该注解用于将Controller的方法返回的对象，通过适当的HttpMessageConverter转换为指定格式后，写入到Response对象的body数据区。

 返回的数据不是html标签的页面，而是其他某种格式的数据时（如json、xml等）使用；

HttpMessageConverter

public interface HttpMessageConverter
     
       {        /**      * 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
      
        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
        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;    }
      
     

该接口定义了四个方法，分别是读取数据时的 canRead(), read() 和 写入数据时的canWrite(), write()方法。

 

在使用 <mvc:annotation-driven />标签配置时，默认配置了RequestMappingHandlerAdapter（注意是RequestMappingHandlerAdapter不是AnnotationMethodHandlerAdapter），并为他配置了一下默认的HttpMessageConverter：

ByteArrayHttpMessageConverter converts byte arrays.    StringHttpMessageConverter converts strings.    ResourceHttpMessageConverter converts to/from org.springframework.core.io.Resource for all media types.    SourceHttpMessageConverter converts to/from a javax.xml.transform.Source.    FormHttpMessageConverter converts form data to/from a MultiValueMap
     
      .    Jaxb2RootElementHttpMessageConverter converts Java objects to/from XML — added if JAXB2 is present on the classpath.    MappingJacksonHttpMessageConverter converts to/from JSON — added if Jackson is present on the classpath.    AtomFeedHttpMessageConverter converts Atom feeds — added if Rome is present on the classpath.    RssChannelHttpMessageConverter converts RSS feeds — added if Rome is present on the classpath.
     

ByteArrayHttpMessageConverter: 负责读取二进制格式的数据和写出二进制格式的数据；

StringHttpMessageConverter：   负责读取字符串格式的数据和写出二进制格式的数据；

 

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

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）；负责写入application/x-www-from-urlencoded和multipart/form-data格式的数据；

 

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

 

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

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

 

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

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

 

当使用@RequestBody和@ResponseBody注解时，RequestMappingHandlerAdapter就使用它们来进行读取或者写入相应格式的数据。

 

HttpMessageConverter匹配过程：

@RequestBody注解时： 根据Request对象header部分的Content-Type类型，逐一匹配合适的HttpMessageConverter来读取数据；

spring 3.1源代码如下：

private Object readWithMessageConverters(MethodParameter methodParam, HttpInputMessage inputMessage, Class paramType)              throws Exception {            MediaType contentType = inputMessage.getHeaders().getContentType();          if (contentType == null) {              StringBuilder builder = new StringBuilder(ClassUtils.getShortName(methodParam.getParameterType()));              String paramName = methodParam.getParameterName();              if (paramName != null) {                  builder.append(' ');                  builder.append(paramName);              }              throw new HttpMediaTypeNotSupportedException(                      "Cannot extract parameter (" + builder.toString() + "): no Content-Type found");          }            List
     
       allSupportedMediaTypes = new ArrayList
      
       ();          if (this.messageConverters != null) {              for (HttpMessageConverter
        messageConverter : this.messageConverters) {                  allSupportedMediaTypes.addAll(messageConverter.getSupportedMediaTypes());                  if (messageConverter.canRead(paramType, contentType)) {                      if (logger.isDebugEnabled()) {                          logger.debug("Reading [" + paramType.getName() + "] as \"" + contentType                                  +"\" using [" + messageConverter + "]");                      }                      return messageConverter.read(paramType, inputMessage);                  }              }          }          throw new HttpMediaTypeNotSupportedException(contentType, allSupportedMediaTypes);      }
      
     

@ResponseBody注解时： 根据Request对象header部分的Accept属性（逗号分隔），逐一按accept中的类型，去遍历找到能处理的HttpMessageConverter；

源代码如下：

private void writeWithMessageConverters(Object returnValue,                  HttpInputMessage inputMessage, HttpOutputMessage outputMessage)                  throws IOException, HttpMediaTypeNotAcceptableException {              List
     
       acceptedMediaTypes = inputMessage.getHeaders().getAccept();              if (acceptedMediaTypes.isEmpty()) {                  acceptedMediaTypes = Collections.singletonList(MediaType.ALL);              }              MediaType.sortByQualityValue(acceptedMediaTypes);              Class
       returnValueType = returnValue.getClass();              List
      
        allSupportedMediaTypes = new ArrayList
       
        ();              if (getMessageConverters() != null) {                  for (MediaType acceptedMediaType : acceptedMediaTypes) {                      for (HttpMessageConverter messageConverter : getMessageConverters()) {                          if (messageConverter.canWrite(returnValueType, acceptedMediaType)) {                              messageConverter.write(returnValue, acceptedMediaType, outputMessage);                              if (logger.isDebugEnabled()) {                                  MediaType contentType = outputMessage.getHeaders().getContentType();                                  if (contentType == null) {                                      contentType = acceptedMediaType;                                  }                                  logger.debug("Written [" + returnValue + "] as \"" + contentType +                                          "\" using [" + messageConverter + "]");                              }                              this.responseArgumentUsed = true;                              return;                          }                      }                  }                  for (HttpMessageConverter messageConverter : messageConverters) {                      allSupportedMediaTypes.addAll(messageConverter.getSupportedMediaTypes());                  }              }              throw new HttpMediaTypeNotAcceptableException(allSupportedMediaTypes);          }
       
      
     

MappingJacksonHttpMessageConverter 调用了 objectMapper.writeValue(OutputStream stream, Object)方法，使用@ResponseBody注解返回的对象就传入Object参数内。若返回的对象为已经格式化好的json串时，不使用@RequestBody注解，而应该这样处理：

1、response.setContentType("application/json; charset=UTF-8");

2、response.getWriter().print(jsonStr);

直接输出到body区，然后的视图为void。