Java编写验证码

package web;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.OutputStream;
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 CheckCodeServlet extends HttpServlet {
    /*
     * 返回一张动态图片
     * 
     */
    public void service(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        response.setContentType("text/jpeg");
        //获取字节流
        OutputStream os=response.getOutputStream();
        //图片映射区，理解成画板就行，画验证码
    //    BufferedImage image=new BufferedImage(width,height,imageType);
        BufferedImage image=new BufferedImage(80,40,BufferedImage.TYPE_INT_RGB);
        //画笔
        Graphics g=image.getGraphics();
        //添加背景颜色（随机）
        Random r=new Random();
        g.setColor(new Color(r.nextInt(256),r.nextInt(256),r.nextInt(256)));//Color(red,green,blue);rgb格式的颜色设置
        g.fillRect(0, 0, 80, 40);//画矩形区域
        //画字符串
        g.setColor(Color.black);//设置验证码的颜色
        g.setFont(new Font(null,Font.BOLD,20));//设置验证码的字体 Font(字体,样式,大小)
        String num=getNum(5);//调入验证码
        g.drawString(num, 5, 30);//(字符串内容,X坐标,Y坐标)
        //划线，干扰机器人识别验证码
        for(int i=0;i<6;i++){
            //画6条线
            g.setColor(new Color(r.nextInt(256),r.nextInt(256),r.nextInt(256)));//设置线的颜色
            g.drawLine(r.nextInt(80),r.nextInt(80),r.nextInt(80),r.nextInt(80));//g.drawLine(起点X坐标,起点Y坐标,终点X坐标,终点Y坐标);
        }
        //压缩 传输（画出的图片比较大，需要压缩，画出的是24位图）
        ImageIO.write(image, "jpeg", os);

    }

    //返回正确的验证码
    //length：验证码的长度
    public String getNum(int length){
        String str="ABCDEFGHIJKLMNOPQRSTUVWXYZ123456789";//常量，为了根据长度挑出随机的验证码
        Random r=new Random();
        StringBuffer sf=new StringBuffer();
        for(int i=0;i<length;i++){
            int index=r.nextInt(str.length());//取str的下标
            char c=str.charAt(index);//根据下标取字符
            sf.append(c);//字符放入字符串缓冲区
        }
        return sf.toString();//转换成字符串返回
    }
}

<servlet>
    <servlet-name>CheckCodeServlet</servlet-name>
    <servlet-class>web.CheckCodeServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>CheckCodeServlet</servlet-name>
    <url-pattern>/checkCode</url-pattern>
  </servlet-mapping>