Java 生成验证码

1、第一步生成背景板

public void generateCheckCode(HttpServletRequest request, HttpServletResponse response) throws IOException {
        //服务器通知浏览器不要缓存
        response.setHeader("pragma", "no-cache");
        response.setHeader("cache-control", "no-cache");
        response.setHeader("expires", "0");

        //在内存中创建一个长80，宽30的图片，默认黑色背景
        //参数一：长
        //参数二：宽
        //参数三：颜色
        int width = 80;
        int height = 30;
        BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

        //获取画笔
        Graphics g = image.getGraphics();
        //设置画笔颜色为灰色
        g.setColor(Color.BLACK);
        //填充图片
        g.fillRect(0, 0, width, height);

        //产生4个随机验证码，12Ey
        String checkCode = getCheckCode();
        //将验证码放入HttpSession中
        request.getSession().setAttribute("checkCode", checkCode);

        //设置画笔颜色为黄色
        g.setColor(Color.WHITE);
        //设置字体的小大
        g.setFont(new Font("黑体", Font.BOLD, 24));
        //向图片上写入验证码
        g.drawString(checkCode, 15, 25);

        //将内存中的图片输出到浏览器
        //参数一：图片对象
        //参数二：图片的格式，如PNG,JPG,GIF
        //参数三：图片输出到哪里去
        ImageIO.write(image, "PNG", response.getOutputStream());
    }

2、第二步生成验证码字符串

private String getCheckCode() {
    String base = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    int size = base.length();
    Random r = new Random();
    StringBuilder sb = new StringBuilder();
    for (int i = 1; i <= 4; i++) {
        //产生0到size-1的随机值
        int index = r.nextInt(size);
        //在base字符串中获取下标为index的字符
        char c = base.charAt(index);
        //将c放入到StringBuffer中去
        sb.append(c);
    }
    return sb.toString();
}