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
Please see self explanatory JSP code below. To run below example add recaptcha4j-0.0.7.jar file in your applications classpath.