Spring MVC之@RequestBody, @ResponseBody 详解

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<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;  
  
}

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Spring 3.1 document “16.14 Configuring Spring 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<String, String>.  
  
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<MediaType> allSupportedMediaTypes = new ArrayList<MediaType>();
        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<MediaType> acceptedMediaTypes = inputMessage.getHeaders().getAccept();
        if (acceptedMediaTypes.isEmpty()) {
            acceptedMediaTypes = Collections.singletonList(MediaType.ALL);
        }
        MediaType.sortByQualityValue(acceptedMediaTypes);
        Class<?> returnValueType = returnValue.getClass();
        List<MediaType> allSupportedMediaTypes = new ArrayList<MediaType>();
        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。
本文转自：http://blog.csdn.net/kobejayandy/article/details/12690555