在servlet中生成网页中的验证码:

package com.ctcsys.util;
    import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
import java.util.*;
import java.awt.image.*;
import java.awt.*;
import javax.imageio.*;

public class CodeMakerServlet extends HttpServlet
...{
      //验证码字体
    private Font[] codeFont =
    ...{
        new Font("Algerian", Font.BOLD,25),
  new Font("Vivaldi", Font.BOLD, 25),
        new Font("Broadway", Font.BOLD,25),
  new Font("Forte", Font.BOLD, 25)
    };
        //验证码数字颜色
    private Color[] color =
    ...{
       Color.BLACK, Color.RED, Color.DARK_GRAY, Color.BLUE
    };

    String codeNumbers = "";

    int width = 90, height = 25;

    // 处理 HTTP get 请求
    public void doGet(HttpServletRequest request, HttpServletResponse response)
                      throws ServletException, IOException
    ...{
        // 清空缓冲区
        response.reset();

        // 注意这里的MIME类型
        response.setContentType("image/png");

        // 设置页面不缓存
        response.setHeader("Pragma", "No-cache");
        response.setHeader("Cache-Control", "no-cache");
        response.setDateHeader("Expires", 0);

        // 创建一个 110X40 的图像,验证码显示的图片大小
        BufferedImage image = new BufferedImage(width, height,
                                                BufferedImage.TYPE_INT_RGB);

        // 得到图形环境对象 g
        Graphics g = image.getGraphics();

        // 填充背景
        g.setColor(new Color(64,219,255));
        g.fillRect(0, 0, width, height);

        for (int i = 0; i < 4; i++)
        ...{
            drawCode(g, i);
        }

        drawNoise(g, 17);

        // 绘制边框
        g.setColor(Color.gray);
        g.drawRect(0, 0, width - 1, height - 1);

        // 将验证码内容保存进session中，用于验证用户输入是否正确时使用
        HttpSession session = request.getSession(true);
        session.removeAttribute("codeNumbers");
        session.setAttribute("codeNumbers", codeNumbers);
       
  // 重设字符串
        codeNumbers = "";

        // 利用ImageIO类的write方法对图像进行编码
        ServletOutputStream sos = response.getOutputStream();
        ImageIO.write(image, "PNG", sos);
        sos.close();
    }

    // 绘制验证码
    public void drawCode(Graphics graphics, int i)
    ...{
        int number = (int)(Math.random() * 10);
        graphics.setFont(codeFont[i]);
        graphics.setColor(color[i]);
        graphics.drawString("" + number, 10 + i * 17,22);

        codeNumbers += number;
    }

    // 绘制干扰线
    public void drawNoise(Graphics graphics, int lineNumber)
    ...{
        graphics.setColor(new Color(9,143,134));
        for (int i = 0; i < lineNumber; i++)
        ...{
            int pointX1 = 1 + (int)(Math.random() * width);
            int pointY1 = 1 + (int)(Math.random() * height);
            int pointX2 = 1 + (int)(Math.random() * width);
            int pointY2 = 1 + (int)(Math.random() * height);
            graphics.drawLine(pointX1, pointY1, pointX2, pointY2);
        }
    }
    // 处理 HTTP post 请求, 和doGet一样
    public void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException
    ...{
        doGet(request, response);
    }
}

在要显示验证码位置的页面代码中加入<img id="code" src="/ctcmcdisplay/codeMaker" title="点击刷新验证码"
       style="cursor : pointer;"  align="absmiddle" onclick="return refreshcode()"/>即可，刷新验证码的javascript脚本如下：

<script type="text/javascript">
function refreshcode(){
document.getElementById("code").src="/ctcmcdisplay/codeMaker?update="+Math.random();
return false;
}
</script>

红色部分是存放servlet的功能名称，蓝色部分就是在web.xml中配置的servlet映射，web.xml中的配置代码段：

<!-- 生成验证码的servlet -->
<servlet>
  <servlet-name>CodeMakerServlet</servlet-name>
  <servlet-class>com.ctcsys.util.CodeMakerServlet</servlet-class>
</servlet>
<servlet-mapping>
  <servlet-name>CodeMakerServlet</servlet-name>
  <url-pattern>/codeMaker</url-pattern>
</servlet-mapping>

这样在页面中就可以随机出现验证码了，点击验证码图片的时候也可以刷新验证码。