HttpMessageConverter用法

HttpMessageConverter接口定义

 * 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。

常用的HttpMessageConverter

在使用 <mvc:annotation-driven />标签配置时，默认配置了RequestMappingHandlerAdapter（注意是RequestMappingHandlerAdapter不是AnnotationMethodHandlerAdapter），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。

spring 3.1 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);
}