@RequestBody, @ResponseBody 註解詳解(轉)

標籤：

引言：

 

接上一篇文章講述處理@RequestMapping的方法參數綁定之後，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對象上；

      ii) 再把HttpMessageConverter返回的對象資料繫結到 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；

說明：request的body部分的資料編碼格式由header部分的Content-Type指定；

 

 

@ResponseBody

 

作用：

      該註解用於將Controller的方法返回的對象，通過適當的HttpMessageConverter轉換為指定格式後，寫入到Response對象的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

 

<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>

 

 

該介面定義了四個方法，分別是讀取資料時的 canRead(), read() 和 寫入資料時的canWrite(), write()方法。

 

在使用 <mvc:annotation-driven />標籤配置時，預設配置了RequestMappingHandlerAdapter（注意是RequestMappingHandlerAdapter不是AnnotationMethodHandlerAdapter,詳情查看Spring 3.1 document “16.14 Configuring Spring MVC”章節），並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: 負責讀取二進位格式的資料和寫出二進位格式的資料；

StringHttpMessageConverter：   負責讀取字串格式的資料和寫出二進位格式的資料；

 

ResourceHttpMessageConverter：負責讀取資源檔和寫出資源檔資料；

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對象header部分的Content-Type類型，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對象header部分的Accept屬性（逗號分隔），逐一按accept中的類型，去遍曆找到能處理的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 調用了 objectMapper.writeValue(OutputStream stream, Object)方法，使用@ResponseBody註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，然後的視圖為void。

@RequestBody, @ResponseBody 註解詳解(轉)