Session实现一次性验证码

<h3>带有验证码的登录页面</h3>
< form action="servlet/LogonFormServlet" method="post">
用户名:<input type="text" name="name"><br>
密码:<input type="password" name="pass"><br>
验证码:<input type="text" name="check_code">
< img src="servlet/CheckCodeServlet"><br>
< input type="submit" value="登录">
< /form>

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.awt.*;
import java.awt.image.*;
import javax.imageio.ImageIO;

public class CheckCodeServlet extends HttpServlet
{
    private static int WIDTH=60;
    private static int HEIGHT=20;
    public void doGet(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException
    {
    HttpSession session = request.getSession();
    response.setContentType("image/jpeg");
    ServletOutputStream sos=response.getOutputStream();

    //设置浏览器不要缓存此图片
    response.setHeader("Pragma","No-cache");
    response.setHeader("Cache-Control","no-cache");
    response.setDateHeader("Expires",0);

    //创建内容图片并获得其图形上下文
    BufferedImage image=new BufferedImage(WIDTH,HEIGHT,BufferedImage.TYPE_INT_RGB);
    Graphics g=image.getGraphics();

    //产生随机的验证码
    char[] rands=generateCheckCode();

    //产生图像
    drawBackground(g);
    drawRands(g,rands);
  
    //结束图像的绘制过程,完成图像
    g.dispose();

    //将图像输出到客户端
    ByteArrayOutputStream bos=new ByteArrayOutputStream();
    ImageIO.write(image,"JPEG",bos);
    byte[] buf=bos.toByteArray();
    response.setContentLength(buf.length);
    //下面的语句也可以写成:bos.writeTo(sos);
    sos.write(buf);
    bos.close();
    sos.close();

    //将当前验证码存入到session中
    session.setAttribute("check_code",new String(rands));
    }
  
    private char[] generateCheckCode()
    {
        //定义验证码的字符表
        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);
        }
        return rands;
    }

    private void drawRands(Graphics g,char[] rands)
    {
        g.setColor(Color.BLACK);
        g.setFont(new Font(null,Font.ITALIC|Font.BOLD,18));
        //在不同的高度上输出验证码的每个字符
        g.drawString(""+rands[0],1,17);
        g.drawString(""+rands[1],16,15);
        g.drawString(""+rands[2],31,87);
        g.drawString(""+rands[3],46,16);
        System.out.println(rands);
    }

    private void drawBackground(Graphics g)
    {
        //画背景
        g.setColor(new Color(0xDCDCDC));
        g.fillRect(0,0,WIDTH,HEIGHT);
        //随即产生120个干扰点
        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);
        }
    }
}

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class LogonFormServlet extends HttpServlet
{
    public void service(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException
    {
        response.setContentType("text/html;charset=GB2312");
        PrintWriter out=response.getWriter();

        HttpSession session=request.getSession(false);
        if(session==null)
        {
            out.println("验证码处理问题!");
            return ;
        }

        String savedCode=(String)session.getAttribute("check_code");
        if(savedCode==null)
        {
            out.println("验证码处理问题!");
            return ;
        }
      
        String checkCode=request.getParameter("check_code");
        if(!savedCode.equals(checkCode))
        {
        /*验证码未通过,不从session中清楚原来的验证码,
        以便用户可以后退回登录页面继续使用原来的验证码进行登录*/
            out.println("验证码无效!");
            return ;
        }
  
        /*验证码检查通过后,从session中清楚原来的验证码,
        以防用户后退回登录页面继续使用原来的验证码进行登录*/
        session.removeAttribute("check_code");
        out.println("验证码通过,服务器正在校验用户名和密码!");
    }
}

转载：http://blog.csdn.net/mr_walter/article/details/7878267