为什么FileItemIterator.hasNext（）返回false？

Question

请帮忙。我几乎看到每个论坛都是徒劳的。我不明白为什么hasNext（）返回false？

public class FileUploadServ extends HttpServlet {

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub

        response.setContentType("text/html");
        boolean isMultipart = ServletFileUpload.isMultipartContent(request);
        if (isMultipart) {
            JOptionPane.showMessageDialog(null,"is multipart");

            ServletFileUpload upload = new ServletFileUpload();
            try {
                FileItemIterator itr = upload.getItemIterator(request);
                boolean c=itr.hasNext(); // why does this return false
                JOptionPane.showMessageDialog(null,"hasNext value:"+c);

                  while (c=itr.hasNext()){

                    FileItemStream item = itr.next(); 
                    if (item.isFormField()) { 


                    }
                    else {
                        String path= getServletContext().getRealPath("/");

                        if (FileUpload.processFile(path, item))
                            response.getWriter().println("Upload Successful");
                        else
                            response.getWriter().println("Upload Failed!");
                     }
                    }
                  //JOptionPane.showMessageDialog(null,"out of while hasNext value:"+c);
            } catch (FileUploadException fue) {
                //fue.printStackTrace();
                JOptionPane.showMessageDialog(null,"error!:"+ fue.getMessage());

            }
        }
    }
}