Question

对于请求的处理，Spring 首先考虑的是对于 Multipart 的处理，如果是 MultipartContent 类型的 request，则转换 request 为 MultipartHttpServletRequest 类型的 request。

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws

MultipartException {

　 if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {

　　 if (request instanceof MultipartHttpServletRequest) {

　　　 logger.debug("Request is already a MultipartHttpServletRequest - if not

　　　 in a forward, " +

　　　 "this typically results from an additional MultipartFilter in web.xml");

　　}

　　 else {

　　　 return this.multipartResolver.resolveMultipart(request);

　　}

　}

　 // If not returned before: return original request.

　 return request;

}