servlet生成验证码

package com.sevend.util;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class CheckNum extends HttpServlet {

	/**
	 * 生成验证码
	 *
	 * This method is called when a form has its tag value method equals to get.
	 *
	 * @param request the request send by the client to the server
	 * @param response the response send by the server to the client
	 * @throws ServletException if an error occurred
	 * @throws IOException if an error occurred
	 */
	public void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		//禁止浏览器缓存图片
		response.setDateHeader("Expiress", -1);
		response.setHeader("Cache-Control", "no-cahce");
		response.setHeader("Pragma", "no-cache");
		//通知客户端以图片的方式打开发送过去的数据
		response.setHeader("Content-Type", "image/jpeg");
		//在内存中创建一副图片
		BufferedImage image=new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);
		//向图片上写数据
		Graphics g=image.getGraphics();
		//设置背景色
		g.setColor(Color.DARK_GRAY);
		g.fillRect(0, 0, 80, 30);
		//设置写入数据的颜色和字体
		g.setColor(Color.BLACK);
		g.setFont(new Font(null,Font.BOLD,20));
		//向图片上写入数据
		String num=createNum();
		//把生成的随机数保存到session中
		request.getSession().setAttribute("checknum", num);
		g.drawString(num, 0, 20);
		//把写好数据的图片输出到浏览器
		ImageIO.write(image, "jpg", response.getOutputStream());
	}
	/**
	 * 生成随机数
	 */
	public String createNum(){
		Random r=new Random();
		//生成1-9999999之间的数
		String num=r.nextInt(9999999)+"";
		StringBuffer sb=new StringBuffer();
		//如果生成随机数的位不够，用0补全
		for(int i=0;i<7-num.length();i++){
			sb.append("0");
		}
		num=sb.toString()+num;
		return num;
		
	}

	/**
	 * The doPost method of the servlet. <br>
	 *
	 * This method is called when a form has its tag value method equals to post.
	 *
	 * @param request the request send by the client to the server
	 * @param response the response send by the server to the client
	 * @throws ServletException if an error occurred
	 * @throws IOException if an error occurred
	 */
	public void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		this.doGet(request, response);
	}

}