spring下HttpMessageConverter核心代码详解

使用spring的小伙伴肯定对下面的配置特别了解：

    <mvc:annotation-driven>
        <mvc:message-converters register-defaults="false">
            <bean class="org.springframework.http.converter.ByteArrayHttpMessageConverter" />
            <bean class="org.springframework.http.converter.StringHttpMessageConverter">
                <property name="supportedMediaTypes" value="text/plain;charset=UTF-8" />
            </bean>
            <bean class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter">
                <property name="supportedMediaTypes">
                    <list>
                        <value>text/plain;charset=UTF-8</value>
                        <value>application/json;charset=UTF-8</value>
                    </list>
                </property>
            </bean>
        </mvc:message-converters>
    </mvc:annotation-driven>

再看下下面的controller代码：

    @GetMapping(value = "/get-person", produces = MediaType.APPLICATION_JSON_VALUE)
    public @ResponseBody Person getPerson() throws IOException {
    	Person per = new Person("Bill","Gates");
    	return per;
    }

这里面就离不开AbstractMessageConverterMethodProcessor这个抽象类，下面我们来看下该类相关代码：

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

		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);
		}

		HttpServletRequest request = inputMessage.getServletRequest();

		//获取前端传递的多媒体类型
		List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);

		//获取该方法支持返回的的多媒体类型，对于getPerson方法来说只有application/json一种类型
		List<MediaType> producibleMediaTypes = getProducibleMediaTypes(request, valueType, declaredType);
		
		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;
		}

		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) {

		//去除权衡因子：比如对application/json;q=0.8来说，调用removeQualityValue后返回application/json
			selectedMediaType = selectedMediaType.removeQualityValue();

			/*接下来的代码的作用就是从mvc:message-converters配置的消息转换器中选择一个可以处理返回值为

			Person类且支持返回类型为application/json的转换器，如果找不到则抛出

			‘HttpMediaTypeNotAcceptableException’异常，找到则调用相应转换器的write方法将Person对象

			按照json格式处理后写入返回消息的body中*/
			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);
		}
	}

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