java web验证码功能实现

<servlet>           <!-- 验证码功能 -->        <servlet-name>KaptchaServlet</servlet-name>        <servlet-class>com.google.code.kaptcha.servlet.KaptchaServlet</servlet-class>        <init-param>    <!-- 定义是否存在有边框 -->            <param-name>kaptcha.border</param-name>            <param-value>no</param-value>        </init-param>        <init-param>    <!-- 定义边框颜色 -->            <param-name>kaptcha.border.color</param-name>            <param-value>105,179,90</param-value>        </init-param>        <init-param>    <!-- 设置字体颜色 -->            <param-name>kaptcha.textproducer.font.color</param-name>            <param-value>red</param-value>        </init-param>        <init-param>    <!-- 设置生成图片宽度 -->            <param-name>kaptcha.image.width</param-name>            <param-value>130</param-value>        </init-param>        <init-param>    <!-- 设置生成图片高度 -->            <param-name>kaptcha.image.height</param-name>            <param-value>38</param-value>        </init-param>        <init-param>    <!-- 设置验证码显示文字尺寸 -->            <param-name>kaptcha.textproducer.font.size</param-name>            <param-value>30</param-value>        </init-param>        <init-param>    <!-- 设置验证码保存到session中的属性名称 -->            <param-name>kaptcha.session.key</param-name>            <param-value>rand</param-value>        </init-param>        <init-param>    <!-- 设置字符间距 -->            <param-name>kaptcha.textproducer.char.space</param-name>            <param-value>2</param-value>        </init-param>        <init-param>    <!-- 设置生成的验证码长度 -->            <param-name>kaptcha.textproducer.char.length</param-name>            <param-value>4</param-value>        </init-param>        <init-param>    <!-- 设置使用的字体 -->            <param-name>kaptcha.textproducer.font.names</param-name>            <param-value>宋体,楷体,微软雅黑</param-value>        </init-param>    </servlet>    <servlet-mapping>        <servlet-name>KaptchaServlet</servlet-name>        <url-pattern>/captcha.jpg</url-pattern>    </servlet-mapping>