spring mvc 上传要配置MultipartResolver的原因

上传的时候我们要先配

原因是什么呢？

看DispatcherServlet源码可以找到这个一个方法，会在转发器初始化时，会尝试通过类的方式获取multipartResolver，设置为类内变量：

private void initMultipartResolver(ApplicationContext context) {
   try {
      this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
      if (logger.isTraceEnabled()) {
         logger.trace("Detected " + this.multipartResolver);
      }
      else if (logger.isDebugEnabled()) {
         logger.debug("Detected " + this.multipartResolver.getClass().getSimpleName());
      }
   }
   catch (NoSuchBeanDefinitionException ex) {
      // Default is no multipart resolver.
      this.multipartResolver = null;
      if (logger.isTraceEnabled()) {
         logger.trace("No MultipartResolver '" + MULTIPART_RESOLVER_BEAN_NAME + "' declared");
      }
   }
}

 拿到multipartResolver后，dispatcher转发会经过这么一段代码

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HttpServletRequest processedRequest = request;
		HandlerExecutionChain mappedHandler = null;
		boolean multipartRequestParsed = false;

		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

		try {
			ModelAndView mv = null;
			Exception dispatchException = null;

			try {
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);
.......下面省略

 而checkMultiPart里面的代码如下

rotected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
		if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
			if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
				if (DispatcherType.REQUEST.equals(request.getDispatcherType())) {
					logger.trace("Request already resolved to MultipartHttpServletRequest, e.g. by MultipartFilter");
				}
			}
			else if (hasMultipartException(request)) {
				logger.debug("Multipart resolution previously failed for current request - " +
						"skipping re-resolution for undisturbed error rendering");
			}
			else {
				try {
					return this.multipartResolver.resolveMultipart(request);
				}
				catch (MultipartException ex) {
					if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {
						logger.debug("Multipart resolution failed for error dispatch", ex);
						// Keep processing error dispatch with regular request handle below
					}
					else {
						throw ex;
					}
				}
			}
		}
		// If not returned before: return original request.
		return request;
	}

可以看到，如果之前我们配了MultipartResolver，该方法就会使用MultipartResolver的isMultipartResolver判断请求是否为multipart型的，是的话就会进一步解析成

MultipartHttpServletRequest 再返回（该类继承了HttpServletRequest, MultipartRequest）。

以上就是配置MultipartResolver的原因。