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

谁践踏了优雅 2022-07-12 07:47 332阅读 0赞

上一篇博客[ springMVC源码分析--HttpMessageConverter数据转化（一）][springMVC_--HttpMessageConverter]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。

![Image 1][]

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）][springMVC_--RequestParamMethodArgumentResolver]，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

[springMVC_--HttpMessageConverter]: http://blog.csdn.net/qq924862077/article/details/55222947
[Image 1]: 
[springMVC_--RequestParamMethodArgumentResolver]: http://blog.csdn.net/qq924862077/article/details/54292082