安装

Add jcaptcha-all.jar (provided in bin-distribution) and ehcache.jar (not provided see ehcache site) to your application class path, ie in you WEB-INF/lib folder.

实例一个jcaptcha服务，注意，必须是单例模式的
import com.octo.captcha.service.image.ImageCaptchaService;
import com.octo.captcha.service.image.DefaultManageableImageCaptchaService;

public class CaptchaServiceSingleton {
   
    private static ImageCaptchaService instance = new DefaultManageableImageCaptchaService();
   
    public static ImageCaptchaService getInstance(){
        return instance;
    }
}
注:以上是默认的一个实现，下面是其他更多的实现

SimpleListSoundCaptchaEngine   //还可以用声音，真爽哦
SpellerSoundCaptchaEngine
SpellerSoundCaptchaEngine
DefaultGimpyEngineCaptcha           
BaffleListGimpyEngineCaptcha           
BasicListGimpyEngineCaptcha           
DeformedBaffleListGimpyEngineCaptcha           
DoubleRandomListGimpyEngineCaptcha           
SimpleListImageCaptchaEngineCaptcha           
SimpleFishEyeEngineCaptcha
具体请参考官方说明

编写一个产生图片的servlet

import com.octo.captcha.service.CaptchaServiceException;
import com.sun.image.codec.jpeg.JPEGCodec;
import com.sun.image.codec.jpeg.JPEGImageEncoder;

import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.IOException;

public class ImageCaptchaServlet extends HttpServlet {

    public void init(ServletConfig servletConfig) throws ServletException {

        super.init(servletConfig);

    }

    protected void doGet(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse) throws ServletException, IOException {
       
       byte[] captchaChallengeAsJpeg = null;
       // the output stream to render the captcha image as jpeg into
        ByteArrayOutputStream jpegOutputStream = new ByteArrayOutputStream();
        try {
        // get the session id that will identify the generated captcha.
        //the same id must be used to validate the response, the session id is a good candidate!
        String captchaId = httpServletRequest.getSession().getId();
        // call the ImageCaptchaService getChallenge method
            BufferedImage challenge =
                    CaptchaServiceSingleton.getInstance().getImageChallengeForID(captchaId,
                            httpServletRequest.getLocale());
           
            // a jpeg encoder
            JPEGImageEncoder jpegEncoder =
                    JPEGCodec.createJPEGEncoder(jpegOutputStream);
            jpegEncoder.encode(challenge);
        } catch (IllegalArgumentException e) {
            httpServletResponse.sendError(HttpServletResponse.SC_NOT_FOUND);
            return;
        } catch (CaptchaServiceException e) {
            httpServletResponse.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
            return;
        }

        captchaChallengeAsJpeg = jpegOutputStream.toByteArray();

        // flush it in the response
        httpServletResponse.setHeader("Cache-Control", "no-store");
        httpServletResponse.setHeader("Pragma", "no-cache");
        httpServletResponse.setDateHeader("Expires", 0);
        httpServletResponse.setContentType("image/jpeg");
        ServletOutputStream responseOutputStream =
                httpServletResponse.getOutputStream();
        responseOutputStream.write(captchaChallengeAsJpeg);
        responseOutputStream.flush();
        responseOutputStream.close();
    }
}

为servlet修改web.xml配置文件
<servlet>
        <servlet-name>jcaptcha</servlet-name>
        <servlet-class>ImageCaptchaServlet</servlet-class>
        <load-on-startup>0</load-on-startup>
    </servlet>

<servlet-mapping>
        <servlet-name>jcaptcha</servlet-name>
        <url-pattern>/jcaptcha</url-pattern>
</servlet-mapping>

编写你的客户端的展示
<img src="jcaptcha">
<input type='text' name='j_captcha_response' value=''>
上面的src="jcaptcha"  就是调用了上面的servlet,text里是用户填写的确认验证码

后台逻辑验证
Boolean isResponseCorrect =Boolean.FALSE;
           //remenber that we need an id to validate!
           String captchaId = httpServletRequest.getSession().getId();
           //retrieve the response
           String response = httpServletRequest.getParameter("j_captcha_response");
           // Call the Service method
            try {
                isResponseCorrect = CaptchaServiceSingleton.getInstance().validateResponseForID(captchaId,
                        response);
            } catch (CaptchaServiceException e) {
                 //should not happen, may be thrown if the id is not valid
            }