http://www.google.com/recaptcha | https://developers.google.com/recaptcha

configuring_recaptcha.txt

1 - Create your private and public key for your domain: https://www.google.com/recaptcha/admin/create

2 - Displaying the reCAPTCHA Widget: https://developers.google.com/recaptcha/docs/display
  2.1 - Once configured the script and iframe you need to submit the value of fields to your servlet:
        - recaptcha_response_field
        - recaptcha_challenge_field

3 - Download and setup de recaptcha lib into your java project: https://code.google.com/p/recaptcha/downloads/list?q=label:java-Latest

4 - Implementing the server side recaptcha validation: 
      4.1 - See the ReCaptchaUtil.java: https://gist.github.com/fforbeck/7137371#file-recaptchautil-java
      4.2 - Here you can find how to verifying the solution: https://developers.google.com/recaptcha/docs/verify

ReCaptchaUtil.java

import static org.apache.commons.lang.StringUtils.isBlank;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.log4j.Logger;

import net.tanesha.recaptcha.ReCaptchaImpl;
import net.tanesha.recaptcha.ReCaptchaResponse;

public class ReCaptchaUtil {
	
	private Logger logger = Logger.getLogger(ReCaptchaUtil.class);
	
	public boolean isValidCaptcha(HttpServletRequest request, HttpServletResponse response) {
		
		try {
			//TODO send the error code to the client
			final String challenge = request.getParameter("recaptcha_challenge_field");
			final String challengeResponse = request.getParameter("recaptcha_response_field");
			final String remoteAddr = request.getRemoteAddr();
			if (isBlank(challenge) || isBlank(challengeResponse) || isBlank(remoteAddr)) {
				logger.error("Invalid captcha data { challenge: " + challenge + ", response: " + response + ", address: " + remoteAddr + "}.");
				return false;
			}
			
			final ReCaptchaImpl reCaptcha = new ReCaptchaImpl();
			reCaptcha.setPrivateKey(Properties.getProperty("recaptcha.private.key"));

			final ReCaptchaResponse reCaptchaResponse = reCaptcha.checkAnswer(remoteAddr, challenge, challengeResponse);  
	        
			if (reCaptchaResponse != null && reCaptchaResponse.isValid())
				return true;
			
			logger.error(reCaptchaResponse.getErrorMessage());
			
			return false;
			
		} catch (Exception e) {
			logger.error(e.getMessage(), e);
			return false;
		}
	}

}