简介：

@RequestBody

作用： 

      i) 该注解用于读取Request请求的body部分数据，使用系统默认配置的HttpMessageConverter进行解析，然后把相应的数据绑定到要返回的对象上；

      ii) 再把HttpMessageConverter返回的对象数据绑定到 controller中方法的参数上。

使用时机：

A) GET、POST方式提时， 根据request header Content-Type的值来判断:

•     application/x-www-form-urlencoded， 可选（即非必须，因为这种情况的数据@RequestParam, @ModelAttribute也可以处理，当然@RequestBody也能处理）；
•     multipart/form-data, 不能处理（即使用@RequestBody不能处理这种格式的数据）；
•     其他格式， 必须（其他格式包括application/json, application/xml等。这些格式的数据，必须使用@RequestBody来处理）；

 

B) PUT方式提交时， 根据request header Content-Type的值来判断:

 

•     application/x-www-form-urlencoded， 必须；
•     multipart/form-data, 不能处理；
•     其他格式， 必须；

说明：request的body部分的数据编码格式由header部分的Content-Type指定；

 

 

@ResponseBody

 

作用： 

      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。

使用时机：

      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

/**   * Strategy interface that specifies a converter that can convert from and to HTTP requests and responses.   *   * @author Arjen Poutsma   * @author Juergen Hoeller   * @since 3.0   */    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,详情查看 3.1 document “16.14 Configuring  MVC”章节），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：

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：

@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 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);            }
        
       
      

补充：

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。