@RequestBody的作用
@RequestBody用于读取Request请求的body数据,然后利用SpringMVC配置的HttpMessageConverter对数据进行转换,最后把转换后的数据绑定到被@RequestBody注解的参数上;
@RequestBody的使用场景
根据request header中 Content-Type和被@RequestBody注解的参数不同,最常见的应用场景如下:
- Content-Type为application/json
- 参数为JavaBean:可实现json反序列化为JavaBean,使用的HttpMessageConverter为 MappingJackson2HttpMessageConverter
- 参数为String:简单将字符串赋值给参数,使用的HttpMessageConverter为 StringHttpMessageConverter
- Content-Type为application/xml
- 参数为JavaBean:可实现xml反序列化为JavaBean,使用的HttpMessageConverter为 Jaxb2RootElementHttpMessageConverter
- 参数为String:简单将字符串赋值给参数,使用的HttpMessageConverter为 StringHttpMessageConverter
- application/x-www-form-urlencoded
- 参数为String:简单将字符串赋值给参数,使用的HttpMessageConverter为 StringHttpMessageConverter
HttpMessageConverter接口
该接口定义了五个方法,分别是读取数据时的 canRead()、read() ,写入数据时的canWrite()、 write()方法以及获取支持类型的 getSupportedMediaTypes()
public interface HttpMessageConverter<T> { // Indicate whether the given class and media type can be read by this converter. boolean canRead(Class<?> clazz, MediaType mediaType); // Indicate whether the given class and media type can be written by this converter. boolean canWrite(Class<?> clazz, MediaType mediaType); // Return the list of MediaType objects supported by this converter. List<MediaType> getSupportedMediaTypes(); // Read an object of the given type from the given input message, and returns it. T read(Class<T> clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException; // Write an given object to the given output message. void write(T t, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException; }
使用 <mvc:annotation-driven />标签配置时,默认配置了RequestMappingHandlerAdapter,并为他配置了一下默认的HttpMessageConverter:
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;
HttpMessageConverter匹配过程
根据Request对象header部分的ContentType类型和被注解参数类型,逐一匹配合适的HttpMessageConverter来读取数据;
protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter, Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException { ...... MediaType contentType; contentType = inputMessage.getHeaders().getContentType(); ....... for (HttpMessageConverter<?> converter : this.messageConverters) { Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass(); if (converter instanceof GenericHttpMessageConverter) { GenericHttpMessageConverter<?> genericConverter = (GenericHttpMessageConverter<?>) converter; if (genericConverter.canRead(targetType, contextClass, contentType)) { if (logger.isDebugEnabled()) { logger.debug("Read [" + targetType + "] as "" + contentType + "" with [" + converter + "]"); } if (inputMessage.getBody() != null) { inputMessage = getAdvice().beforeBodyRead(inputMessage, parameter, targetType, converterType); body = genericConverter.read(targetType, contextClass, inputMessage); body = getAdvice().afterBodyRead(body, inputMessage, parameter, targetType, converterType); } else { body = getAdvice().handleEmptyBody(null, inputMessage, parameter, targetType, converterType); } break; } } else if (targetClass != null) { if (converter.canRead(targetClass, contentType)) { if (logger.isDebugEnabled()) { logger.debug("Read [" + targetType + "] as "" + contentType + "" with [" + converter + "]"); } if (inputMessage.getBody() != null) { inputMessage = getAdvice().beforeBodyRead(inputMessage, parameter, targetType, converterType); body = ((HttpMessageConverter<T>) converter).read(targetClass, inputMessage); body = getAdvice().afterBodyRead(body, inputMessage, parameter, targetType, converterType); } else { body = getAdvice().handleEmptyBody(null, inputMessage, parameter, targetType, converterType); } break; } } } ....... return body; }
注意
- 在一个方法的参数列表中,@RequestBody只能使用一次;
- json字符串中,如果value为""的话,后端对应属性如果是String类型的,那么接受到的就是"",如果是后端属性的类型是Integer、Double等类型,那么接收到的就是null;
- json字符串中,如果value为null的话,后端对应收到的就是null;
- 在传json字符串给后端时,如果某个key没有value的话,要么干脆就不写该key,要么就将value赋值null 或"",不能有类似 {......,"key":,.....}, 这样的写法