生成随机数验证码的工具类（from韩顺平）

  生成随机数验证码的工具类

package com.cx;
//生成随机数的图片
import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.PrintWriter;
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 CreateCode extends HttpServlet {

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

        //7.禁止浏览器缓存随机图片
        response.setDateHeader("Expires", -1);
        response.setHeader("Cache-Control", "no-cache");
        response.setHeader("Pragma", "no-cache");
        
        //6.通知客户机以图片方式打开发送过去的数据
        response.setHeader("Content-Type", "image/jpeg");
        //1.在内存中创建一副图片
        BufferedImage image=new BufferedImage(80, 30, BufferedImage.TYPE_INT_RGB);
        //2.在图片上写数据
        Graphics g=image.getGraphics();
        
        //设背景色
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, 80, 30);
        
        //3.设置写入数据的颜色和字体
        g.setColor(Color.BLACK);
        g.setFont(new Font(null,Font.BOLD, 20));
        
        //4.向图片上写数据
        String num=makeNum();
        //将随机生成的数值保存到session
        request.getSession().setAttribute("checkcode", num);
        g.drawString(num, 0, 20);
        
        //5.把写好的数据的图片输出给浏览器
        ImageIO.write(image, "jpg", response.getOutputStream());
        
    }
    //该函数生成随机7位数
    public String makeNum(){
        Random r=new Random();
        //这里9999999可以生成7位
        String num=r.nextInt(9999)+"";
        StringBuffer sb=new StringBuffer();
        //如果不够4位，前面不补零
        for(int i=0;i<4-num.length();i++){
            sb.append("0");
        }
        num=sb.toString()+num;
        return num;
    }

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

        this.doGet(request, response);
    }

}
//使用
//<img src="/验证码的url"/>