驗證碼的作用：通常的登錄或者註冊系統時，都會要求用戶輸入驗證碼，以此區別用戶行為和計算機程序行為，目的是有人防止惡意註冊、暴力破解密碼等。
實現驗證碼的思路：用server 實現隨機生成數字和字母組成圖片的功能，用jsp 頁面實現顯示驗證碼和用戶輸入驗證碼的功能，再用server 類分別獲取圖片和用戶輸入的數據，判斷兩個數據是否一致。
代碼實現
1.編寫數字、英文隨機生成的server 類，源碼：
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); } /*實現的核心代碼*/ public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("image/jpeg"); HttpSession session=request.getSession(); int width=60; int height=20; //設置瀏覽器不要緩存此圖片response.setHeader("Pragma", "No-cache"); response.setHeader("Cache-Control", "no-cache"); response.setDateHeader("Expires", 0); //創建內存圖像並獲得圖形上下文BufferedImage image=new BufferedImage(width, height,BufferedImage.TYPE_INT_RGB); Graphics g=image.getGraphics(); /* * 產生隨機驗證碼* 定義驗證碼的字符表*/ 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); } /* * 產生圖像* 畫背景*/ g.setColor(new Color(0xDCDCDC)); g.fillRect(0, 0, width, height); /* * 隨機產生120個乾擾點*/ 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)); //在不同高度輸出驗證碼的不同字符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(); //將圖像傳到客戶端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.用於顯示驗證碼的頁面：
<%@ 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.用於檢驗輸入的驗證碼是否正確：
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); } /*核心代碼*/ public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String info=null; /*獲取輸入的值*/ String value1=request.getParameter("name"); /*獲取圖片的值*/ HttpSession session=request.getSession(); String value2=(String)session.getAttribute("checkcode"); /*對比兩個值（字母不區分大小寫）*/ if(value2.equalsIgnoreCase(value1)){ info="驗證碼輸入正確"; }else{ info="驗證碼輸入錯誤"; } 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.顯示輸入結構界面（輸入驗證碼是否正確）：
<%@ 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.項目結構、效果截圖：
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