生成验证码原理

课程代码：

package com.kuang.servlet;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

public class ImageServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 让浏览器3秒自动刷新一次
        resp.setHeader("refresh","3");

        // 在内存中创建一个图片
        BufferedImage image = new BufferedImage(80, 20, BufferedImage.TYPE_INT_RGB);

        // 得到图片
        Graphics2D g = (Graphics2D) image.getGraphics(); //可以理解为画笔

        //设置图片的背景颜色
        g.setColor(Color.WHITE);
        g.fillRect(0,0,80,20);

        //给图片写随机数
        //设置字体颜色
        g.setColor(Color.BLUE);
        //设置字体样式
        g.setFont(new Font(null, Font.BOLD, 20));
        // 画图
        g.drawString(makeNum(), 0, 20);

        //告诉浏览器，请求用图片形式打开
        resp.setContentType("image/jpeg");
        // 网站存在缓存，不让浏览器缓存
        resp.setDateHeader("expires", -1);
        resp.setHeader("Cache-Control", "no-cache");
        resp.setHeader("Pragma", "no-cache");

        // 把图片写给浏览器
        ImageIO.write(image, "jpg", resp.getOutputStream());

    }

    // 生成随机数
    private String makeNum(){
        Random random = new Random();
        String num = random.nextInt(99999999) + "";
        StringBuffer sb = new StringBuffer();
        for (int i = 0; 0 < 7-num.length(); i++) {
            sb.append("0");
        }
        num = sb.toString() + num;
        return num;
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}