spring multipart源码分析：

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

1. 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。
2. 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。