话不多说，直接上代码
package cn.stormtides.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.BufferedReader;
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;
//创建一个对象，在内存中存图片
BufferedImage image=new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
//美化
//填充背景色
Graphics g = image.getGraphics();
g.setColor(Color.PINK);
g.fillRect(0,0,width,height);
//画边框
g.setColor(Color.BLUE);
g.drawRect(0,0,width-1,height-1);
//写验证码
String str="ABCDEFGHIJKLMNOPQRSTUVWXYZabcefghijklmnopqrstuvwxyz0123456789";
Random ran=new Random();
for (int i=1;i<=4;i++){
//生成随机角标
int index=ran.nextInt(str.length());
//获取字符
char ch=str.charAt(index);
//写验证码
g.drawString(ch+"",width/5*i,height/2);
}
//干扰线
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,x2,y2);
}
//将图片输出到页面
ImageIO.write(image,"jpg",response.getOutputStream());
}
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
this.doPost(request,response);
}
}
前端页面代码（比较简易大家别嫌弃）:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<script>
window.onload=function () {
var img=document.getElementById("checkCode");
img.onclick=function () {
//加时间戳
var date=new Date().getTime();
img.src="/day15/checkCodeServlet?"+date;
}
}
</script>
</head>
<body>
<img id="checkCode" src="/day15/checkCodeServlet" />
<a id="change" href="">看不清换一张？</a>
</body>
</html>
至于为什么要加时间戳，在打开页面加载图片时会存储图片路径和图片（由数字字母干扰线组成的图片），再当图片刷新时还是会加载该图片，加上一个时间戳会图片路径不会重复，从而加载新的图片，"/day15"为虚拟路径