What is Captcha?

The term CAPTCHA (for Completely Automated Public Turing Test To Tell Computers and Humans Apart) was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas Hopper and John Langford of Carnegie Mellon University.

A CAPTCHA is a program that can tell whether its user is a human or a computer. You've probably seen them — colorful images with distorted text at the bottom of Web registration forms. CAPTCHAs are used by many websites to prevent abuse from "bots," or automated programs usually written to generate spam. No computer program can read distorted text as well as humans can, so bots cannot navigate sites protected by CAPTCHAs.

Example captcha image:

How to add a captcha to JSP(Adding captcha)

A captcha is an image with some modified text on it. They're usually used to prevent automated scripts from accessing certain parts of your site. For example, you'd add a captcha on a wiki edit page or a forum reply page so a spammer doesn't make a bunch of fake content on your site.

This page will teach you how to add a captcha to your site by using reCAPTCHA. reCAPTCHA is a free (and ad-free) widget that will generate captchas for you.

First things first; you will need to sign up for a reCAPTCHA account. Don't worry, this is very straightforward. Sign up for an account here:http://recaptcha.net/whyrecaptcha.html. After you sign up, you will be given a public and a private key. Write these down because you'll need them later.

The second thing you'll want to do is download a reCAPTCHA library that simplifies using the reCAPTCHA API in java. You can download that here:http://code.google.com/p/recaptcha/downloads/list. Download the zip that says "for Java". Extract the jar and put it in your classpath.

Now for some code. Add this to your ActionBean:

public String getReCaptchaHtml() {
  ReCaptcha recaptcha = createReCaptcha();
  return recaptcha.createRecaptchaHtml("You did not type the captcha correctly", new Properties());
}

private ReCaptcha createReCaptcha() {
  String publicKey = //your public key
    String privateKey = //your private key
    return ReCaptchaFactory.newReCaptcha(publicKey, privateKey, true);
}

With this, we can add the reCAPTCHA widget to our JSP like so:

${actionBean.reCaptchaHtml}

But one thing remains: We don't have a way to tell if the user submitted the correct value. Lets assume the method that gets called on submit is named "submit". Here's how you would add a validation method that ensures that the captcha code was entered correctly:

@ValidationMethod(on = "submit")
public void captchaValidation(ValidationErrors errors) {
  ReCaptchaResponse response = createReCaptcha().checkAnswer(context.getRequest().getRemoteAddr(),
          context.getRequest().getParameter("recaptcha_challenge_field"),
          context.getRequest().getParameter("recaptcha_response_field"));
  if (!response.isValid()) {
      errors.add("Captcha", new SimpleError("You didn't type the captcha correctly!"));
  }
}

This will give you a completely functional captcha.