java验证码组件kaptcha使用方法

<!-- 登陆验证码Kaptcha -->
    <servlet>
        <servlet-name>Kaptcha</servlet-name>
        <servlet-class>com.google.code.kaptcha.servlet.KaptchaServlet</servlet-class>
                   <init-param>
            <param-name>kaptcha.producer.impl</param-name>
            <param-value>com.google.code.kaptcha.impl.DefaultKaptcha </param-value>
        </init-param>
                   <init-param>
                            <param-name>kaptcha.textproducer.font.color</param-name>
                            <param-value>black</param-value>
                   </init-param>
                   <init-param>
                            <param-name>kaptcha.textproducer.char.space</param-name>
                            <param-value>10</param-value>
                   </init-param>
                   <init-param>
                            <param-name>kaptcha.border</param-name>
                            <param-value>no</param-value>
                   </init-param>
                   <init-param>
                            <param-name>kaptcha.border.thickness</param-name>
                            <param-value>0</param-value>
                   </init-param>
                   <init-param>
                            <param-name>kaptcha.textproducer.font.size</param-name>
                            <param-value>35</param-value>
                   </init-param>
                   <init-param>
                            <param-name>kaptcha.textproducer.char.length</param-name>
                            <param-value>4</param-value>
                   </init-param>
                   <init-param>
            <param-name>kaptcha.image.width</param-name>
            <param-value>150</param-value>
        </init-param>
        <init-param>
            <param-name>kaptcha.image.height</param-name>
            <param-value>50</param-value>
        </init-param>
        <init-param>
                            <param-name>kaptcha.background.clear.from</param-name>
                            <param-value>255,255,255</param-value>
                   </init-param>
                   <init-param>
                            <param-name>kaptcha.background.clear.to</param-name>
                            <param-value>255,255,255</param-value>
                   </init-param>
                   <init-param>
            <param-name>kaptcha.textproducer.char.string</param-name>
            <param-value>0123456789</param-value>
        </init-param>
        <init-param>
            <param-name>kaptcha.textproducer.font.names</param-name>
            <param-value>Arial Black</param-value>
        </init-param>
         <init-param>
            <param-name>kaptcha.noise.impl</param-name>
            <param-value>com.google.code.kaptcha.impl.NoNoise</param-value>
        </init-param>
    </servlet>
    <servlet-mapping>
        <servlet-name>Kaptcha</servlet-name>
        <url-pattern>/kaptcha.jpg</url-pattern>
</servlet-mapping>

<img id="img" src="kaptcha.jpg" width="90" height="25" onclick="refresh();" alt="点击更换验证码"/>
function refresh() {
                  document.getElementById('img').src="kaptcha.jpg?"+Math.random();
}

request.getSession().getAttribute(com.google.code.kaptcha.Constants.KAPTCHA_SESSION_KEY);