@responsebody表示該方法的返回結果直接寫入HTTP response body中
一般在異 步獲取數據時使用，在使用@RequestMapping后，返回值通常解析為跳轉路徑，加上@responsebody后返回結果不會被解析為跳轉路 徑，而是直接寫入HTTP response body中。比如異步獲取json數據，加上@responsebody后，會直接返回json數據。
引言：
接上一篇文章講述處理@RequestMapping的方法參數綁定之后，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) 該注解用於讀取Request請求的body部分數據，使用系統默認配置的HttpMessageConverter進行解析，然后把相應的數據綁定到要返回的對象上；
ii) 再把HttpMessageConverter返回的對象數據綁定到 controller中方法的參數上。
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，因為這種情況的數據@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數據）；
- 其他格式， 必須（其他格式包括application/json, application/xml等。這些格式的數據，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；
說明：request的body部分的數據編碼格式由header部分的Content-Type指定；
@ResponseBody
作用：
該注解用於將Controller的方法返回的對象，通過適當的HttpMessageConverter轉換為指定格式后，寫入到Response對象的body數據區。
使用時機：
返回的數據不是html標簽的頁面，而是其他某種格式的數據時（如json、xml等）使用；
HttpMessageConverter
<span style="font-family:Microsoft YaHei;">/** * 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; } </span>
該接口定義了四個方法，分別是讀取數據時的 canRead(), read() 和 寫入數據時的canWrite(), write()方法。
在使用 <mvc:annotation-driven />標簽配置時，默認配置了RequestMappingHandlerAdapter（注意是RequestMappingHandlerAdapter不是AnnotationMethodHandlerAdapter,詳情查看Spring 3.1 document “16.14 Configuring Spring MVC”章節），並為他配置了一下默認的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: 負責讀取二進制格式的數據和寫出二進制格式的數據；
StringHttpMessageConverter： 負責讀取字符串格式的數據和寫出二進制格式的數據；
ResourceHttpMessageConverter：負責讀取資源文件和寫出資源文件數據；
FormHttpMessageConverter： 負責讀取form提交的數據（能讀取的數據格式為 application/x-www-form-urlencoded，不能讀取multipart/form-data格式數據）；負責寫入 application/x-www-from-urlencoded和multipart/form-data格式的數據；
MappingJacksonHttpMessageConverter: 負責讀取和寫入json格式的數據；
SouceHttpMessageConverter： 負責讀取和寫入 xml 中javax.xml.transform.Source定義的數據；
Jaxb2RootElementHttpMessageConverter: 負責讀取和寫入xml 標簽格式的數據；
AtomFeedHttpMessageConverter: 負責讀取和寫入Atom格式的數據；
RssChannelHttpMessageConverter: 負責讀取和寫入RSS格式的數據；
當使用@RequestBody和@ResponseBody注解時，RequestMappingHandlerAdapter就使用它們來進行讀取或者寫入相應格式的數據。
HttpMessageConverter匹配過程：
@RequestBody注解時： 根據Request對象header部分的Content-Type類型，逐一匹配合適的HttpMessageConverter來讀取數據；
spring 3.1源代碼如下：
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注解時： 根據Request對象header部分的Accept屬性（逗號分隔），逐一按accept中的類型，去遍歷找到能處理的HttpMessageConverter；
源代碼如下：
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 調用了 objectMapper.writeValue(OutputStream stream, Object)方法，使用@ResponseBody注解返回的對象就傳入Object參數內。若返回的對象為已經格式化好的json串時，不使用@RequestBody注解，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，然后的視圖為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
http://www.byywee.com/page/M0/S702/702424.html