比较好用的验证码库 支持 声音验证码
克隆 https://sourceforge.net/p/simplecaptcha/code/ci/master/tree/

SimpleCaptcha - A Java library for creating CAPTCHAs for J2EE applications,
including Spring MVC, Struts, and Grails.

Example implementations are located in the examples/ directory. The more robust
examples were done using Grails; this is the recommended starting point for
learning how to implement and use SimpleCaptcha.

To build, simply run ant from this directory. The only requirement is that the 
$CATALINA_HOME environment variable be set:

    # set $CATALINA_HOME=/usr/apache-tomcat-6.0.18/
    # ant