JS图片验证码换一张不显示

开始能显示](https://img-ask.csdn.net/upload/201904/12/1555048415_771910.jpg)

代码

class="verifyimg"
src="${pageContext.request.contextPath}/UserServlet?method=CheckCode" />
换一张

            function changeImg() {
    var img = document.getElementById("imgId");
    img.src = "${pageContext.request.contextPath}/UserServlet?method=CheckCode?" + new Date();
}

后台代码
public void CheckCode(HttpServletRequest req, HttpServletResponse resp) throws IOException {

    // 使用java图形界面技术绘制一张图片

    int charNum = 4;
    int width = 30 * 4;
    int height = 30;

    // 1. 创建一张内存图片
    BufferedImage bufferedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

    // 2.获得绘图对象
    Graphics graphics = bufferedImage.getGraphics();

    // 3、绘制背景颜色
    graphics.setColor(Color.YELLOW);
    graphics.fillRect(0, 0, width, height);

    // 4、绘制图片边框
    graphics.setColor(Color.BLUE);
    graphics.drawRect(0, 0, width - 1, height - 1);

    // 5、输出验证码内容
    graphics.setColor(Color.RED);
    graphics.setFont(new Font("宋体", Font.BOLD, 20));

    // 随机输出4个字符
    Graphics2D graphics2d = (Graphics2D) graphics;
    String s = "ABCDEFGHGKLMNPQRSTUVWXYZ23456789";
    Random random = new Random();
    // session中要用到
    String msg = "";
    int x = 5;
    for (int i = 0; i < 4; i++) {
        int index = random.nextInt(32);
        String content = String.valueOf(s.charAt(index));
        msg += content;
        double theta = random.nextInt(45) * Math.PI / 180;
        // 让字体扭曲
        graphics2d.rotate(theta, x, 18);
        graphics2d.drawString(content, x, 18);
        graphics2d.rotate(-theta, x, 18);
        x += 30;
    }

    // 6、绘制干扰线
    graphics.setColor(Color.GRAY);
    for (int i = 0; i < 5; i++) {
        int x1 = random.nextInt(width);
        int x2 = random.nextInt(width);

        int y1 = random.nextInt(height);
        int y2 = random.nextInt(height);
        graphics.drawLine(x1, y1, x2, y2);
    }

    // 释放资源
    graphics.dispose();

    // 图片输出 ImageIO
    ImageIO.write(bufferedImage, "jpg", resp.getOutputStream());

}

给url带一个随机数

可能是网的问题，重新刷新试试

添加了点击事件了么？