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 src="logcheck"></a><br><br> <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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