jcaptcha

web.xml:
    <servlet>
        <servlet-name>jcaptcha</servlet-name>
        <servlet-class>com.octo.captcha.module.servlet.image.SimpleImageCaptchaServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>jcaptcha</servlet-name>
        <url-pattern>/jcaptcha.jpg</url-pattern>
    </servlet-mapping>

web-inf/lib:
commons-collections-3.2
commons-logging-1.0.4
filters-2.0.235
jcaptcha-2.0-alpha-1-SNAPSHOT
jcaptcha-api-1.0
jcaptcha-integration-simple-servlet-2.0-alpha-1-SNAPSHOT

jsp:
<script type="text/javascript">
    function refresh() {
        var image = document.getElementById("jcaptcha.jpg");
        image.src = "jcaptcha.jpg?" + Math.floor(Math.random() * 1000)
    }
</script>
...
<img src="jcaptcha.jpg" id="jcaptcha.jpg" />
<input type="text" name="jcaptcha" value="" class="required" />
<input type="button" value="更換驗證碼" onClick="javascipt:refresh()">

servlet check:
String userCaptchaResponse = request.getParameter("jcaptcha");
boolean captchaPassed = SimpleImageCaptchaServlet.validateResponse(request, userCaptchaResponse);
if(captchaPassed){
// proceed to submit action
}else{
// return error to user
}

Reference
https://jcaptcha.atlassian.net/wiki/display/general/Simple+Servlet+Integration+documentation