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
- <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:
- 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:
- <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:
- <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。