關於隨機生成驗證碼的問題

<%@ page contentType="image/jpeg" import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*" %>
<%
 // 在內存中創建圖象
 int width=60, height=20;
 BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
 // 獲取圖形上下文
 Graphics g = image.getGraphics();
 //Graphics2D graphics = image.createGraphics();
 // 設定背景色
 g.setColor(Color.white);
 g.fillRect(0, 0, width, height);
 //畫邊框
 g.setColor(Color.black);
 g.drawRect(0,0,width-1,height-1);
 // 取隨機產生的認證碼(4位數字)
 String rand = request.getParameter("rand");
 rand = rand.substring(0,rand.indexOf("."));
 switch(rand.length()) {
  
  case 1:
   rand = "000"+rand;
   break;
  case 2:
   rand = "00"+rand;
   break;
  case 3:
   rand = "0"+rand;
   break;
  default:
   rand = rand.substring(0,4);
   break;
 }
 // 將認證碼存入SESSION
 session.setAttribute("rand",rand);
 // 將認證碼顯示到圖象中
 g.setColor(Color.black);
 g.setFont(new Font("Times New Roman",Font.PLAIN,18));
 g.drawString(rand,10,15);
 // 隨機產生88個干擾點，使圖象中的認證碼不易被其它程序探測到
 Random random = new Random();
 for (int i=0;i<88;i++) {
  int x = random.nextInt(width);
  int y = random.nextInt(height);
  g.drawLine(x,y,x,y);
 }
 // 圖象生效
 g.dispose();
 // 輸出圖象到頁面
 ImageIO.write(image, "JPEG", response.getOutputStream());
%>