Below is the example for Google Recaptcha validation using jquery(AJAX).
What is reCAPTCHA?
reCAPTCHA is a free CAPTCHA service that protects your site against spam, malicious registrations and other forms of attacks where computers try to disguise themselves as a human; a CAPTCHA is a Completely Automated Public Turing test to tell Computers and Human Apart. reCAPTCHA comes in the form of a widget that you can easily add to your blog, forum, registration form, etc.
To know more about Google recaptcha, please see below links
http://www.google.com/recaptcha
https://developers.google.com/recaptcha/
Please see self explanatory JSP code below. To run below example add recaptcha4j-0.0.7.jar file in your applications classpath.
test.jsp
validateRecaptcha.jsp
Output