代码如下所示：
代码如下：

package com.hoo.util;
 

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

import javax.imageio.ImageIO;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 /**
 * <b>function:</b> 验证码生成工具类
 * @project NetWorkService
 * @package com.hoo.util
 * @fileName ValidCodeUtils.java
 * @createDate 2010-8-3 下午03:05:50
 * @author hoojo
 */
 @SuppressWarnings("unused")
 public class ValidCodeUtils {
 /*********************************************************************
  * 验证码宽度
  */
 public static int WIDTH = 60;
 /***
  * 验证码高度
  */
 public static int HEIGHT = 20;

 /**********************************************************************
  * 验证码背景颜色COLOR_FC_BG 应当小于COLOR_BC_BG
  */
 public static int COLOR_FC_BG = 200;
 /***
  * 验证码背景颜色COLOR_FC_BG 应当小于COLOR_BC_BG
  */
 public static int COLOR_BC_BG = 250;

 /**********************************************************************
  * 验证码背景干扰线颜色COLOR_FC_LINE 应当小于COLOR_BC_LINE
  */
 public static int COLOR_FC_LINE = 160;
 /***
  * 验证码背景干扰线颜色COLOR_FC_LINE 应当小于COLOR_BC_LINE
  */
 public static int COLOR_BC_LINE = 200;

 /***************************************************************************
  * 验证码颜色COLOR_FC_CODE 应当小于COLOR_BC_CODE
  */
 public static int COLOR_FC_CODE = 20;
 /***
  * 验证码颜色COLOR_FC_CODE 应当小于COLOR_BC_CODE
  */
 public static int COLOR_BC_CODE = 170;

 /***************************************************************************
  * 生成在指定范围内的颜色
  * @param fc 范围fc color值 小于255
  * @param bc 范围bc color值 小于255
  * @return Color
  */
 private static Color getRandColor(int fc, int bc) {
     Random random = new Random();
     if (fc < 0)
  fc = 0;
     if (bc < 0)
  bc = 1;
     if (fc > 255)
  fc = 255;
     if (bc > 255)
  bc = 255;
     if (bc == fc) 
  bc += 10;
     int temp = 0;
     if (bc < fc) {
  temp = bc;
  bc = fc;
  fc = temp;
     }
     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);
 }

/**
  * <b>function:</b> 生成图片方法
  * @createDate 2010-8-3 下午03:06:22
  * @author hoojo
  * @param request HttpServletRequest
  * @param response HttpServletResponse
  * @return boolean
  * @throws Exception
  */
 public static boolean getImage(HttpServletRequest request, HttpServletResponse response) throws Exception{
     response.reset();
     response.setContentType("image/jpeg");
     // 设置页面不缓存
     response.setHeader("Pragma", "No-cache");
     response.setHeader("Cache-Control", "no-cache");
     response.setDateHeader("Expires", 0);
     // 在内存中创建图象     
     BufferedImage image = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);

    // 获取图形上下文
     Graphics img = image.getGraphics();
     // 生成随机类
     Random random = new Random();

    // 设定背景色
     img.setColor(getRandColor(COLOR_FC_BG, COLOR_BC_BG));
     img.fillRect(0, 0, WIDTH, HEIGHT);

    // 设定字体
     img.setFont(new Font("Times New Roman", Font.PLAIN, 18));

    // 画边框
     // g.setColor(new Color());
     // g.drawRect(0,0,width-1,height-1);

    // 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到
     img.setColor(getRandColor(COLOR_FC_LINE, COLOR_BC_LINE));
     for (int i = 0; i < 155; i++) {
  int x = random.nextInt(WIDTH);
  int y = random.nextInt(HEIGHT);
  int xl = random.nextInt(12);
  int yl = random.nextInt(12);
  img.drawLine(x, y, x + xl, y + yl);
     }

    // 取随机产生的认证码(4位数字)
     String codeValue = "";
     for (int i = 0; i < 4; i++) {
  //String rand = String.valueOf(random.nextInt(10));
  String rand = getRandomChar();
  codeValue = codeValue.concat(rand);
  img.setFont(getRandomFont());//随机字体
  // 将认证码显示到图象中
  img.setColor(getRandColor(COLOR_FC_CODE, COLOR_BC_CODE));
  img.drawString(rand, 13 * i + 6, 16);
     }
     request.getSession().setAttribute("codeValue", codeValue);
     // 图象生效
     img.dispose();
     // 输出图象到页面
     return ImageIO.write(image, "JPEG", response.getOutputStream());
 }

 /**
  * 随机生成字符，含大写、小写、数字
  * <b>function:</b> 功能
  * @createDate 2010-8-23 上午10:33:55
  * @author hoojo
  * @return
  */
 public static String getRandomChar() {
     int index = (int) Math.round(Math.random() * 2);
     String randChar = "";
     switch (index) {
     case 0://大写字符
  randChar = String.valueOf((char)Math.round(Math.random() * 25 + 65));
  break;
     case 1://小写字符
  randChar = String.valueOf((char)Math.round(Math.random() * 25 + 97));
  break;
     default://数字
  randChar = String.valueOf(Math.round(Math.random() * 9));
  break;
     }
     return randChar;
 }

 /**
  * <b>function:</b> 随机生成字体、文字大小
  * @createDate 2010-8-23 上午10:44:22
  * @author hoojo
  * @return
  */
 public static Font getRandomFont() {
     String[] fonts = {"Georgia", "Verdana", "Arial", "Tahoma", "Time News Roman", "Courier New", "Arial Black", "Quantzite"};
     int fontIndex = (int)Math.round(Math.random() * (fonts.length - 1));
     int fontSize = (int) Math.round(Math.random() * 4 + 16);
     return new Font(fonts[fontIndex], Font.PLAIN, fontSize);
 }
 }

其中验证码的值是保存在session中：request.getSession().setAttribute("codeValue", codeValue);
 比较用户输入的值和session中的codeValue是否相等即可；

下面是jsp页面调用servlet：ValidCodeServlet.java

ValidCodeServlet中调用了上面的ValidCodeUtils 验证码生成工具类

package com.hoo.servlet;

import java.io.IOException;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;

import com.hoo.util.ValidCodeUtils;

@SuppressWarnings("serial")
 public class ValidCodeServlet extends HttpServlet {

 public void doGet(HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException {
     try {
  ValidCodeUtils.getImage(request, response);
     } catch (Exception e) {
  e.printStackTrace();
     }
 }

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

 

jsp页面调用servlet方法即可

 

js：reloadValidCode方法

function reloadValidCode(o) {
 o.src = "${pageContext.request.contextPath }/validCodeServlet?timed=" + new Date().getMilliseconds();
 }
 这里的"timed=" + new Date().getMilliseconds();是需要防止IE缓存用的

html标签：
 <img src="${pageContext.request.contextPath }/validCodeServlet" onclick="reloadValidCode(this)"/>
 直接跟Servlet名称配置的url即可，和web.xml配置对应。主要调用路径${pageContext.request.contextPath }/validCodeServlet这样会带上根目录，比较保险。

web.xml中validCodeServlet配置
 <servlet>
 <servlet-name>validCodeServlet</servlet-name>
 <servlet-class>com.hoo.servlet.ValidCodeServlet</servlet-class>
 </servlet>

<servlet-mapping>
 <servlet-name>validCodeServlet</servlet-name>
 <url-pattern>/validCodeServlet</url-pattern>
 </servlet-mapping>