例子：
生成验证码的servlet：
package yzy.servlet; import javax.imageio.ImageIO; import javax.servlet.ServletException; import javax.servlet.http.Cookie; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpSession; import java.awt.*; import java.awt.geom.AffineTransform; import java.awt.image.BufferedImage; import java.io.IOException; import java.io.OutputStream; import java.util.Arrays; import java.util.Random; /** * 验证码Servlet * * @version 1.0 */ public class VerificationCodeImageServlet extends HttpServlet { //使用到Algerian字体，系统里没有的话需要安装字体，字体只显示大写，去掉了1,0,i,o几个容易混淆的字符 private final String VERIFY_CODES = "23456789ABCDEFGHJKLMNPQRSTUVWXYZ"; private Random random = new Random(); public void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setHeader("Pragma", "No-cache"); response.setHeader("Cache-Control", "no-cache"); response.setDateHeader("Expires", 0); response.setContentType("image/jpeg"); //生成随机字串 String verifyCode = generateVerifyCode(4, VERIFY_CODES); //存入会话session HttpSession session = request.getSession(true); //删除以前的 session.removeAttribute("verCode"); session.setAttribute("verCode", verifyCode.toLowerCase()); //生成图片 int w = 100, h = 30; Cookie cookie = new Cookie("v_c_v", verifyCode); cookie.setPath("/"); response.addCookie(cookie); outputImage(w, h, response.getOutputStream(), verifyCode); } /** * 使用指定源生成验证码 * @param verifySize 验证码长度 * @param sources 验证码字符源 * @return */ private String generateVerifyCode(int verifySize, String sources){ int codesLen = sources.length(); Random rand = new Random(System.currentTimeMillis()); StringBuilder verifyCode = new StringBuilder(verifySize); for(int i = 0; i < verifySize; i++){ verifyCode.append(sources.charAt(rand.nextInt(codesLen-1))); } return verifyCode.toString(); } /** * 输出指定验证码图片流 * @param w * @param h * @param os * @param code * @throws IOException */ private void outputImage(int w, int h, OutputStream os, String code) throws IOException{ int verifySize = code.length(); BufferedImage image = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB); Random rand = new Random(); Graphics2D g2 = image.createGraphics(); g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,RenderingHints.VALUE_ANTIALIAS_ON); Color[] colors = new Color[5]; Color[] colorSpaces = new Color[] { Color.WHITE, Color.CYAN, Color.GRAY, Color.LIGHT_GRAY, Color.MAGENTA, Color.ORANGE, Color.PINK, Color.YELLOW }; float[] fractions = new float[colors.length]; for(int i = 0; i < colors.length; i++){ colors[i] = colorSpaces[rand.nextInt(colorSpaces.length)]; fractions[i] = rand.nextFloat(); } Arrays.sort(fractions); g2.setColor(Color.GRAY);// 设置边框色 g2.fillRect(0, 0, w, h); Color c = getRandColor(200, 250); g2.setColor(c);// 设置背景色 g2.fillRect(0, 2, w, h-4); //绘制干扰线 Random random = new Random(); g2.setColor(getRandColor(160, 200));// 设置线条的颜色 for (int i = 0; i < 20; i++) { int x = random.nextInt(w - 1); int y = random.nextInt(h - 1); int xl = random.nextInt(6) + 1; int yl = random.nextInt(12) + 1; g2.drawLine(x, y, x + xl + 40, y + yl + 20); } // 添加噪点 float yawpRate = 0.05f;// 噪声率 int area = (int) (yawpRate * w * h); for (int i = 0; i < area; i++) { int x = random.nextInt(w); int y = random.nextInt(h); int rgb = getRandomIntColor(); image.setRGB(x, y, rgb); } shear(g2, w, h, c);// 使图片扭曲 g2.setColor(getRandColor(100, 160)); int fontSize = h-4; Font font = new Font("Algerian", Font.ITALIC, fontSize); g2.setFont(font); char[] chars = code.toCharArray(); for(int i = 0; i < verifySize; i++){ AffineTransform affine = new AffineTransform(); affine.setToRotation(Math.PI / 4 * rand.nextDouble() * (rand.nextBoolean() ? 