要导入servlet-api.jar包

创建验证码的Util工具类：

先创建图片并生成随机的验证码字母

设置图片的底色，并用setFont函数将验证码画在图片上，如果想让验证码难一点，可以添加for循环的代码给图片增加旋转角度

给验证码增加干扰线，提高安全性

设置边框

创建VerifyPic的servlet，通过Util类获取代码，并存入session中，然后输送去前端页面

前端页面验证码如下：

完整Util工具类代码如下：

package com.sport.util;

import java.awt.*;
import java.awt.image.BufferedImage;
import java.util.HashMap;
import java.util.Map;

public class VerifyUtil {
 public static Map generateVerify(){
  //创建一张图片
  BufferedImage verifyPic = new BufferedImage(120,40,BufferedImage.TYPE_3BYTE_BGR);
  //通过图片获取画笔
  Graphics2D g = verifyPic.createGraphics();
  //准备一个字母+数字的字典
  String letters = "23456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
  //规定验证码的位数
  int verifyLength = 4;
  //生成随机验证码
  String verifyCode = "";
  //循环取值
  for(int i=0; i data = new HashMap();
  data.put("verifyCode", verifyCode);
  data.put("verifyPic", verifyPic);

  return data;
 }
}

完整VerifyPic的代码如下：

package com.sport.servlet;

import com.sport.util.VerifyUtil;

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 javax.servlet.http.HttpSession;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Map;

@WebServlet("/VerifyPic")
public class VerifyPic extends HttpServlet {
 protected void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  Map data = VerifyUtil.generateVerify();
  //获取验证码
  String verifyCode = (String) data.get("verifyCode");
  //将验证码存入session
  HttpSession session = request.getSession();
  session.setAttribute("verifyCode", verifyCode);
  //获取图片
  BufferedImage verifyPic = (BufferedImage) data.get("verifyPic");
  //输出图片给前端，使用工具类ImageIO
  ImageIO.write(verifyPic, "jpg", response.getOutputStream());

 }

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