1：首先实现一个servlet用来生成图片（当然也可以用jsp实现）：
/*
 * ClassName: ValidateCode.java
 * Author: qiujy
 * Version: v1.0
 * CreateDate: 2005-12-30
 *
 * Copyright 2005-2006 www.mobstar.cn All rights reserved.
 */
package com.mob.common;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Date;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ValidateCode extends HttpServlet {

 // Process the HTTP Get request

 public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  // 设置页面不缓存
  response.setHeader("Pragma", "No-cache");
  response.setHeader("Cache-Control", "no-cache");
  response.setDateHeader("Expires", 0);

  // 设置图片的长宽
  int width = 62, height = 22;

  // ////// 创建内存图像
  BufferedImage image = new BufferedImage(width, height,
    BufferedImage.TYPE_INT_RGB);
  // 获取图形上下文
  Graphics g = image.createGraphics();

  // 设定图像背景色(因为是做背景，所以偏淡)
  g.setColor(getRandColor(180, 250));
  g.fillRect(0, 0, width, height);
  // 设置字体
  g.setFont(new Font("Times New Roman", Font.PLAIN, 17));

  // /////设置默认生成4个验证码
  int length = 4;
  java.util.Random rand = new Random(); // 设置随机种子

  /*
   * if (request.getParameter("length") != null) { try { length =
   * Integer.parseInt(request.getParameter("length")); }catch
   * (NumberFormatException e) {} }
   */
  // 设置备选验证码:包括"a-z"和数字"0-9"
  String base = "abcdefghijklmnopqrstuvwxyz0123456789";
  int size = base.length();
  StringBuffer str = new StringBuffer();
  for (int i = 0; i < length; i++) {
   int start = rand.nextInt(size);
   String tmpStr = base.substring(start, start + 1);

   str.append(tmpStr);
   // 生成随机颜色(因为是做前景，所以偏深)
   g.setColor(getRandColor(10, 150));

   // 将此字画到图片上
   // g.drawString(str.toString(), 4, 17);
   g.drawString(tmpStr, 13 * i + 6 + rand.nextInt(5), 14 + rand
     .nextInt(6));

  }

  // 将认证码存入session
  request.getSession().setAttribute("valiCode", str.toString());

  // 图象生效
  g.dispose();

  // 输出图象到页面
  ImageIO.write(image, "JPEG", response.getOutputStream());

 }

 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  this.doGet(request, response);
 }

 // 给定范围获得一个随机颜色
 Color getRandColor(int fc, int bc) {
  Random random = new Random();
  if (fc > 255)
   fc = 255;
  if (bc > 255)
   bc = 255;
  int r = fc + random.nextInt(bc - fc);
  int g = fc + random.nextInt(bc - fc);
  int b = fc + random.nextInt(bc - fc);
  return new Color(r, g, b);
 }

}

2：在web.xml中做如下配置
 <!-- validate code maker begin -->
 <servlet>
  <servlet-name>CodeMakerServlet</servlet-name>
  <servlet-class>com.mob.common.ValidateCode</servlet-class>
 </servlet>
 <servlet-mapping>
  <servlet-name>CodeMakerServlet</servlet-name>
  <url-pattern>/codeMaker</url-pattern>
 </servlet-mapping>
  <!--　validate code maker end -->

3：然后在需要显示验证码的地方加入以下代码就可以了

<img alt="" src="/codeMaker" width="60" height="22"/>

4：若要验证是否填写正确：可如下做法

　在输入页面加入如下代码

<tr>
          <td align="right">输入验证码:&nbsp;&nbsp;</td>
          <td><input type="text" name="check_num" id="check_num" size="10" maxlength="10">&nbsp;
           <img src="/blog/codeMaker"/>
           <!--<img alt="" src="/blog/com/mob/common/ValidateCode" width="40" height="16"/>-->
           </td>
</tr> 

在处理请求的java类（如struts中的action中）中只需要这样做这要可以了：

String valiCode = null;
  Object obj = request.getSession().getAttribute("valiCode");
  if (obj != null && !obj.equals("")) {
   valiCode = obj.toString();
  }

  String check_num = loginForm.getCheck_num();
  if (check_num != null && !check_num.equals("") && !check_num.trim().equals(valiCode)) {
   request
     .setAttribute("errorMessage",
       "请注意正确输入登录验证码!");
   return mapping.getInputForward();
  }

最终效果如下：