图形验证码的实现

package work;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.*;

import javax.imageio.ImageIO;
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;

public class checkcode extends HttpServlet {
 public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  request.setCharacterEncoding("UTF-8");
  response.setContentType("image/jpeg");
  HttpSession session=request.getSession();
  int width=60;
  int height=20;
  response.setHeader("Pragma","NO-cache");
  response.setHeader("Cache-Control","no-cache");
  response.setDateHeader("Expires",0);
  BufferedImage image=new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
  Graphics g=image.getGraphics();
  String chars="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  char[] rands=new char[4];
  for(int i=0;i<4;i++){
   int rand=(int)(Math.random()*36);
   rands[i]=chars.charAt(rand);
  }
  g.setColor(new Color(0xDCDCDC));
  g.fillRect(0, 0, width, height);
  for(int i=0;i<120;i++){
   int x=(int)(Math.random()*width);
   int y=(int)(Math.random()*height);
   int red=(int)(Math.random()*255);
   int green=(int)(Math.random()*255);
   int blue=(int)(Math.random()*255);
   g.setColor(new Color(red,green,blue));
   g.drawOval(x, y, 1, 0);
  }
  g.setColor(Color.BLACK);
  g.setFont(new Font(null,Font.ITALIC|Font.BOLD,18));
  g.drawString(""+rands[0], 1, 17);
  g.drawString(""+rands[1], 16, 15);
  g.drawString(""+rands[2], 31, 18);
  g.drawString(""+rands[3], 46, 16);
  g.dispose();
  ServletOutputStream sos=response.getOutputStream();
  ByteArrayOutputStream baos=new ByteArrayOutputStream();
  ImageIO.write(image, "JPEG", baos);
  byte[] buffer=baos.toByteArray();
  response.setContentLength(buffer.length);
  sos.write(buffer);
  baos.close();
  sos.close();
  session.setAttribute("checkcode", new String(rands));
 }
}