converter instanceof GenericHttpMessageConverter ?
               (GenericHttpMessageConverter<?>) converter : null);
         if (genericConverter != null ?
               ((GenericHttpMessageConverter) converter).canWrite(targetType, valueType, selectedMediaType) :
               converter.canWrite(valueType, selectedMediaType)) {
            body = getAdvice().beforeBodyWrite(body, returnType, selectedMediaType,
                  (Class<? extends HttpMessageConverter<?>>) converter.getClass(),
                  inputMessage, outputMessage);
            if (body != null) {
               Object theBody = body;
               LogFormatUtils.traceDebug(logger, traceOn ->
                     "Writing [" + LogFormatUtils.formatValue(theBody, !traceOn) + "]");
               addContentDispositionHeader(inputMessage, outputMessage);
               if (genericConverter != null) {
                  genericConverter.write(body, targetType, selectedMediaType, outputMessage);
               }
               else {
                  ((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);
               }
            }
            else {
               if (logger.isDebugEnabled()) {
                  logger.debug("Nothing to write: null body");
               }
            }
            return;
         }
      }
   }

   if (body != null) {
      if (isContentTypePreset) {
         throw new HttpMessageNotWritableException(
               "No converter for [" + valueType + "] with preset Content-Type '" + contentType + "'");
      }
      throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
   }
}

总结

HttpMessageConverter流程示意图

参考于：[https://www.jianshu.com/p/3e1de3d02dd8]
作者：日拱一兵