上一篇文章 springMVC源码分析--HttpMessageConverter（一）之数据转化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。
springmvc读写数据的处理过程.png
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
@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
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