========================jsp==============================
 function changeImg(){
   var a=document.getElementById("mySpan");

//最好在"/ImageServlet?now='+new Date()+'"/>否則有些瀏覽器支援的不夠好
   a.innerHTML='<img align="middle" src="ImageServlet"/>  <a href="#" onclick="changeImg();">看不清，換一張！</a>'; //會覆蓋“mySpan”位置
  }

<td width="350"><div id="kan"  style="text-align:left;">
  <span id="mySpan"><img alt="點選更換驗證碼" align="middle" title="點選更換驗證碼"  src="ImageServlet" /><a  href="#" onclick="changeImg();">看不清，換一張！</a> </span>
  </div></td>
  </tr>

=====================web.xml=================
<servlet>
    <servlet-name>ImageServlet</servlet-name>
    <servlet-class>com.e8online2.comm.util.IdentifyingServlet</servlet-class>
  </servlet>
   <servlet-mapping>
    <servlet-name>ImageServlet</servlet-name>
    <url-pattern>/ImageServlet</url-pattern>
  </servlet-mapping>

/=======================最後從session中獲取原來的驗證碼與jsp中上傳的驗證碼比較就可以了=================

package com.e8online2.comm.util;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
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 IdentifyingServlet extends HttpServlet {

 /**
  *
  */
 private static final long serialVersionUID = 1L;

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

  response.setContentType("image/jpeg");

  // 給定範圍獲得隨機顏色
  // 設定頁面不快取
  response.setHeader("Pragma", "No-cache");
  response.setHeader("Cache-Control", "no-cache");
  response.setDateHeader("Expires", 0);

  // 在記憶體中建立圖象
  int width = 60, height = 17;
  BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

  // 獲取圖形上下文
  Graphics g = image.getGraphics();

  // 生成隨機類
  Random random = new Random();

  // 設定背景色
  g.setColor(getRandom(254, 255));
  g.fillRect(0, 0, width, height);

  // 設定字型
  g.setFont(new Font("Cooper Black", Font.PLAIN, 18));

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

  // 隨機產生155條幹擾線，使圖象中的認證碼不易被其它程式探測到
  g.setColor(getRandom(230, 240));
  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);
   g.drawLine(x, y, x + xl, y + yl);
  }

  // 取隨機產生的認證碼(4位數字)
  String sRand = "";
  for (int i = 0; i < 4; i++) {
   String rand = String.valueOf(random.nextInt(10));
   sRand += rand;
   // 將認證碼顯示到圖象中
   g.setColor(new Color(100 + random.nextInt(130), 100 + random.nextInt(130), 100 + random.nextInt(130)));// 呼叫函數出來的顏色相同，可能是因為種子太接近，所以只能直接生成
   g.drawString(rand, 13 * i + 6, 14);
  }

  // 將認證碼存入SESSION
  request.getSession().setAttribute("IdentifyingCode", sRand);

  // 圖象生效
  g.dispose();

  // 輸出圖象到頁面
  ImageIO.write(image, "JPEG", response.getOutputStream());
 }

 private Color getRandom(int mixValue, int maxValue) {
  Random random = new Random();
  if (mixValue > 255)
   mixValue = 255;
  if (maxValue > 255)
   maxValue = 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);
  int rgb = mixValue + random.nextInt(maxValue - mixValue);
  return new Color(rgb, rgb, rgb);
 }

}