使用session实现一次性验证码

       在登录页面和各种页面，会看到有验证码输入，这样做的目的是为了防止密码猜测工具破解密码，保护了用户密码安全，验证码只能使用一次，这样就给密码猜测工具带来了很大的困难，基本上阻断了密码猜测工具的使用。

       可以使用session获得一次性验证码。先看一下登录页面，即显示验证码的页面，代码为：

  
    
    
  
  
  
    

    用户名：

    密     码:

    验证码：
    

    
    
  

验证码存放在一张图片上，那图片是通过servlet产生的，在servlet中先产生验证码存放到session中，供以后验证使用，然后在画一张图片，将验证码无规则的放在图片上，在图片上画上干扰字符，然后就可以啦。代码如下：

package com.you.servlet;

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;

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);
		
		//创建内存图像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);
		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));

	}

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

		doGet(request, response);
	}
	
	private char[] generateCheckCode() {
		//定义验证码的字符集
		String chars = "0123456789abcdefghigklmnopqrstuvwxyz";
		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, 18);
		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);
		//随机产生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);
		}
	}

}

然后是登录之后处理用户是否登录成功的servlet，在这个servlet中通过比较session中存放的验证码和用户输入的验证码，如果匹配则进行下一步判断，如果不匹配直接输出验证码不匹配的问题。代码为：

package com.you.servlet;

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 LoginFormServlet extends HttpServlet {

	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		response.setContentType("text/html;charset=utf-8");
		PrintWriter out = response.getWriter();
		
		HttpSession session = request.getSession(false);
		if(session == null) {
			out.print("验证码处理问题");
			return;
		}
		
		String saveCode = (String)session.getAttribute("check_code");
		if(saveCode == null) {
			out.print("验证码处理问题");
			return;
		}
		
		String checkCode = request.getParameter("check_code");
		if(!saveCode.equals(checkCode)) {
			out.print("验证码无效!");
			return;
		}
		
		session.removeAttribute("check_code");
		out.print("验证码通过，服务器正在校验用户名和密码！");

	}

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

		doGet(request, response);
	}

}