Spring MVC之@RequestBody, @ResponseBody 详解

淡淡的烟草味﹌ 2022-09-05 00:09 178阅读 0赞

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

/** * 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;
     
    }

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：

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：

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);
    		}