一、验证码简介

验证码的本质：是一个图片，但是是存在内存的图片。（还是字母东倒西歪的图片）
目的：防止恶意的表单注册

二、代码展示

1、生成验证码的逻辑CheckCodeServlet

package cn.itcast.web.servlet;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
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;

@WebServlet("/checkCodeServlet")
public class CheckCodeServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        int width = 100;
        int height = 50;

        //1.创建一对象，在内存中生成图片(验证码图片对象)
        BufferedImage image = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);//参数：宽，高，图片类型


        //2.美化图片
        //2.1 填充背景色
        Graphics g = image.getGraphics();//画笔对象
        g.setColor(Color.PINK);//设置画笔颜色
        g.fillRect(0,0,width,height);//前两个参数x，y为起始点

        //2.2画边框
        g.setColor(Color.BLUE);
        g.drawRect(0,0,width - 1,height - 1);
        //设置字体大小。好像没成
        Font font = new Font("copperplate",Font.BOLD,50);
        g.setFont(font);

        String str = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghigklmnopqrstuvwxyz0123456789";
        //生成随机角标(下标)
        Random ran = new Random();

        //四位验证码，循环四次
        for (int i = 1; i <= 4; i++) {
            int index = ran.nextInt(str.length());
            //获取字符
            char ch = str.charAt(index);//随机字符
            //2.3写验证码
            g.drawString(ch+"",width/5*i,height/2);
        }


        //2.4画干扰线
        g.setColor(Color.GREEN);
        //随机生成坐标点
        for (int i = 0; i < 10; i++) {//画十条线
            int x1 = ran.nextInt(width);
            int x2 = ran.nextInt(width);

            int y1 = ran.nextInt(height);
            int y2 = ran.nextInt(height);
            g.drawLine(x1,y1,x2,y2);
        }


        //3.将图片输出到页面展示
        ImageIO.write(image,"jpg",response.getOutputStream());
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request,response);
    }
}

简单的页面展示，单击切换验证码

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>验证码</title>

    <script>
        /*
            分析：
                点击超链接或者图片，需要换一张
                1.给超链接和图片绑定单击事件

                2.重新设置图片的src属性值
         */
    window.onload = function(){
        //1.获取图片对象
        var img = document.getElementById("checkCode");
        //2.绑定单击事件
        img.onclick = function(){
            //加时间戳
            var date = new Date().getTime();

            img.src = "/day15/checkCodeServlet?"+date;//后面加一个永不重复的无意义的数，欺骗电脑缓存。因为？后没用
        }

        var a = document.getElementById("change");
        a.onclick = function (){
            var data = new Date().getTime();
            a.href = "/day15/checkCodeServlet?"+date;
        }

    }


    </script>


</head>
<body>

    <!--访问动态生成的图片-->
    <img id="checkCode" src="/day15_response_war_exploded/checkCodeServlet" />

    <a id="change" href="">看不清换一张？</a>

</body>
</html>