springMVC源码分析--HttpMessageConverter参数read操作（二）

上一篇博客springMVC源码分析--HttpMessageConverter数据转化（一）中我们简单介绍了一下HttpMessageConverter接口提供的几个方法，主要有以下几个方法：

（1）canRead 是否可以读

（2）canWrite 是否可以写

（3）read() 读数据

（4）write() 写数据

接下来我们介绍一下读取数据的处理操作。

首先要去重温一下参数处理器需要做的操作springMVC源码分析--RequestParamMethodArgumentResolver参数解析器（三），AbstractNamedValueMethodArgumentResolver中的resolveArgument方法，解析参数值

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

简单的提交数据的示例：

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

我们可以在HandlerMethodArgumentResolver的子类RequestResponseBodyMethodProcessor的方法resolveArgument看到如下处理，这里就是对提交的参数的处理

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

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

接下来在readWithMessageConverters中的处理如下：

@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;
	}

在父类AbstractMessageConverterMethodArgumentResolver的方法readWithMessageConverters中的处理如下：简单来说就是根据提供的实现的HttpMessageConverter来对提交的RequestBody中的数据进行转换处理。

@SuppressWarnings("unchecked")
	protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter param,
			Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
		//获取数据类型
		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);
			//选择适合的消息处理器来处理参数
			for (HttpMessageConverter<?> converter : this.messageConverters) {
				Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
				//判断是否适合处理参数
				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);
							//处理参数
							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);
							//处理数据，对于简单的String数据使用StringHttpMessageConverter进行处理
							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);
		}
		//最终返回处理的结果值
		return body;
	}

springMVC默认提供了很多参数和结果值处理器，包括如下：

（1）MappingJackson2HttpMessageConverter

（2）GsonHttpMessageConverter

（3）ByteArrayHttpMessageConverter

（4）ObjectToStringHttpMessageConverter

（5）ProtobufHttpMessageConverter

（6）ResourceHttpMessageConverter

（7）StringHttpMessageConverter

（8）AllEncompassingFormHttpMessageConverter