在很多时候，Spring的注解为我们提供了很多方便，但只知道其用法，不懂其执行原理，有时候出错了，很难快速的定位出错原因，今天我想把自己对于@Requestbody这个注解的一点想法和大家分享下。

   首先Spring处理一个请求时,请求的入口就是大家在配置文件中配置的 DispathcherServlet 这分发类，其实这个类能够接受到request的原理就是它实现了Servlet的doGet,doPost等方法，在没有正式达到Controller代码时，在它处理逻辑时，会获取Controller的反射实例，通过反射实例获取它的注解参数，执行完注解方法后，才会返回到Controller中，所以配置了@Requeat,@Valid 等注解时，返回到Controller中的都是已经经过数据绑定和校验后的对象，当Controller配置@Requestbody这个注解时，Spring会调用  AbstractMessageConverterMethodArgumentResolver 这个父类的 readWithMessageConverters 方法 通过 HttpMessageConverter类来进行解析，然后把数据要返回的对象上，再把绑定后的对象返回到Controller.

protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage,
      MethodParameter methodParam, Type targetType) throws IOException, HttpMediaTypeNotSupportedException {

   MediaType contentType;
   try {
      contentType = inputMessage.getHeaders().getContentType();
   }
   catch (InvalidMediaTypeException ex) {
      throw new HttpMediaTypeNotSupportedException(ex.getMessage());
   }
   if (contentType == null) {
      contentType = MediaType.APPLICATION_OCTET_STREAM;
   }

   Class<?> contextClass = methodParam.getContainingClass();
   Class<T> targetClass = (Class<T>)
         ResolvableType.forMethodParameter(methodParam, targetType).resolve(Object.class);

   for (HttpMessageConverter<?> converter : this.messageConverters) {
      if (converter instanceof GenericHttpMessageConverter) {
         GenericHttpMessageConverter<?> genericConverter = (GenericHttpMessageConverter<?>) converter;
         if (genericConverter.canRead(targetType, contextClass, contentType)) {
            if (logger.isDebugEnabled()) {
               logger.debug("Reading [" + targetType + "] as \"" +
                     contentType + "\" using [" + converter + "]");
            }
            return genericConverter.read(targetType, contextClass, inputMessage);
         }
      }
      if (converter.canRead(targetClass, contentType)) {
         if (logger.isDebugEnabled()) {
            logger.debug("Reading [" + targetClass.getName() + "] as \"" +
                  contentType + "\" using [" + converter + "]");
         }
         return ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage);
      }
   }

   throw new HttpMediaTypeNotSupportedException(contentType, this.allSupportedMediaTypes);
}

其实重要的是俩个参数，第一个是 contentType ，这个参数是从request的header中取出来的，比如你的请求header是 json ,那么这个参数的类型就是 application/json ,

第二个重要的参数就是 HttpMessageConverter 这个接口，这个接口的核心作用是，通过 contentType 判断是否request的值是否可读可写，Spring默认提供了7种messageConverters分别实现HttpMessageConverter 这个接口，我们来看下HttpMessageConverter 提供的几个接口，

public interface HttpMessageConverter<T> {

   /**
    * Indicates whether the given class can be read by this converter.
    * @param clazz the class to test for readability
    * @param mediaType the media type to read, can be {@code null} if not specified.
    * Typically the value of a {@code Content-Type} header.
    * @return {@code true} if readable; {@code false} otherwise
    */
   boolean canRead(Class<?> clazz, MediaType mediaType);

   /**
    * Indicates whether the given class can be written by this converter.
    * @param clazz the class to test for writability
    * @param mediaType the media type to write, can be {@code null} if not specified.
    * Typically the value of an {@code Accept} header.
    * @return {@code true} if writable; {@code false} otherwise
    */
   boolean canWrite(Class<?> clazz, MediaType mediaType);

   /**
    * Return the list of {@link MediaType} objects supported by this converter.
    * @return the list of supported media types
    */
   List<MediaType> getSupportedMediaTypes();

   /**
    * Read an object of the given type form the given input message, and returns it.
    * @param clazz the type of object to return. This type must have previously been passed to the
    * {@link #canRead canRead} method of this interface, which must have returned {@code true}.
    * @param inputMessage the HTTP input message to read from
    * @return the converted object
    * @throws IOException in case of I/O errors
    * @throws HttpMessageNotReadableException in case of conversion errors
    */
   T read(Class<? extends T> clazz, HttpInputMessage inputMessage)
         throws IOException, HttpMessageNotReadableException;

   /**
    * Write an given object to the given output message.
    * @param t the object to write to the output message. The type of this object must have previously been
    * passed to the {@link #canWrite canWrite} method of this interface, which must have returned {@code true}.
    * @param contentType the content type to use when writing. May be {@code null} to indicate that the
    * default content type of the converter must be used. If not {@code null}, this media type must have
    * previously been passed to the {@link #canWrite canWrite} method of this interface, which must have
    * returned {@code true}.
    * @param outputMessage the message to write to
    * @throws IOException in case of I/O errors
    * @throws HttpMessageNotWritableException in case of conversion errors
    */
   void write(T t, MediaType contentType, HttpOutputMessage outputMessage)
         throws IOException, HttpMessageNotWritableException;

}

其实就是read和write的判断，分别实现这个接口的 七个类是：

1.ResourceHttpMessageConverter：负责读取资源文件和写出资源文件数据； 

2.FormHttpMessageConverter：       负责读取form提交的数据（能读取的数据格式为 application/x-www-form-urlencoded，不能读取multipart/form-data格式数据）

3.MappingJacksonHttpMessageConverter:  负责读取和写入json格式的数据；

4.SouceHttpMessageConverter：                   负责读取和写入 xml 中javax.xml.transform.Source定义的数据；

5.Jaxb2RootElementHttpMessageConverter:  负责读取和写入xml 标签格式的数据；

6.AtomFeedHttpMessageConverter:              负责读取和写入Atom格式的数据；

7.RssChannelHttpMessageConverter:           负责读取和写入RSS格式的数据；

如果请求的contentType是json的话，那么通过循环判断可读会定位到 MappingJacksonHttpMessageConverter，其实Spring默认解析json用的是 jackson.然后会调用jackson的ObjectMapper去解析json,然后写入到要绑定的对象上。

private Object readJavaType(JavaType javaType, HttpInputMessage inputMessage) {
   try {
      return this.objectMapper.readValue(inputMessage.getBody(), javaType);
   }
   catch (IOException ex) {
      throw new HttpMessageNotReadableException("Could not read document: " + ex.getMessage(), ex);
   }
}