验证码的作用：通常的登录或者注册系统时，都会要求用户输入验证码，以此区别用户行为和计算机程序行为，目的是有人防止恶意注册、暴力破解密码等。

实现验证码的思路：用 server 实现随机生成数字和字母组成图片的功能，用 jsp 页面实现显示验证码和用户输入验证码的功能，再用 server 类分别获取图片和用户输入的数据，判断两个数据是否一致。

代码实现

1.编写数字、英文随机生成的 server 类，源码：

 package com;

 import java.awt.Color;

 import java.awt.Font;

 import java.awt.Graphics;

 import java.awt.image.BufferedImage;

 import java.io.ByteArrayOutputStream;

 import java.io.IOException;

 import java.io.PrintWriter;

 import javax.imageio.ImageIO;

 import javax.servlet.ServletException;

 import javax.servlet.ServletOutputStream;

 import javax.servlet.http.HttpServlet;

 import javax.servlet.http.HttpServletRequest;

 import javax.servlet.http.HttpServletResponse;

 import javax.servlet.http.HttpSession;

 public class logcheck extends HttpServlet {

     public logcheck() {

         super();

     }

     public void destroy() {

         super.destroy();

     }

     public void doGet(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         doPost(request, response);

     }

     /*实现的核心代码*/

     public void doPost(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         response.setContentType("image/jpeg");

         HttpSession session=request.getSession();

         int width=60;

         int height=20;

         //设置浏览器不要缓存此图片

         response.setHeader("Pragma", "No-cache");

         response.setHeader("Cache-Control", "no-cache");

         response.setDateHeader("Expires", 0);

         //创建内存图像并获得图形上下文

         BufferedImage image=new BufferedImage(width, height,BufferedImage.TYPE_INT_RGB);

         Graphics g=image.getGraphics();

         /*

          * 产生随机验证码

          * 定义验证码的字符表

          */

         String chars="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

         char[] rands=new char[4];

         for(int i=0;i<4;i++){

             int rand=(int) (Math.random() *36);

             rands[i]=chars.charAt(rand);

         }

         /*

          * 产生图像

          * 画背景

          */

         g.setColor(new Color(0xDCDCDC));

         g.fillRect(0, 0, width, height);

         /*

          * 随机产生120个干扰点

          */

         for(int i=0;i<120;i++){

             int x=(int)(Math.random()*width);

             int y=(int)(Math.random()*height);

             int red=(int)(Math.random()*255);

             int green=(int)(Math.random()*255);

             int blue=(int)(Math.random()*255);

             g.setColor(new Color(red,green,blue));

             g.drawOval(x, y, 1, 0);

         }

         g.setColor(Color.BLACK);

         g.setFont(new Font(null, Font.ITALIC|Font.BOLD,18));

         //在不同高度输出验证码的不同字符

         g.drawString(""+rands[0], 1, 17);

         g.drawString(""+rands[1], 16, 15);

         g.drawString(""+rands[2], 31, 18);

         g.drawString(""+rands[3], 46, 16);

         g.dispose();

         //将图像传到客户端

         ServletOutputStream sos=response.getOutputStream();

         ByteArrayOutputStream baos=new ByteArrayOutputStream();

         ImageIO.write(image, "JPEG", baos);

         byte[] buffer=baos.toByteArray();

         response.setContentLength(buffer.length);

         sos.write(buffer);

         baos.close();

         sos.close();

         session.setAttribute("checkcode", new String(rands));

     }

     public void init() throws ServletException {

         // Put your code here

     }

 }

2.用于显示验证码的页面：

 <%@ 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>index</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>

   <form action="yanzheng" method="post">

       <input type="text" name="name" size="5" maxlength="4">

     <a href="index.jsp"><img border="0" src="logcheck"></a><br><br>

     &nbsp&nbsp&nbsp&nbsp&nbsp<input type="submit" value="提交">

   </form>

   </body>

 </html>

3.用于检验输入的验证码是否正确：

 package com;

 import java.io.IOException;

 import java.io.PrintWriter;

 import javax.jms.Session;

 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 yanzheng extends HttpServlet {

     public yanzheng() {

         super();

     }

     public void destroy() {

         super.destroy();

     }

     public void doGet(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         doPost(request, response);

     }

     /*核心代码*/

     public void doPost(HttpServletRequest request, HttpServletResponse response)

             throws ServletException, IOException {

         String info=null;

         /*获取输入的值*/

         String value1=request.getParameter("name");

         /*获取图片的值*/

         HttpSession session=request.getSession();

         String value2=(String)session.getAttribute("checkcode");

         /*对比两个值（字母不区分大小写）*/

         if(value2.equalsIgnoreCase(value1)){

             info="验证码输入正确";

         }else{

             info="验证码输入错误";

         }

         System.out.println(info);

         request.setAttribute("info", info);

         request.getRequestDispatcher("/login.jsp").forward(request, response);

     }

     public void init() throws ServletException {

         // Put your code here

     }

 }

4.显示输入结构界面（输入验证码是否正确）：

 <%@ 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 'login.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>

     <%=request.getAttribute("info") %>

   </body>

 </html>

5.项目结构、效果截图：

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