@RequestBody, @ResponseBody 註解詳解
引言：
接上一篇文章講述處理@RequestMapping的方法引數繫結之後，詳細介紹下@RequestBody、@ResponseBody的具體用法和使用時機；同時對曾經看的一篇文章中講述的某些部分進行澄清 （文章地址：http://www.byywee.com/page/M0/S702/702424.html）。
簡介：
@RequestBody
作用：
i) 該註解用於讀取Request請求的body部分資料，使用系統預設配置的HttpMessageConverter進行解析，然後把相應的資料繫結到要返回的物件上；
ii) 再把HttpMessageConverter返回的物件資料繫結到 controller中方法的引數上。
使用時機：
A) GET、POST方式提時， 根據request header Content-Type的值來判斷:
- application/x-www-form-urlencoded， 可選（即非必須，因為這種情況的資料@RequestParam, @ModelAttribute也可以處理，當然@RequestBody也能處理）；
- multipart/form-data, 不能處理（即使用@RequestBody不能處理這種格式的資料）；
- 其他格式， 必須（其他格式包括application/json, application/xml等。這些格式的資料，必須使用@RequestBody來處理）；
B) PUT方式提交時， 根據request header Content-Type的值來判斷:
- application/x-www-form-urlencoded， 必須；
- multipart/form-data, 不能處理；
- 其他格式， 必須；
@ResponseBody
作用：
該註解用於將Controller的方法返回的物件，通過適當的HttpMessageConverter轉換為指定格式後，寫入到Response物件的body資料區。
使用時機：
返回的資料不是html標籤的頁面，而是其他某種格式的資料時（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;
}
在使用 <mvc:annotation-driven />標籤配置時，預設配置了RequestMappingHandlerAdapter（注意是RequestMappingHandlerAdapter不是AnnotationMethodHandlerAdapter,詳情檢視Spring 3.1 document
“16.14 Configuring Spring MVC”章節），併為他配置了一下預設的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: 負責讀取二進位制格式的資料和寫出二進位制格式的資料；
StringHttpMessageConverter： 負責讀取字串格式的資料和寫出二進位制格式的資料；
ResourceHttpMessageConverter：負責讀取資原始檔和寫出資原始檔資料；
FormHttpMessageConverter： 負責讀取form提交的資料（能讀取的資料格式為 application/x-www-form-urlencoded，不能讀取multipart/form-data格式資料）；負責寫入application/x-www-from-urlencoded和multipart/form-data格式的資料；
MappingJacksonHttpMessageConverter: 負責讀取和寫入json格式的資料；
SouceHttpMessageConverter： 負責讀取和寫入 xml 中javax.xml.transform.Source定義的資料；
Jaxb2RootElementHttpMessageConverter: 負責讀取和寫入xml 標籤格式的資料；
AtomFeedHttpMessageConverter: 負責讀取和寫入Atom格式的資料；
RssChannelHttpMessageConverter: 負責讀取和寫入RSS格式的資料；
當使用@RequestBody和@ResponseBody註解時，RequestMappingHandlerAdapter就使用它們來進行讀取或者寫入相應格式的資料。
HttpMessageConverter匹配過程：
@RequestBody註解時： 根據Request物件header部分的Content-Type型別，逐一匹配合適的HttpMessageConverter來讀取資料；
spring 3.1原始碼如下：
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註解時： 根據Request物件header部分的Accept屬性（逗號分隔），逐一按accept中的型別，去遍歷找到能處理的HttpMessageConverter；
原始碼如下：
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);
}補充：
MappingJacksonHttpMessageConverter 呼叫了 objectMapper.writeValue(OutputStream stream, Object)方法，使用@ResponseBody註解返回的物件就傳入Object引數內。若返回的物件為已經格式化好的json串時，不使用@RequestBody註解，而應該這樣處理：
1、response.setContentType("application/json; charset=UTF-8");
2、response.getWriter().print(jsonStr);
直接輸出到body區，然後的檢視為void。
參考資料：
1、 Spring 3.1 Doc：
spring-3.1.0/docs/spring-framework-reference/html/mvc.html
2、Spring 3.x MVC 入門4 -- ＠ResponseBody & ＠RequestBody