，java servlet 下传文件异常。

<form action="/vats/upfileServlet.do" method="post" encType=multipart/form-data >         图片：<input type="file" name="img"/>     <input type="submit" value="提交"/>     </form>

public void doPost(HttpServletRequest request, HttpServletResponse response)             throws ServletException, IOException {         response.setContentType("text/html");         request.setCharacterEncoding("UTF-8");         response.setCharacterEncoding("UTF-8");                  DiskFileUpload du = new DiskFileUpload();         du.setSizeMax(4194304); // 设置最大文件尺寸，这里是4MB         du.setSizeThreshold(4096);// 设置缓存区大小 ，4 kb;         // up 为 /WebRoot 下的 一个目录         du.setRepositoryPath(request.getSession().getServletContext().getRealPath("/up"));// 设置缓存目录         // 得到所有文件         try {             List list = du.parseRequest(request);             Iterator it = list.iterator();             while (it.hasNext()) {                 FileItem fileItem = (FileItem) it.next();                     String fileName = fileItem.getName();                     int index = fileName.lastIndexOf('.');                     fileName = fileName.substring(index);                     fileItem.write(new File(request.getSession()                             .getServletContext().getRealPath("/file")+ "/" + fileName));                     System.out.println("上传成功");             }         } catch (Exception e) {             e.printStackTrace();         }         PrintWriter out = response.getWriter();         out.flush();         out.close();     }