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；
  

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

<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。