int width = 100;
int height = 50;

BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

Graphics g = image.getGraphics();
g.setColor(Color.WHITE);
g.fillRect(0, 0, width, height);
g.setColor(Color.BLACK);
g.drawRect(0, 0, width - 1, height - 1);

//字符集
String str = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefgjijklmnopqrstuvwxyz";

//随机数
Random ran = new Random();

private Color getRandomColor(Random random) {
  //获取随机颜色
  int colorIndex = random.nextInt(3);
  switch (colorIndex) {
    case 0:
      return Color.BLUE;
    case 1:
      return Color.GREEN;
    case 2:
      return Color.RED;
    case 3:
      return Color.YELLOW;
    default:
      return Color.MAGENTA;
  }
}

//绘制验证码
for (int i = 0; i < 4; i++) {
  //获取随机字符
  int index = ran.nextInt(str.length());
  char ch = str.charAt(index);
  //获取随机色
  Color randomColor = getRandomColor(ran);
  g.setColor(randomColor);
  //设置字体
  Font font = new Font("宋体", Font.BOLD, height / 2);
  g.setFont(font);
  //写入验证码
  g.drawString(ch + "", (i == 0) ? width / 4 * i + 2 : width / 4 * i, height - height / 4);
}

//干扰线
for (int i = 0; i < 10; i++) {
  int x1 = ran.nextInt(width);
  int x2 = ran.nextInt(width);
  int y1 = ran.nextInt(height);
  int y2 = ran.nextInt(height);
  Color randomColor = getRandomColor(ran);
  g.setColor(randomColor);
  g.drawLine(x1, x2, y1, y2);
}

ImageIO.write(image, "jpg", resp.getOutputStream());

看不清，换一张

//点击图片时
var img = document.getElementById("identcode");
img.onclick = function (){
  refesh();
}

//点击连接时
var a = document.getElementById("refesh");
a.onclick = function (){
  refesh();
  //返回false防止a标签默认href行为
  return false;
}

function refesh() {
  /**
   * 由于路径相同时浏览器会自动调用缓存中的图片
   * 所以在连接后加时间戳解决此问题
   */
  var date = new Date().getTime();
  img.src = "identcode?" + date;
}