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
plaincopyByteArrayHttpMessageConverter 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。