kaptcha工作的原理是调用com.google.code.kaptcha.servlet.KaptchaServlet，生成一个图片。同时将生成的验证码字符串放到HttpSession中。
kaptcha可以配置一下信息：
验证码的字体
验证码字体的大小
验证码字体的字体颜色
验证码内容的范围(数字，字母，中文汉字！)
验证码图片的大小，边框，边框粗细，边框颜色
验证码的干扰线(可以自己继承com.google.code.kaptcha.NoiseProducer写一个自定义的干扰线)
验证码的样式(鱼眼样式、3D、普通模糊……当然也可以继承com.google.code.kaptcha.GimpyEngine自定义样式)
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
version="2.4">
<servlet>
<servlet-name>Kaptcha</servlet-name>
<servlet-class>com.google.code.kaptcha.servlet.KaptchaServlet</servlet-class>
<init-param>
<description> Border around kaptcha. Legal values are yes or no. </description>
<param-name>kaptcha.border</param-name>
<param-value>no</param-value>
</init-param>
<init-param>
<description>Color of the border. Legal values are r,g,b (and optional alpha) or white,black,blue. </description>
<param-name>kaptcha.border.color</param-name>
<param-value>red</param-value>
</init-param>
<init-param>
<description>Thickness of the border around kaptcha. Legal values are > 0. </description>
<param-name>kaptcha.border.thickness</param-name>
<param-value>5</param-value>
</init-param>
<init-param>
<description>Width in pixels of the kaptcha image. </description>
<param-name>kaptcha.image.width</param-name>
<param-value>80</param-value>
</init-param>
<init-param>
<description>Height in pixels of the kaptcha image. </description>
<param-name>kaptcha.image.height</param-name>
<param-value>40</param-value>
</init-param>
<init-param>
<description>The image producer. </description>
<param-name>kaptcha.producer.impl</param-name>
<param-value>com.google.code.kaptcha.impl.DefaultKaptcha </param-value>
</init-param>
<init-param>
<description>The text producer. </description>
<param-name>kaptcha.textproducer.impl</param-name>
<param-value>com.google.code.kaptcha.text.impl.DefaultTextCreator</param-value>
</init-param>
<init-param>
<description>The characters that will create the kaptcha. </description>
<param-name>kaptcha.textproducer.char.string</param-name>
<param-value>abcde2345678gfynmnpwx </param-value>
</init-param>
<init-param>
<description>The number of characters to display. </description>
<param-name>kaptcha.textproducer.char.length</param-name>
<param-value>5</param-value>
</init-param>
<init-param>
<description>A list of comma separated font names.</description>
<param-name>kaptcha.textproducer.font.names</param-name>
<param-value>Arial, Courier</param-value>
</init-param>
<init-param>
<description>The size of the font to use. </description>
<param-name>kaptcha.textproducer.font.size</param-name>
<param-value>23</param-value>
</init-param>
<init-param>
<description>The color to use for the font. Legal values are r,g,b. </description>
<param-name>kaptcha.textproducer.font.color</param-name>
<param-value>black</param-value>
</init-param>
<init-param>
<description>The noise producer. </description>
<param-name>kaptcha.noise.impl</param-name>
<param-value>com.google.code.kaptcha.impl.NoNoise </param-value>
</init-param>
<init-param>
<description>The noise color. Legal values are r,g,b. </description>
<param-name>kaptcha.noise.color</param-name>
<param-value>black</param-value>
</init-param>
<init-param>
<description>The obscurificator implementation. </description>
<param-name>kaptcha.obscurificator.impl</param-name>
<param-value>com.google.code.kaptcha.impl.ShadowGimpy</param-value>
</init-param>
<init-param>
<description>The background implementation. </description>
<param-name>kaptcha.background.impl</param-name>
<param-value>com.google.code.kaptcha.impl.DefaultBackground</param-value>
</init-param>
<init-param>
<description>Ending background color. Legal values are r,g,b. </description>
<param-name>kaptcha.background.clear.to</param-name>
<param-value>white</param-value>
</init-param>
<init-param>
<description>The word renderer implementation. </description>
<param-name>kaptcha.word.impl</param-name>
<param-value>com.google.code.kaptcha.text.impl.DefaultWordRenderer</param-value>
</init-param>
<init-param>
<description>The value for the kaptcha is generated and is put into the HttpSession. This is the key value for that item in the session. </description>
<param-name>kaptcha.session.key</param-name>
<param-value>KAPTCHA_SESSION_KEY</param-value>
</init-param>
<init-param>
<description>The date the kaptcha is generated is put into the HttpSession. This is the key value for that item in the session. </description>
<param-name>kaptcha.session.date</param-name>
<param-value>KAPTCHA_SESSION_DATE</param-value>
</init-param>
</servlet>
<servlet-mapping>
<servlet-name>Kaptcha</servlet-name>
<url-pattern>/Kaptcha.jpg</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>KaptchaExample.jsp</welcome-file>
</welcome-file-list>
</web-app>
详细信息请看下面的web.xml文件
下面介绍一下用法:
1.首先去官网下载jar:http://code.google.com/p/kaptcha/
2.建立一个web项目，导入kaptcha-2.3.jar到环境变量中。
3.配置web.xml文件
其实就是配置com.google.code.kaptcha.servlet.KaptchaServlet