效果图：
loginProcess.jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"pageEncoding="UTF-8"%><% String captcha=request.getParameter("captcha");%><% String captcha_session=(String)session.getAttribute("captchaCode");%><!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
<%if (captcha.equals(captcha_session))out.print("验证码正确~！");elseout.print("验证码不正确~！");
%>
</body>
</html><%@ page language="java" contentType="text/html; charset=UTF-8"pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head><meta charset="UTF-8"><title>登录页面之验证码原理</title><script>// JavaScript code to refresh captcha every 30 secondsfunction refreshCaptcha() {var img = document.getElementById("captchaImg");img.src = "./abc?" + new Date().getTime(); // add timestamp to force reload}setInterval(refreshCaptcha, 30000); // refresh captcha every 30 seconds</script>
</head>
<body><h1>登录页面之验证码原理</h1><form action="loginProcess.jsp" method="post"><div><label for="username">姓名:</label><input type="text" id="username" name="username"></div><div><label for="password">密码:</label><input type="password" id="password" name="password"></div><div><label for="captcha">验证码:</label><input type="text" id="captcha" name="captcha"><img id="captchaImg" width=100 src="./abc" alt="Captcha Image" onclick="refreshCaptcha()"><button type="button" onclick="refreshCaptcha()">刷新验证码</button></div><div><button type="submit">登录</button></div></form>
</body>
</html>package src;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.OutputStream;
import java.util.Base64;
import java.util.Random;
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;
@WebServlet("/abc")
public class GenerateCaptchaServlet extends HttpServlet {/*** @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)*/protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {// Set content type of the responseresponse.setContentType("image/jpeg");// Create a random captcha codeString captchaCode = generateRandomCode(6); // Generate 6-character random code// Store the captcha code in session for validationrequest.getSession().setAttribute("captchaCode", captchaCode);// Create a BufferedImage for captcha imageint width = 200;int height = 50;BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);Graphics2D g2d = image.createGraphics();// Draw the captcha code on the imageg2d.setFont(new Font("Arial", Font.BOLD, 24));g2d.setColor(Color.RED);g2d.drawString(captchaCode, 50, 30);// Dispose the graphicsg2d.dispose();// Output the image as JPEG to the clientOutputStream out = response.getOutputStream();ImageIO.write(image, "jpeg", out);out.close();}private String generateRandomCode(int length) {String charset = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";Random random = new Random();StringBuilder sb = new StringBuilder(length);for (int i = 0; i < length; i++) {int index = random.nextInt(charset.length());sb.append(charset.charAt(index));}return sb.toString();}
}
