首先从网上下载一个生成验证码的Servlet：
package com.train.controller;import java.awt.Color;import java.awt.Font;import java.awt.Graphics;import java.awt.Graphics2D;import java.awt.image.BufferedImage;import java.io.BufferedReader;import java.io.FileInputStream;import java.io.IOException;import java.io.InputStreamReader;import java.util.ArrayList;import java.util.List;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("/CodeServlet")public class CodeServlet extends HttpServlet {private static final long serialVersionUID = 1L;// 集合中保存所有成语private List<String> words = new ArrayList<String>();public CodeServlet() {super();}@Overridepublic void init() throws ServletException {// 初始化阶段，读取new_words.txt// web工程中读取 文件，必须使用绝对磁盘路径String path = getServletContext().getRealPath("/WEB-INF/new_words.txt");try {BufferedReader reader = new BufferedReader(new InputStreamReader(new FileInputStream(path), "utf-8"));String line;while ((line = reader.readLine()) != null) {words.add(line);}reader.close();} catch (IOException e) {e.printStackTrace();}}/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse * response) */protected void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {int width = 120;int height = 36;// 步骤一 绘制一张内存中图片BufferedImage bufferedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);// 步骤二 图片绘制背景颜色 ---通过绘图对象Graphics graphics = bufferedImage.getGraphics();// 得到画图对象 --- 画笔// 绘制任何图形之前 都必须指定一个颜色graphics.setColor(getRandColor(200, 250));graphics.fillRect(0, 0, width, height);// 步骤三 绘制边框graphics.setColor(Color.WHITE);graphics.drawRect(0, 0, width - 1, height - 1);// 步骤四 四个随机数字Graphics2D graphics2d = (Graphics2D) graphics;// 设置输出字体graphics2d.setFont(new Font("宋体", Font.BOLD, 18));Random random = new Random();// 生成随机数int index = random.nextInt(words.size());String word = words.get(index);// 获得成语// 定义x坐标int x = 10;for (int i = 0; i < word.length(); i++) {// 随机颜色graphics2d.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));// 旋转 -30 --- 30度int jiaodu = random.nextInt(60) - 30;// 换算弧度double theta = jiaodu * Math.PI / 180;// 获得字母数字char c = word.charAt(i);// 将c 输出到图片graphics2d.rotate(theta, x, 20);graphics2d.drawString(String.valueOf(c), x, 20);graphics2d.rotate(-theta, x, 20);x += 30;}// 将验证码内容保存session作用域request.getSession().setAttribute("checkcode_session", word);// 步骤五 绘制干扰线graphics.setColor(getRandColor(160, 200));int x1;int x2;int y1;int y2;for (int i = 0; i < 30; i++) {x1 = random.nextInt(width);x2 = random.nextInt(12);y1 = random.nextInt(height);y2 = random.nextInt(12);graphics.drawLine(x1, y1, x1 + x2, x2 + y2);}// 将上面图片输出到浏览器 ImageIOgraphics.dispose();// 释放资源ImageIO.write(bufferedImage, "jpg", response.getOutputStream());}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse * response) */protected void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {// TODO Auto-generated method stubdoGet(request, response);}private Color getRandColor(int fc, int bc) {// 取其随机颜色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);}}如代码里写的，我们需要一个new_words.txt用来生成图片验证码：
在登录页面添加验证码：
<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>Insert title here</title><link rel="stylesheet" rel="external nofollow" href="css/style.css" /></head><body><div class="register"><form action="/Train1/loginServlet" method="post"><div class="login-box"><table width="350" height="200"><tr align="center"><td><div>用户名：</div></td><td colspan="2"><input type="text" name="userName" /></td></tr><tr align="center"><td><div>密 码：</div></td><td colspan="2"><input type="password" name="passWord" /></td></tr><tr><td>验证码：<input type="text" id="checkCode" name="checkCode" /></td><td><img id="imageCode" οnclick="changeImage()"src="CodeServlet"></td></tr><tr align="center"><td colspan="3"><input type="submit"style="width: 50px; background-color: red;" value="登录" /></td></tr><tr><td><%=request.getAttribute("msg")%></td></tr></table></div></form></div><script type="text/javascript">function changeImage() {document.getElementById("imageCode").src="CodeServlet?"+new Date().getTime();}</script></body></html>changeImage()方法里的+new Date().getTime()是为了让用户点击一下就能换图片。
在Servlet里验证用户输入的验证码是否正确，如果不正确就给出提示，拒绝登录：
protected void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("utf-8");String userName = request.getParameter("userName");String passWord = MD5Utils.md5(request.getParameter("passWord"));userModel model = new userModel();model.setUserName(userName);model.setPassWord(passWord);userService service = new userService();if (request.getSession().getAttribute("checkcode_session").equals(request.getParameter("checkCode"))) {try {userModel md = service.selectByNameAndPass(model);if (md != null) {// 找到记录，登录成功request.getSession().setAttribute("userName", md.getUserName());request.getRequestDispatcher("welcome.jsp").forward(request, response);} else {request.setAttribute("msg", "登录失败！请重试！");request.getRequestDispatcher("login.jsp").forward(request, response);}} catch (Exception e) {e.printStackTrace();}} else {request.setAttribute("msg", "验证码输入有误！请重试！");request.getRequestDispatcher("login.jsp").forward(request, response);}}即可实现登录图片验证码功能，点击一次换一次图片。