转载 https://blog.csdn.net/qq924862077/article/details/55271959

同样返回值的操作操作也是在HandlerMethodReturnValueHandler中处理的，可以参考一下老田写的springMVC源码分析--HandlerMethodReturnValueHandler返回值解析器（一）

简单的返回值处理示例使用@ResponseBody进行注解：

@ResponseBody
	@RequestMapping("/get")
	public Object get(){
		Product product = new Product();
		product.setDescription("hello  springMVC  RestFul");
		product.setId(10);
		product.setName("springMVC");
		product.setPrice(10);
		return product; //在页面中返回json数据
	}

这里返回值是一个Product对象，但真正浏览器获取的数据是json数据，处理的过程就是在HttpMessageConverter中实现的 。

配置一下消息处理使用FastJSON处理的配置

<mvc:annotation-driven>
		<mvc:message-converters register-defaults="true">
			<bean
				class="com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter">
				<property name="supportedMediaTypes">
					<array>
						<value>text/html;charset=UTF-8</value>
					</array>
				</property>
				<property name="features">
					<array>
						<value>WriteMapNullValue</value>
						<value>WriteNullStringAsEmpty</value>
						<value>DisableCircularReferenceDetect</value>
					</array>
				</property>
			</bean>
		</mvc:message-converters>
	</mvc:annotation-driven>

这样返回值的处理操作就是使用FastJsonHttpMessageConverter来进行处理，将返回值转为json数据返回。

返回值的处理是在HandlerMethodReturnValueHandlerComposite中handleReturnValue中实现的。

@Override
public void handleReturnValue(Object returnValue, MethodParameter returnType,
	ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {
	HandlerMethodReturnValueHandler handler = selectHandler(returnValue, returnType);
	Assert.notNull(handler, "Unknown return value type [" + returnType.getParameterType().getName() + "]");
	handler.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
	}

在RequestResponseBodyMethodProcessor的父类AbstractMessageConverterMethodProcessor的writeWithMessageConverters中处理操作

protected <T> void writeWithMessageConverters(T returnValue, MethodParameter returnType, NativeWebRequest webRequest)
			throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
 
		ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
		ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
		writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
	}

接下来就是通过职责链模式选择HttpMessageConverter的实现类来进行数据转换操作。

@SuppressWarnings("unchecked")
	protected <T> void writeWithMessageConverters(T returnValue, MethodParameter returnType,
			ServletServerHttpRequest inputMessage, ServletServerHttpResponse outputMessage)
			throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
 
		Class<?> returnValueClass = getReturnValueType(returnValue, returnType);
		Type returnValueType = getGenericType(returnType);
		HttpServletRequest servletRequest = inputMessage.getServletRequest();
		List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(servletRequest);
		List<MediaType> producibleMediaTypes = getProducibleMediaTypes(servletRequest, returnValueClass, returnValueType);
 
		if (returnValue != null && producibleMediaTypes.isEmpty()) {
			throw new IllegalArgumentException("No converter found for return value of type: " + returnValueClass);
		}
 
		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 (returnValue != null) {
				throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
			}
			return;
		}
 
		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;
			}
		}
 
		if (selectedMediaType != null) {
			selectedMediaType = selectedMediaType.removeQualityValue();
			//匹配消息数据转换器
			for (HttpMessageConverter<?> messageConverter : this.messageConverters) {
				if (messageConverter instanceof GenericHttpMessageConverter) {
					if (((GenericHttpMessageConverter<T>) messageConverter).canWrite(returnValueType,
							returnValueClass, selectedMediaType)) {
						returnValue = (T) getAdvice().beforeBodyWrite(returnValue, returnType, selectedMediaType,
								(Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
								inputMessage, outputMessage);
						if (returnValue != null) {
							addContentDispositionHeader(inputMessage, outputMessage);
							//进行消息转换成配置的格式
							((GenericHttpMessageConverter<T>) messageConverter).write(returnValue,
									returnValueType, selectedMediaType, outputMessage);
							if (logger.isDebugEnabled()) {
								logger.debug("Written [" + returnValue + "] as \"" +
										selectedMediaType + "\" using [" + messageConverter + "]");
							}
						}
						return;
					}
				}
				else if (messageConverter.canWrite(returnValueClass, selectedMediaType)) {
					returnValue = (T) getAdvice().beforeBodyWrite(returnValue, returnType, selectedMediaType,
							(Class<? extends HttpMessageConverter<?>>) messageConverter.getClass(),
							inputMessage, outputMessage);
					if (returnValue != null) {
						addContentDispositionHeader(inputMessage, outputMessage);
						//将数据转换为配置的数据格式
						((HttpMessageConverter<T>) messageConverter).write(returnValue,
								selectedMediaType, outputMessage);
						if (logger.isDebugEnabled()) {
							logger.debug("Written [" + returnValue + "] as \"" +
									selectedMediaType + "\" using [" + messageConverter + "]");
						}
					}
					return;
				}
			}
		}
 
		if (returnValue != null) {
			throw new HttpMediaTypeNotAcceptableException(this.allSupportedMediaTypes);
		}
	}

最终会选择FastJsonHttpMessageConverter 的write方法中进行处理操作，就是将数据转换为json写到输出流中

@Override
	protected void writeInternal(Object obj, HttpOutputMessage outputMessage)
			throws IOException, HttpMessageNotWritableException {
        HttpHeaders headers = outputMessage.getHeaders();
        ByteArrayOutputStream outnew = new ByteArrayOutputStream();
        int len = JSON.writeJSONString(outnew, //
                                       fastJsonConfig.getCharset(), //
                                       obj, //
                                       fastJsonConfig.getSerializeConfig(), //
                                       fastJsonConfig.getSerializeFilters(), //
                                       fastJsonConfig.getDateFormat(), //
                                       JSON.DEFAULT_GENERATE_FEATURE, //
                                       fastJsonConfig.getSerializerFeatures());
        headers.setContentLength(len);
        OutputStream out = outputMessage.getBody();
        outnew.writeTo(out);
        outnew.close();
    }