（1）canRead 是否可以讀

（2）canWrite 是否可以寫

（3）read() 讀資料

（4）write() 寫資料

接下來我們介紹一下讀取資料的處理操作。

public final Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
			NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
		.......
		Object arg = resolveName(namedValueInfo.name, parameter, webRequest);
		......
	}
 

簡單的提交資料的示例：

 @RequestMapping(value="/save")
    public String saveProduct( String name,@RequestBody String description,@RequestBody float price, Model model)
   .......
}

我們可以在HandlerMethodArgumentResolver的子類RequestResponseBodyMethodProcessor的方法resolveArgument看到如下處理，這裡就是對提交的引數的處理

public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
			NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {

		Object arg = readWithMessageConverters(webRequest, parameter, parameter.getGenericParameterType());
		.........
	}
 

接下來在readWithMessageConverters中的處理如下：

@Override
	protected <T> Object readWithMessageConverters(NativeWebRequest webRequest, MethodParameter methodParam,
			Type paramType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
		HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
		ServletServerHttpRequest inputMessage = new ServletServerHttpRequest(servletRequest);
		Object arg = readWithMessageConverters(inputMessage, methodParam, paramType);
		if (arg == null) {
			if (methodParam.getParameterAnnotation(RequestBody.class).required()) {
				throw new HttpMessageNotReadableException("Required request body is missing: " +
						methodParam.getMethod().toGenericString());
			}
		}
		return arg;
	}
 

在父類AbstractMessageConverterMethodArgumentResolver的方法readWithMessageConverters中的處理如下：簡單來說就是根據提供的實現的HttpMessageConverter來對提交的RequestBody中的資料進行轉換處理。

@SuppressWarnings("unchecked")
	protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter param,
			Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
		//獲取資料型別
		MediaType contentType;
		boolean noContentType = false;
		try {
			contentType = inputMessage.getHeaders().getContentType();
		}
		catch (InvalidMediaTypeException ex) {
			throw new HttpMediaTypeNotSupportedException(ex.getMessage());
		}
		if (contentType == null) {
			noContentType = true;
			contentType = MediaType.APPLICATION_OCTET_STREAM;
		}

		Class<?> contextClass = (param != null ? param.getContainingClass() : null);
		Class<T> targetClass = (targetType instanceof Class<?> ? (Class<T>) targetType : null);
		if (targetClass == null) {
			ResolvableType resolvableType = (param != null ?
					ResolvableType.forMethodParameter(param) : ResolvableType.forType(targetType));
			targetClass = (Class<T>) resolvableType.resolve();
		}

		HttpMethod httpMethod = ((HttpRequest) inputMessage).getMethod();
		Object body = NO_VALUE;

		try {
			inputMessage = new EmptyBodyCheckingHttpInputMessage(inputMessage);
			//選擇適合的訊息處理器來處理引數
			for (HttpMessageConverter<?> converter : this.messageConverters) {
				Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
				//判斷是否適合處理引數
				if (converter instanceof GenericHttpMessageConverter) {
					GenericHttpMessageConverter<?> genericConverter = (GenericHttpMessageConverter<?>) converter;
					if (genericConverter.canRead(targetType, contextClass, contentType)) {
						if (logger.isDebugEnabled()) {
							logger.debug("Read [" + targetType + "] as \"" + contentType + "\" with [" + converter + "]");
						}
						if (inputMessage.getBody() != null) {
							inputMessage = getAdvice().beforeBodyRead(inputMessage, param, targetType, converterType);
							//處理引數
							body = genericConverter.read(targetType, contextClass, inputMessage);
							body = getAdvice().afterBodyRead(body, inputMessage, param, targetType, converterType);
						}
						else {
							body = null;
							body = getAdvice().handleEmptyBody(body, inputMessage, param, targetType, converterType);
						}
						break;
					}
				}
				else if (targetClass != null) {
					if (converter.canRead(targetClass, contentType)) {
						if (logger.isDebugEnabled()) {
							logger.debug("Read [" + targetType + "] as \"" + contentType + "\" with [" + converter + "]");
						}
						if (inputMessage.getBody() != null) {
							inputMessage = getAdvice().beforeBodyRead(inputMessage, param, targetType, converterType);
							//處理資料，對於簡單的String資料使用StringHttpMessageConverter進行處理
							body = ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage);
							body = getAdvice().afterBodyRead(body, inputMessage, param, targetType, converterType);
						}
						else {
							body = null;
							body = getAdvice().handleEmptyBody(body, inputMessage, param, targetType, converterType);
						}
						break;
					}
				}
			}
		}
		catch (IOException ex) {
			throw new HttpMessageNotReadableException("Could not read document: " + ex.getMessage(), ex);
		}

		if (body == NO_VALUE) {
			if (httpMethod == null || !SUPPORTED_METHODS.contains(httpMethod) ||
					(noContentType && inputMessage.getBody() == null)) {
				return null;
			}
			throw new HttpMediaTypeNotSupportedException(contentType, this.allSupportedMediaTypes);
		}
		//最終返回處理的結果值
		return body;
	}

springMVC預設提供了很多引數和結果值處理器，包括如下：

（1）MappingJackson2HttpMessageConverter

（2）GsonHttpMessageConverter

（3）ByteArrayHttpMessageConverter

（4）ObjectToStringHttpMessageConverter

（5）ProtobufHttpMessageConverter

（6）ResourceHttpMessageConverter

（7）StringHttpMessageConverter

（8）AllEncompassingFormHttpMessageConverter