Captcha Image

Example

Image Service

public class ValidateCodeService {
 static final String AB = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
 static Random rnd = new Random();
 public static String createRandomString(int length) {
  StringBuilder sb = new StringBuilder(length);
  for (int i = 0; i < length; i++)
   sb.append(AB.charAt(rnd.nextInt(AB.length())));
  return sb.toString();
 }

 public static Image getImage(String code) {
  BufferedImage image = new BufferedImage(80, 30,
    BufferedImage.TYPE_INT_RGB);
  Graphics g = image.getGraphics();
  g.setColor(Color.WHITE);
  g.fillRect(0, 0, 80, 30);
  g.setColor(Color.BLACK);
  g.setFont(new Font(null, Font.BOLD, 20));
  g.drawString(code, 0, 20);
  return image;
 }
}

Output servlet

URL: /ValidateCode

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  response.setDateHeader("Expires", -1);
  response.setHeader("Cache-Control","no-cache");
  response.setHeader("Pragma","no-cache");
  response.setHeader("Content-Type", "image/jped");
  String code = ValidateCodeService.createRandomString(4);
  request.getSession().setAttribute("ValidateCode", code);
  Image image = ValidateCodeService.getImage(code);
  ImageIO.write((RenderedImage) image, "jpg", response.getOutputStream());
 }

View

 out.println(
  "Validation Code: <input type='text' name='ValidateCode'/>"
  +"<img id='ValidateCodeImg' src='"+request.getContextPath()+"/ValidateCode'>"
  +"<a href='#' rel='nofollow' title='Refresh Image' onclick=\"document.getElementById('ValidateCodeImg').src = '"+request.getContextPath()+"/ValidateCode?'+Date.now();return false;\">"
    +"Refresh"
         +"</a>"
    +"<br>");