需求分析

本质上是图片，用来防止恶意表单注册。

通过img标签，src为servlet的路径

步骤

书写静态页面login.html

img标签，src为servlet路径。再写一个a标签。两者都绑定点击事件，点击后图片进行刷新。
书写servlet

通过ImageIO将写好的img输出到页面流里面去（response.getOutPutStream）
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//将图片写入到浏览器页面上去
ImageIO.write(img,"jpg",response.getOutputStream());

代码

登录页面login.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>注册</title>
    <script>
        window.onload=function () {
            var img=document.getElementById("image");
            var change=document.getElementById("change");
            img.onclick=function() {
                var date=new Date().getTime();
                img.src="/day/CheckCodeServlet?"+date;
            }
            change.onclick=function() {
                var date=new Date().getTime();
                img.src="/day/CheckCodeServlet?"+date;
            }
        }
    </script>
</head>
<body>

<img src="/day/CheckCodeServlet" id="image">
<a href="" id="change">看不见？点击换一张</a>
</body>
</html>

CheckCodeServlet

package cn.itcast.web.servlet;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
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 {
        this.doGet(request, response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        int height=50;
        int width=100;
        //创建对象，在内存中图片对象
        BufferedImage img = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);


        //美化图片
        Graphics g = img.getGraphics();
        g.setColor(Color.pink);
        g.drawRect(0,0,width-1,height-1);

        String str="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
        Random ran=new Random();
        for (int i = 1; i <= 4; i++) {
            int index = ran.nextInt(str.length());
            char c = str.charAt(index);
            g.drawString(c+"",width/5*i,height/2);
        }
        g.setColor(Color.GREEN);
        for (int i = 0; i < 10; i++) {
            int x1 = ran.nextInt(width);
            int x2 = ran.nextInt(width);
            int y1 = ran.nextInt(height);
            int y2 = ran.nextInt(height);
            g.drawLine(x1,y1,x1,x2);

        }


        //将图片写入到浏览器页面上去
        ImageIO.write(img,"jpg",response.getOutputStream());


    }
}