方法写的一个验证码（可以修改为JSP形式的）

public String validId(){
  HttpServletResponse response = this.getResponse();
  HttpServletRequest request = this.getRequest();
  //设置页面不缓存 
  response.setHeader("Pragma","No-cache"); 
  response.setHeader("Cache-Control","no-cache"); 
  response.setDateHeader("Expires", 0);
  //设置宽与高度
  int width = 60, height = 20;
  //在内存中创建图片
  BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
  //获取图形上下文
  Graphics g = image.getGraphics();
  //生成随机类
  Random r = new Random();
  //设置字体
  g.setColor(new Color(160, 200, 100));
  g.fillRect(0, 0, width, height);
  //设置字体
  g.setFont(new Font("Times New Roman", Font.BOLD, height));
  //随机产生50条干扰线，使图形中的验证码不易被其他的程序探测
  g.setColor(new Color(160, 200, 200));
  for (int i=0; i<50; i++) {
  int x = r.nextInt(width);
  int y = r.nextInt(height);
  int x1 = r.nextInt(width);
  int y1 = r.nextInt(height);
  g.drawLine(x, y, x1, y1);
  }
  //随机产生验证码（4位数字）
  StringBuffer sb = new StringBuffer("");
  for (int i=0; i<4; i++) {
  String s = String.valueOf(r.nextInt(10)); sb.append(s);
  g.setColor( new Color(20+ r.nextInt(110), 20+r.nextInt(110), 20+r.nextInt(110)) );
  g.drawString(s, 13*i+6, 16);
  }
  g.dispose();
  try {
   ImageIO.write(image, "JPEG", response.getOutputStream());
  } catch (IOException e) {
   e.printStackTrace();
  }
  request.getSession().setAttribute("validateId", sb.toString());
  return null;
 }