1、MultipartResolver
MultipartResolver接口提供了spring mvc的上传视图，MultipartResolver实例在请求转交给handlermapping之前。 也就是说DispatcherServlet ，在请求映射的时候，若找到MultipartResolver这个实例的话，
就直接把请求转交给MultipartResolver
/**
* Determine if the given request contains multipart content.
* <p>Will typically check for content type "multipart/form-data", but the actually
* accepted requests might depend on the capabilities of the resolver implementation.
* @param request the servlet request to be evaluated
* @return whether the request contains multipart content
*/
boolean isMultipart(HttpServletRequest request); /**
* Parse the given HTTP request into multipart files and parameters,
* and wrap the request inside a
* {@link org.springframework.web.multipart.MultipartHttpServletRequest}
* object that provides access to file descriptors and makes contained
* parameters accessible via the standard ServletRequest methods.
* @param request the servlet request to wrap (must be of a multipart content type)
* @return the wrapped servlet request
* @throws MultipartException if the servlet request is not multipart, or if
* implementation-specific problems are encountered (such as exceeding file size limits)
* @see MultipartHttpServletRequest#getFile
* @see MultipartHttpServletRequest#getFileNames
* @see MultipartHttpServletRequest#getFileMap
* @see javax.servlet.http.HttpServletRequest#getParameter
* @see javax.servlet.http.HttpServletRequest#getParameterNames
* @see javax.servlet.http.HttpServletRequest#getParameterMap
*/
MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException; /**
* Cleanup any resources used for the multipart handling,
* like a storage for the uploaded files.
* @param request the request to cleanup resources for
*/
void cleanupMultipart(MultipartHttpServletRequest request);
MultipartResolver接口提供了3个方法：
boolean isMultipart(HttpServletRequest request)： 检查当前请求是否是multipart content type
resolveMultipart： 将httprequest转换成MultipartHttpServletRequest
已知的子类： CommonsMultipartResolver, StandardServletMultipartResolver
- 第一个需要使用 Apache 的 commons-fileupload 等 jar 包支持，但它能在比较旧的 servlet 版本中使用。
- 第二个不需要第三方 jar 包支持，它使用 servlet 内置的上传功能，但是只能在 Servlet 3 以上的版本使用。
2、multipartRequest
multipartRequest接口可以讲request对象封装成multipartRequest，提取请求的信息，MultipartReques类主要是对文件上传进行的处理。