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 RadomCodeServlet extends HttpServlet
{

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

 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  //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
  int width=60, height=20;
  BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);

  //获取图形上下文
  Graphics g = image.getGraphics();

  //生成随机类
  Random random = new Random();

  //设定背景色
  g.setColor(getRandColor(200,250));
  g.fillRect(0, 0, width, height);

  //设定字体
  g.setFont(new Font("Times New Roman",Font.PLAIN,18));
  
  //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
  g.setColor(getRandColor(160,200));
  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);
  }
  
  //取随机产生的认证码(4位数字)
  String sRand="";
  for (int i=0;i<4;i++)
  {
      String rand=String.valueOf(random.nextInt(10));
      sRand+=rand;
      // 将认证码显示到图象中
      g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));//调用函数出来的颜色相同，可能是因为种子太接近，所以只能直接生成
      g.drawString(rand,13*i+6,16);
  }

  //将认证码存入SESSION
  request.getSession().setAttribute("rand",sRand);

  //图象生效
  g.dispose();

  //输出图象到页面
  ImageIO.write(image, "JPEG", response.getOutputStream());  
 }
 
 //给定范围获得随机颜色
 private Color getRandColor(int fc,int bc)
 {
        Random random = new Random();
        if(fc>255) fc=255;
        if(bc>255) bc=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);
    }
 调用:
 <td height="25">
            <div align="right">
             <font color="#000000">验证码：</font>
            </div>
           </td>
           <td align="left">
            <input type="text" name="randCode" size=10></input>
            <img style="cursor:hand" name="validatecode"
             src=" servlet/RadomCodeServlet" height="17" align="middle"
             onclick="document.validatecode.src=' servlet/RadomCodeServlet'">
           </td>