Using servlet to implement captcha mainly inherits httpServlet class

package com.zyc.demo;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class DrewImage extends HttpServlet{
  /**
   *
   */
  private static final long serialVersionUID = 1505032428319459075L;
  private final Font mFont =
      new Font("Arial Black", Font.PLAIN, 16);
    private final int IMG_WIDTH = 100;
    private final int IMG_HEIGTH = 18;
    private Color getRandColor(int fc,int bc)
    {
      Random random = new Random();
      if(fc > 255) fc = 255;
      if(bc > 255) bc=255;
      int r = fc + random.nextInt(bc - fc);
      int g = fc + random.nextInt(bc - fc);
      int b = fc + random.nextInt(bc - fc);
      return new Color(r , g , b);
    }
    public void service(HttpServletRequest request,
      HttpServletResponse response)
      throws ServletException, IOException
    {
      response.setHeader("Pragma","No-cache");
      response.setHeader("Cache-Control","no-cache");
      response.setDateHeader("Expires", 0);
      response.setContentType("image/jpeg");
      BufferedImage image = new BufferedImage
        (IMG_WIDTH , IMG_HEIGTH , BufferedImage.TYPE_INT_RGB);
      Graphics g = image.getGraphics();
      Random random = new Random();
      g.setColor(getRandColor(200 , 250));
      g.fillRect(1, 1, IMG_WIDTH - 1, IMG_HEIGTH - 1);
      g.setColor(new Color(102 , 102 , 102));
      g.drawRect(0, 0, IMG_WIDTH - 1, IMG_HEIGTH - 1);
      g.setColor(getRandColor(160,200));
      for (int i = 0 ; i < 30 ; i++)
      {
        int x = random.nextInt(IMG_WIDTH - 1);
        int y = random.nextInt(IMG_HEIGTH - 1);
        int xl = random.nextInt(6) + 1;
        int yl = random.nextInt(12) + 1;
        g.drawLine(x , y , x + xl , y + yl);
      }
      g.setColor(getRandColor(160,200));
      for (int i = 0 ; i < 30 ; i++)
      {
        int x = random.nextInt(IMG_WIDTH - 1);
        int y = random.nextInt(IMG_HEIGTH - 1);
        int xl = random.nextInt(12) + 1;
        int yl = random.nextInt(6) + 1;
        g.drawLine(x , y , x - xl , y - yl);
      }
      g.setFont(mFont);
      String sRand = "";
      for (int i = 0 ; i < 4 ; i++)
      {
        String tmp = getRandomChar();
        sRand += tmp;
        g.setColor(new Color(20 + random.nextInt(110)
          ,20 + random.nextInt(110)
          ,20 + random.nextInt(110)));
        g.drawString(tmp , 15 * i + 10,15);
      }
      HttpSession session = request.getSession(true);
      session.setAttribute("rand" , sRand);
//     System.out.println(" write session"+sRand);
      g.dispose();
      ImageIO.write(image, "JPEG", response.getOutputStream());
    }
    private String getRandomChar()
    {
      int rand = (int)Math.round(Math.random() * 2);
      long itmp = 0;
      char ctmp = '\u0000';
      switch (rand)
      {
        case 1:
          itmp = Math.round(Math.random() * 25 + 65);
          ctmp = (char)itmp;
          return String.valueOf(ctmp);
        case 2:
          itmp = Math.round(Math.random() * 25 + 97);
          ctmp = (char)itmp;
          return String.valueOf(ctmp);
        default :
          itmp = Math.round(Math.random() * 9);
          return itmp + "";
      }
    }
}

Below is the web.xml configuration

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
 <display-name>IndustryDemo</display-name>
 <welcome-file-list>
  <welcome-file>index.html</welcome-file>
  <welcome-file>index.htm</welcome-file>
  <welcome-file>index.jsp</welcome-file>
  <welcome-file>default.html</welcome-file>
  <welcome-file>default.htm</welcome-file>
  <welcome-file>default.jsp</welcome-file>
 </welcome-file-list>

 <servlet>
   <servlet-name>img</servlet-name>
   <servlet-class>com.zyc.demo.DrewImage</servlet-class>
 </servlet>

 <servlet-mapping>
   <servlet-name>img</servlet-name>
   <url-pattern>/img.do</url-pattern>
 </servlet-mapping>

</web-app>

jsp file

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
 <head>
  <base href="<%=basePath%>">

  <title>My JSP 'yanzhengma.jsp' starting page</title>

  <meta http-equiv="pragma" content="no-cache">
  <meta http-equiv="cache-control" content="no-cache">
  <meta http-equiv="expires" content="0">
  <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
  <meta http-equiv="description" content="This is my page">
  <!--
  <link rel="stylesheet" type="text/css" href="styles.css">
  -->

 </head>

 <body>
  <img alt=" Verification code " src="img.do"> <button onclick="window.location.reload();"> The refresh </button>
 </body>
</html>

Simple and practical.