<Spring>Spring MVC之@RequestBody, @ResponseBody 详解

引言：

接上一篇文章讲述处理@RequestMapping的方法参数绑定之后，详细介绍下@RequestBody、@ResponseBody的具体用法和使用时机；

简介：

@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；

@ResponseBody

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；

HttpMessageConverter

[java] view plaincopy

<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>

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 <mvc:annotation-driven />标签配置时，默认配置了

RequestMappingHandlerAdapter（注意是RequestMappingHandlerAdapter

不是AnnotationMethodHandlerAdapter,详情查看Spring 3.1 document “16.14 Configuring Spring MVC”章节），并为他配置了一下默认的HttpMessageConverter：

[java] view plaincopy

ByteArrayHttpMessageConverter 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: 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。

HttpMessageConverter匹配过程：

@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

<span style="font-family:Microsoft YaHei;">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);

}</span>

@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

<span style="font-family:Microsoft YaHei;">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);

}</span>

补充：

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。