@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；
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；
[java]view plaincopy
/**
*?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
*/
publicinterfaceHttpMessageConverter?{
/**
*?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
*/
booleancanRead(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
*/
booleancanWrite(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)
throwsIOException,?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
*/
voidwrite(T?t,?MediaType?contentType,?HttpOutputMessage?outputMessage)
throwsIOException,?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 标签配置时，默认配置了RequestMappingHandlerAdapter（注意是RequestMappingHandlerAdapter不是AnnotationMethodHandlerAdapter,详情查看spring3.1 document “16.14?Configuring Spring MVC”章节），并为他配置了一下默认的HttpMessageConverter：
[java]view plaincopy
ByteArrayHttpMessageConverter?convertsbytearrays.
StringHttpMessageConverter?converts?strings.
ResourceHttpMessageConverter?converts?to/from?org.springframework.core.io.Resourceforall?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?—?addedifJAXB2?is?present?on?the?classpath.
MappingJacksonHttpMessageConverter?converts?to/from?JSON?—?addedifJackson?is?present?on?the?classpath.
AtomFeedHttpMessageConverter?converts?Atom?feeds?—?addedifRome?is?present?on?the?classpath.
RssChannelHttpMessageConverter?converts?RSS?feeds?—?addedifRome?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。
@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：
[java]view plaincopy
privateObject?readWithMessageConverters(MethodParameter?methodParam,?HttpInputMessage?inputMessage,?Class?paramType)
throwsException?{
MediaType?contentType?=?inputMessage.getHeaders().getContentType();
if(contentType?==null)?{
StringBuilder?builder?=newStringBuilder(ClassUtils.getShortName(methodParam.getParameterType()));
String?paramName?=?methodParam.getParameterName();
if(paramName?!=null)?{
builder.append('?');
builder.append(paramName);
}
thrownewHttpMediaTypeNotSupportedException(
"Cannot?extract?parameter?("+?builder.toString()?+"):?no?Content-Type?found");
}
List?allSupportedMediaTypes?=newArrayList();
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?+"]");
}
returnmessageConverter.read(paramType,?inputMessage);
}
}
}
thrownewHttpMediaTypeNotSupportedException(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：
[java]view plaincopy
privatevoidwriteWithMessageConverters(Object?returnValue,
HttpInputMessage?inputMessage,?HttpOutputMessage?outputMessage)
throwsIOException,?HttpMediaTypeNotAcceptableException?{
List?acceptedMediaTypes?=?inputMessage.getHeaders().getAccept();
if(acceptedMediaTypes.isEmpty())?{
acceptedMediaTypes?=?Collections.singletonList(MediaType.ALL);
}
MediaType.sortByQualityValue(acceptedMediaTypes);
Class?returnValueType?=?returnValue.getClass();
List?allSupportedMediaTypes?=newArrayList();
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());
}
}
thrownewHttpMediaTypeNotAcceptableException(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。
