CAPTCHA Image Code - JAVA, JSP, AJAX

CAPTCHA - Completely Automated Public Turing test to tell Computers and Humans Apart
A CAPTCHA is used to ensure that the response is not generated by a computer, means response from human. It is used to distinguish between human and bots.

HTML Form Code

<script>
 function resetCaptcha()
 {
  document.getElementById('imgCaptcha').src = 'captchaimage.aspx?' + Math.random();
 }
</script>
<div>
<img src="captchaimage.jsp" id="imgCaptcha">
<img src="refresh.jpeg" border=0 id="imgRefresh" onclick="setTimeout('resetCaptcha()', 300); return false;">
</div>

JSP CODE - captchaimage.jsp

<%@ page import="java.util.*, java.io.*,java.awt.*,java.awt.image.*,javax.imageio.*,java.awt.geom.*"%>
<%
 response.setContentType("image/jpg");
  /* Define number characters contains the captcha image, declare global */
  int iTotalChars= 6;

  /* Size image iHeight and iWidth, declare globl */
  int iHeight=40;
  int iWidth=170;

  /* font style */
  Font fntStyle1 = new Font("Arial", Font.BOLD, 30);
  Font fntStyle2 = new Font("Verdana", Font.BOLD, 20);

  /* Possible random characters in the image */
  Random randChars = new Random();
  String sImageCode = (Long.toString(Math.abs(randChars.nextLong()), 36)).substring(0,iTotalChars);

  /* BufferedImage is used to create a create new image*/
  /* TYPE_INT_RGB - does not support transpatency, TYPE_INT_ARGB - support transpatency*/
  BufferedImage biImage = new BufferedImage(iWidth, iHeight, BufferedImage.TYPE_INT_RGB);
  Graphics2D g2dImage = (Graphics2D) biImage.getGraphics();

  // Draw background rectangle and noisey filled round rectangles
  int iCircle = 15;
  g2dImage.fillRect(0, 0, iWidth, iHeight);
  for ( int i = 0; i < iCircle; i++ )
  {
    g2dImage.setColor(new Color(randChars.nextInt(255),randChars.nextInt(255),randChars.nextInt(255)));
    int iRadius = (int) (Math.random() * iHeight / 2.0);
    int iX = (int) (Math.random() * iWidth - iRadius);
    int iY = (int) (Math.random() * iHeight - iRadius);
    g2dImage.fillRoundRect(iX, iY, iRadius * 2, iRadius * 2,100,100);
  }
  g2dImage.setFont(fntStyle1);
  for ( int i = 0; i < iTotalChars; i++ )
  {
    g2dImage.setColor(new Color(randChars.nextInt(255),randChars.nextInt(255),randChars.nextInt(255)));
    if (i%2==0)
      g2dImage.drawString(sImageCode.substring(i,i+1),25*i,24);
    else
      g2dImage.drawString(sImageCode.substring(i,i+1),25*i,35);
  }

  /* create jpeg image and display on the screen*/
  OutputStream osImage = response.getOutputStream();
  ImageIO.write(biImage, "jpeg", osImage);
  osImage.close();

  /* Dispose function is used destory an image object */
  g2dImage.dispose();

  session.setAttribute("dns_security_code", sImageCode);
%>

Use AJAX code to Reload CAPTCHA image in the form. Once form submitted,use JavaScript to validate CAPTCHA image text value is not empty. Then use JSP Or Servlet code to validate user input CAPTCHA image text value and SESSION value, if it both are same, then process further oterwise throw verification is not valid in the error message.