javaweb生成一个随机验证码(代码)

生成一个随机验证码:

以图片的方式在浏览器上打印出来

public class ImageServlet extends HttpServlet{
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //如何让浏览器5秒刷新一次；
        resp.setHeader("refresh","3");
        //在内存中创建图片
        BufferedImage image = new BufferedImage(80,20, BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g = (Graphics2D) image.getGraphics();//笔
        //设置图片的背景颜色
        g.setColor(Color.white);
        g.fillRect(0,0,80,20);
        //给图片写数据
        g.setColor(Color.blue);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeNum(),0,20);
        //告诉浏览器这个请求用图片的方式打开
        resp.setContentType("image/jpeg");
        //网站存在缓存，不让浏览器缓存
        resp.setDateHeader("expires",-1);
        resp.setHeader("Cache-Control","no-cache");
        resp.setHeader("Pragma","no-cache");

        //把图片写给浏览器
        ImageIO.write(image,"jpg",resp.getOutputStream());

    }
    //生成随机数
    private String makeNum(){
        Random random = new Random();
         String num = random.nextInt(9999999)+"";
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 7-num.length(); i++) {
            sb.append("0");
        }
        num = sb.toString() + num;
        return num;
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doGet(req, resp);
    }
}

<servlet>
    <servlet-name>imageservlet-name>
    <servlet-class>com.maple.servlet.ImageServletservlet-class>
  servlet>
  <servlet-mapping>
    <servlet-name>imageservlet-name>
    <url-pattern>/imageurl-pattern>
  servlet-mapping>