<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();
     }