import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.util.Random;

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import javax.imageio.*;

public class RandomCodeServlet extends HttpServlet ...{

/** *//**
* Constructor of the object.
*/
public RandomCodeServlet() ...{
super();
}

//驗證碼圖片的寬度
private int width=60;
//驗證碼圖片的高度
private int height=20;

protected void service(HttpServletRequest request,HttpServletResponse response)
throws ServletException,java.io.IOException...{
BufferedImage buffImg=new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
Graphics2D g=buffImg.createGraphics();

//創建一個隨機數生成器
Random random=new Random();

g.setColor(Color.WHITE);
g.fillRect(0, 0, width, height);

//創建字體，字體的大小應該根據圖片的高度來定
Font font=new Font("Times New Roman",Font.PLAIN,18);
//設置字體
g.setFont(font);

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

//隨機產生160條干擾線
g.setColor(Color.GRAY);
for(int i=0;i<160;i++)...{
int x=random.nextInt(width);
int y=random.nextInt(height);
int x1=random.nextInt(12);
int y1=random.nextInt(12);
g.drawLine(x, y, x+x1, y+y1);
}

//randomCode用于保存隨機產生的驗證碼
StringBuffer randomCode=new StringBuffer();
int red=0,green=0,blue=0;

//隨機產生4位數字的驗證碼
for(int i=0;i<4;i++)...{
//得到隨機產生的驗證碼數字
String strRand=String.valueOf(random.nextInt(10));

//產生隨機的顏色分量來構造顏色值
red=random.nextInt(110);
green=random.nextInt(50);
blue=random.nextInt(50);

//用隨機產生的顏色將驗證碼繪制到圖像中
g.setColor(new Color(red,green,blue));
g.drawString(strRand, 13*i+6, 16);

randomCode.append(strRand);
}

//將四位數字的驗證碼保存到session中
HttpSession session=request.getSession();
session.setAttribute("randomCode", randomCode.toString());

//禁止圖像緩存
response.setHeader("Pragma", "no-cache");
response.setHeader("Cache-Control", "no-cache");
response.setDateHeader("Expires", 0);

response.setContentType("image/jpeg");
//將圖像輸出到servlet輸出流中
ServletOutputStream sos=response.getOutputStream();
ImageIO.write(buffImg, "jpeg", sos);
sos.close();

}

}

判斷驗證碼是否匹配的LoginServlet.java代碼如下：
package iss;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class LoginServlet extends HttpServlet ...{

/** *//**
* The doPost method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to post.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException ...{

HttpSession session=request.getSession();
String randomCode=(String)session.getAttribute("randomCode");
if(null==randomCode)...{
response.sendRedirect("login.html");
return;
}
String reqRandom=request.getParameter("random");
response.setCharacterEncoding("gbk");
response.setContentType("text/html;charset=gb2312");
PrintWriter out = response.getWriter();
if(randomCode.equals(reqRandom))...{
out.println("驗證碼匹配！");
}else...{
out.println("驗證碼不匹配！");
}

out.close();
}

}

web.xml如下：
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4"
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">
<servlet>
<description>This is the description of my J2EE component</description>
<display-name>This is the display name of my J2EE component</display-name>
<servlet-name>RandomCodeServlet</servlet-name>
<servlet-class>iss.RandomCodeServlet</servlet-class>
</servlet>
<servlet>
<description>This is the description of my J2EE component</description>
<display-name>This is the display name of my J2EE component</display-name>
<servlet-name>LoginServlet</servlet-name>
<servlet-class>iss.LoginServlet</servlet-class>
</servlet>

<servlet-mapping>
<servlet-name>RandomCodeServlet</servlet-name>
<url-pattern>/imgcode</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>LoginServlet</servlet-name>
<url-pattern>/login</url-pattern>
</servlet-mapping>
</web-app>