JCaptcha - refresh image

Question

Hi,
I'm trying to build a registration page with captcha image, and enables user to change the captcha image by clicking a link.
I tried the open-source JCaptcha project. (5 minutes application integration tutorial http://luminal.gotdns.com/confluence/display/general/5+minutes+application+integration+tutorial)
It worked fine when first load the page. But when I click reload, exception happends:
If I try to reload after ,say 5 mins, the page works again.
java.lang.ClassCastException: java.lang.String
     net.sf.ehcache.Element.equals(Element.java:165)
     java.util.HashMap.eq(Unknown Source)
     java.util.HashMap.removeEntryForKey(Unknown Source)
     java.util.HashMap.remove(Unknown Source)
     net.sf.ehcache.store.MemoryStore.remove(MemoryStore.java:219)
     net.sf.ehcache.Cache.remove(Cache.java:716)
     net.sf.ehcache.Cache.remove(Cache.java:683)
     com.octo.captcha.service.EhcacheManageableCaptchaService.generateAndStoreCaptcha(EhcacheManageableCaptchaService.java:826)
     com.octo.captcha.service.AbstractCaptchaService.getChallengeForID(AbstractCaptchaService.java:538)
     com.octo.captcha.service.image.EhcacheManageableImageCaptchaService.getImageChallengeForID(EhcacheManageableImageCaptchaService.java:505)
     ImageCaptchaServlet.doGet(ImageCaptchaServlet.java:36)
     javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
     javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
I read the example code, found that it takes session id as key to recognize the request. So, it seems to be: when reload attempts, the same session id is sent to the service to acquire an image, but the captcha service doesn't support one session id with different images.
Can anybody guide me to make it work? any other resource can help to solve the problem?

Answer 1

I use current time (System.currentTimeMillis()) instead of session id as a key for captcha image generation, and a new captcha image is generated everytime I relaod the page. :)
However, I don't know how to pass current time from servlet to the request page, so that the request page can laterly send the key back to servlet for captcha validation.
My jsp pages is like:
<form name="LoginPage" action="/jcaptcha/ImageCaptchaServlet" method="get">
<img src="jcaptcha">
<input type='text' name='j_captcha_response' value=''>ImageCaptchaServlet ( copied from http://luminal.gotdns.com/confluence/display/general/5+minutes+application+integration+tutorial)
        //the same id must be used to validate the response, the session id is a good candidate!
//        String captchaId = httpServletRequest.getSession().getId();
          String captchaId = Long.toString(System.currentTimeMillis());     
        // call the ImageCaptchaService getChallenge method
            BufferedImage challenge =                     CaptchaServiceSingleton.getInstance().getImageChallengeForID(captchaId,httpServletRequest.getLocale());
        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();