spring multipart源码分析：

/**
     * 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);