登录验证码的生成

HTML页面表单：

 <form action="servlet/LogonFormServlet" method="post">
    user  name: <input type="text" name="name"/><br/>
    password  : <input type="password" name="pass"/><br/>
    check code: <input type="text" name="check_code"/>
    <img src="servlet/CheckCodeServlet"><br/>
    <input type="submit" value="login"/>
  </form>

生成验证码的Servlet:

package com.frank.liu.checkcode;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.IOException;

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;

/**
 * Check code class.
 * */
public class CheckCodeServlet extends HttpServlet{

    private static int WIDTH = 60;
    private static int HEIGHT = 20;
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        
        HttpSession session = req.getSession();
        resp.setContentType("img/jpeg");
        ServletOutputStream sos = resp.getOutputStream();
        //set the client brower do not use cache
        resp.setHeader("pragma", "No-cache");
        resp.setHeader("Cache-Control", "no-cache");
        resp.setDateHeader("Expires", 0);
        
        BufferedImage image = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
        Graphics graphic = image.getGraphics();
        char [] rands = generateCheckCode();
        
        drawBackground(graphic);
        drawRands(graphic,rands);
        
        graphic.dispose();
        
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ImageIO.write(image, "JPEG", bos);
        byte [] buf = bos.toByteArray();
        resp.setContentLength(buf.length);
        sos.write(buf);
        bos.close();
        sos.close();
        
        session.setAttribute("check_code", new String(rands));
    }
    
    private char [] generateCheckCode(){
        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 drawBackground(Graphics graphic){
        graphic.setColor(new Color(0xDCDCDC));
        graphic.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);
            graphic.setColor(new Color(red,green,blue));
            graphic.drawOval(x, y, 1, 0);
        }
    }
    
    private void drawRands(Graphics graphic, char [] rands){
        graphic.setColor(Color.BLACK);
        graphic.setFont(new Font(null, Font.ITALIC|Font.BOLD, 18));
        graphic.drawString(""+rands[0], 1, 17);
        graphic.drawString(""+rands[1], 16, 15);
        graphic.drawString(""+rands[2], 31, 18);
        graphic.drawString(""+rands[3], 46, 16);
        System.out.println(rands);
    }
}

判断验证码正确性的Servlet:

package com.frank.liu.checkcode;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class LogonFormServlet extends HttpServlet{

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        // TODO Auto-generated method stub
        resp.setContentType("text/html;charset=GB2312");
        PrintWriter out = resp.getWriter();
        
        HttpSession session = req.getSession(false);
        if(session==null){
            out.println("check code handle error!");
            return;
        }
        String savedCode = (String) session.getAttribute("check_code");
        if(savedCode==null){
            out.println("check code handle error!");
            return;
        }
        String checkcode = req.getParameter("check_code");
        if(!savedCode.equals(checkcode)){
            out.println("Input wrong check code!");
            return;
        }
        session.removeAttribute("check_code");
        out.println("Right check code! Login now...");
    }

}