spring中HttpMessageConverter详解

HttpMessageConverter 介绍

Strategy interface that specifies a converter that can convert from and to HTTP requests and responses.

简单来说，HttpMessageConverter就是一个策略接口， 用来将http请求转为某个对象， 或许将http响应转为指定类型的对象。

HttpMessageConverter接口声明如下：

public interface HttpMessageConverter<T> {

   //检测参数clazz指定的类型是否可以被该转换器读取 
trueboolean canRead(Class<?> clazz, MediaType mediaType);

   //检测参数clazz指定的类型是否可以被该转换器写入
trueboolean canWrite(Class<?> clazz, MediaType mediaType);

   //获取该转换器支持的媒体类型列表
trueList<MediaType> getSupportedMediaTypes();

   //从输入信息中读取指定类型对象 
trueT read(Class<? extends T> clazz, HttpInputMessage inputMessage)
			throws IOException, HttpMessageNotReadableException;

   // 将指定的对象写入到输出
truevoid write(T t, MediaType contentType, HttpOutputMessage outputMessage)
			throws IOException, HttpMessageNotWritableException;

}

什么时候使用？

通常，我们在Controller里带有返回值得请求处理方法上加上注解@ResponseBody注解后，springMVC内部就会使用到HttpMessageConverter。 原理如下：

AnnotationMethodHandlerAdapter接口是专门用来处理加了@Controller注解的Controller的。

@Override
public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)throws Exception {
    //省略
    return invokeHandlerMethod(request, response, handler);
}			
protected ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, Object handler)throws Exception {
truetrue// 省略
    Object result = methodInvoker.invokeHandlerMethod(handlerMethod, handler, webRequest, implicitModel);
truetrueModelAndView mav = methodInvoker.getModelAndView(handlerMethod, handler.getClass(), result, implicitModel, webRequest);
truetruemethodInvoker.updateModelAttributes(handler, (mav != null ? mav.getModel() : null), implicitModel, webRequest);
    return mav;
}
@SuppressWarnings("unchecked")
truetruepublic ModelAndView getModelAndView(Method handlerMethod, Class<?> handlerType, Object returnValue,ExtendedModelMap implicitModel, ServletWebRequest webRequest) throws Exception {
    // 省略
    if (returnValue instanceof HttpEntity) {
    	handleHttpEntityResponse((HttpEntity<?>) returnValue, webRequest);
    	return null;
    }
    else if (AnnotationUtils.findAnnotation(handlerMethod, ResponseBody.class) != null) {
    	handleResponseBody(returnValue, webRequest);
    	return null;
    }			
    // 省略	
}
private void handleResponseBody(Object returnValue, ServletWebRequest webRequest) throws Exception {
trueif (returnValue == null) {
truetruereturn;
true}
trueHttpInputMessage inputMessage = createHttpInputMessage(webRequest);
trueHttpOutputMessage outputMessage = createHttpOutputMessage(webRequest);
truewriteWithMessageConverters(returnValue, inputMessage, outputMessage);
}

@SuppressWarnings({ "unchecked", "rawtypes" })
private void writeWithMessageConverters(Object returnValue,HttpInputMessage inputMessage, HttpOutputMessage outputMessage) throws IOException, HttpMediaTypeNotAcceptableException {
    List<MediaType> acceptedMediaTypes = inputMessage.getHeaders().getAccept();
truetrueif (acceptedMediaTypes.isEmpty()) {
truetruetrueacceptedMediaTypes = Collections.singletonList(MediaType.ALL);
truetrue}
truetrueMediaType.sortByQualityValue(acceptedMediaTypes);
truetrueClass<?> returnValueType = returnValue.getClass();
truetrueList<MediaType> allSupportedMediaTypes = new ArrayList<MediaType>();
truetrueif (getMessageConverters() != null) {
truetruetruefor (MediaType acceptedMediaType : acceptedMediaTypes) {
truetruetruetruefor (HttpMessageConverter messageConverter : getMessageConverters()) {
truetruetruetrue    // 判断是否支持 
truetruetruetruetrueif (messageConverter.canWrite(returnValueType, acceptedMediaType)) {
truetruetruetruetrue   // 写入输出
truetruetruetruetruetruemessageConverter.write(returnValue, acceptedMediaType, outputMessage);
truetruetruetruetruetrueif (logger.isDebugEnabled()) {
truetruetruetruetruetruetrueMediaType contentType = outputMessage.getHeaders().getContentType();
truetruetruetruetruetruetrueif (contentType == null) {
truetruetruetruetruetruetruetruecontentType = acceptedMediaType;
truetruetruetruetruetruetrue}
truetruetruetruetruetruetruelogger.debug("Written [" + returnValue + "] as \"" + contentType +
truetruetruetruetruetruetruetruetrue"\" using [" + messageConverter + "]");
truetruetruetruetruetrue}
truetruetruetruetruetruethis.responseArgumentUsed = true;
truetruetruetruetruetruereturn;
truetruetruetruetrue}
truetruetruetrue}
truetruetrue}
truetruetruefor (HttpMessageConverter messageConverter : messageConverters) {
truetruetruetrueallSupportedMediaTypes.addAll(messageConverter.getSupportedMediaTypes());
truetruetrue}
truetrue}
truetruethrow new HttpMediaTypeNotAcceptableException(allSupportedMediaTypes);
}

输出json

SpringMVC默认是使用Jackson（MappingJackson2HttpMessageConverter）来将返回对象转为json的。当然了，你也可以配置其它的类来实现Json的输出。

输出XML

具体的类是：MappingJackson2XmlHttpMessageConverter。 请求头Accept是”application/*+xml” 或 “application/xml”

输出feed

具体的类是：RssChannelHttpMessageConverter。 请求头Accept是：”application/rss+xml”