JavaWeb入门day04-Response验证码实现

1、验证码怎么来的？

• 前端实现
• 后端实现，需要用到java图片类，生产一个图片

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //如何让浏览器5s自动刷新一次
        response.setHeader("refresh", "3");
        //如何在内存中创建图片
        BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g = (Graphics2D) image.getGraphics();//设置画笔
        //设置图片背景颜色
        g.setColor(Color.white);
        g.fillRect(0, 0, 80, 20);//设置矩形
        //给图片写数据
        g.setColor(Color.blue);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(maknum(),0,20);
        //告诉浏览器 要用图片的方式打开
        response.setContentType("image/jpeg");
        //网站存在缓存,不让浏览器缓存
        response.setDateHeader("expires",-1);
        response.setHeader("Cache-Control","no-cache");
        response.setHeader("Pragram","no-cache");
        //把图片写给浏览器
        ImageIO.write(image,"jpg",response.getOutputStream());
    }

    //生成随机数
    private String maknum() {
        Random random = new Random();
        String num = random.nextInt(99999999) + "";
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < 7-num.length(); i++) {
            //不足七位数的时候自动补零
            sb.append("0");
        }
      num= sb.toString() + num;
        return num;
    }