引言：
接上一篇文章讲述处理@RequestMapping的方法参数绑定之后，详细介绍下@RequestBody、@ResponseBody的具体用法和使用时机；
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。