Java Web后端动态生成验证码

package 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;

/**
 * 验证码
 */
@WebServlet("/createCodeServlet")
public class CreateCodeServlet extends HttpServlet {
   public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
      
      //服务器通知浏览器不要缓存
      response.setHeader("pragma","no-cache");
      response.setHeader("cache-control","no-cache");
      response.setHeader("expires","0");
      
      //在内存中创建一个长80，宽30的图片，默认黑色背景
      //参数一：长
      //参数二：宽
      //参数三：颜色
      int width = 80;
      int height = 30;
      BufferedImage image = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
      
      //获取画笔
      Graphics g = image.getGraphics();
      //设置画笔颜色为灰色
      g.setColor(Color.GRAY);
      //填充图片
      g.fillRect(0,0, width,height);
      
      //产生4个随机验证码，12Ey
      String checkCode = getCheckCode();
      //将验证码放入HttpSession中
      request.getSession().setAttribute("CHECKCODE",checkCode);
      //设置画笔颜色为黄色
      g.setColor(Color.YELLOW);
      //设置字体的小大
      g.setFont(new Font("黑体",Font.BOLD,24));
      //向图片上写入验证码
      g.drawString(checkCode,15,25);
      
      //将内存中的图片输出到浏览器
      //参数一：图片对象
      //参数二：图片的格式，如PNG,JPG,GIF
      //参数三：图片输出到哪里去
      ImageIO.write(image,"PNG",response.getOutputStream());
   }
   /**
    * 产生4位随机字符串 
    */
   private String getCheckCode() {
      String base = "0123456789ABCDEFGabcdefg";
      int size = base.length();
      Random r = new Random();
      StringBuffer sb = new StringBuffer();
      for(int i=1;i<=4;i++){
         //产生0到size-1的随机值
         int index = r.nextInt(size);
         //在base字符串中获取下标为index的字符
         char c = base.charAt(index);
         //将c放入到StringBuffer中去
         sb.append(c);
      }
      return sb.toString();
   }
   public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
      this.doGet(request,response);
   }
}