1 : -1), (w / verifySize) * i + fontSize/2, h/2); g2.setTransform(affine); g2.drawChars(chars, i, 1, ((w-10) / verifySize) * i + 5, h/2 + fontSize/2 - 10); } g2.dispose(); ImageIO.write(image, "jpg", os); } private Color getRandColor(int fc, int bc) { 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); } private int getRandomIntColor() { int[] rgb = getRandomRgb(); int color = 0; for (int c : rgb) { color = color << 8; color = color | c; } return color; } private int[] getRandomRgb() { int[] rgb = new int[3]; for (int i = 0; i < 3; i++) { rgb[i] = random.nextInt(255); } return rgb; } private void shear(Graphics g, int w1, int h1, Color color) { shearX(g, w1, h1, color); shearY(g, w1, h1, color); } private void shearX(Graphics g, int w1, int h1, Color color) { int period = random.nextInt(2); boolean borderGap = true; int frames = 1; int phase = random.nextInt(2); for (int i = 0; i < h1; i++) { double d = (double) (period >> 1) * Math.sin((double) i / (double) period + (6.2831853071795862D * (double) phase) / (double) frames); g.copyArea(0, i, w1, 1, (int) d, 0); if (borderGap) { g.setColor(color); g.drawLine((int) d, i, 0, i); g.drawLine((int) d + w1, i, w1, i); } } } private void shearY(Graphics g, int w1, int h1, Color color) { int period = random.nextInt(40) + 10; // 50; boolean borderGap = true; int frames = 20; int phase = 7; for (int i = 0; i < w1; i++) { double d = (double) (period >> 1) * Math.sin((double) i / (double) period + (6.2831853071795862D * (double) phase) / (double) frames); g.copyArea(i, 0, 1, h1, 0, (int) d); if (borderGap) { g.setColor(color); g.drawLine(i, (int) d, i, 0); g.drawLine(i, (int) d + h1, i, h1); } } } }
有验证码的jsp页面（前端验证）：
<form action="${pageContext.request.contextPath}/main.do" method="post"> <div class="name"> <input type="text" id="name" name="username" placeholder="请输入登录用户名"> <p></p> </div> <div class="pwd"> <input type="password" id="pwd" name="password" placeholder="6-16位密码，区分大小写，不能用空格"> <p></p> </div> <div class="idcode"> <input type="text" id="verificationCode" placeholder="请输入验证码"> <a href='#' onclick="javascript:changeImg()"> 换一张</a> <span><img id="img" src="${pageContext.request.contextPath}/verificationCode.do"/></span> <div class="clear"></div> </div> <div class="autoLogin"> <label for=""> <input type="checkbox" checked="checked"> 下次自动登录 </label> <a href="">忘记密码</a> </div> <div class="btn-red"> <input onclick="return checkVerificationCode();" type="submit" value="登录" id="login-btn"> </div> </form>
对应的js代码：
<script type="text/javascript"> function changeImg() { var img = document.getElementById("img"); img.src = "${pageContext.request.contextPath}/verificationCode.do?date=" + new Date(); } function checkVerificationCode() { var verificationCode = document.getElementById('verificationCode').value; var flag = (getCookie('v_c_v').toLowerCase() == verificationCode.toLowerCase()); if (!flag) { alert('验证码输入错误'); } return flag; } function getCookie(cookie_name) { var allCookies = document.cookie; var cookie_pos = allCookies.indexOf(cookie_name); //如果找到了索引，就代表cookie存在 if (cookie_pos != -1) { cookie_pos += cookie_name.length + 1; var cookie_end = allCookies.indexOf(";", cookie_pos); if (cookie_end == -1) { cookie_end = allCookies.length; } return unescape(allCookies.substring(cookie_pos, cookie_end)); } return null; } </script>