Recommendations for java captcha libraries

What happens when ReCaptcha is down/unavailable? Does your service simply stop? Do you simply stop signing people up when it's down? Do you allow users to sign up even if ReCaptcha isn't running? If so, what are the security implications of this? Especially if you use CAPTCHA for more than just signup, e.g. reset password forms, login forms, ... which would not be acceptable to use without the CAPTCHA component.

The Java world of CAPTCHAs is in a sad state, with SimpleCaptcha seemingly the best solution for those of us out there that cannot accept a hosted service.

ReCaptcha is the only captcha you should use, because it's the only captcha that makes the world better (improve OCR results to old text), with almost unlimited database.

All other captchas are usually limited by its database, or do nothing good to this world.

EDIT :: I found steps how to implement captcha using recaptcha.

You can check both Online and Offline captcha using java here

I am the author of SimpleCaptcha. While I would recommend -- for humanity's sake -- using ReCaptcha where you can, I provided SimpleCaptcha because some organizations have policies which prohibit libraries like ReCaptcha. SimpleCaptcha is meant to be entirely stand-alone, with no external dependencies: as long as you are in a J2EE container, you should be good.

Also, SimpleCaptcha is now available for either Java 1.5 or Java 6.