java实现网页验证码功能

package cn.bdqn.servlet; import javax.imageio.ImageIO;import javax.servlet.ServletException;import javax.servlet.annotation.WebServlet;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import java.awt.*;import java.awt.font.ImageGraphicAttribute;import java.awt.image.BufferedImage;import java.io.IOException;import java.util.Random;  @WebServlet(name = "Servlet",urlPatterns = "/yanCode")public class Servlet extends HttpServlet { public void doPost(javax.servlet.http.HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {  this.doGet(request,response);  }  public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {   String s="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";  Random random = new Random();  int width=500;  int height=50;  BufferedImage image = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);  Graphics graphics = image.getGraphics();//相当于画笔  //画背景  graphics.setColor(Color.gray);  graphics.fillRect(0,0,width,height);  graphics.setColor(Color.black);  for (int i = 0; i <4 ; i++) {   int index = random.nextInt(s.length());   String c = s.substring(index, index + 1);   graphics.drawString(c,width/5*(i+1),15);  }   ImageIO.write(image,"jpg",response.getOutputStream());   }}