纯Java实现CAPTCHA验证码，验证码被机器破解由易到难。
图片如下：
eazy：
normal：
hard：
very hard：
1.指明验证码的宽和高，以及画板和画笔
// 验证码图片的宽度。int width = 70;// 验证码图片的高度。int height = 30;BufferedImage image=new BufferedImage(width, height, BufferedImage.TYPE_3BYTE_BGR);Graphics g=image.getGraphics();// 随机操作對象Random random=new Random();
2.生成随机字符串（验证码）
String s ="";for(int i=0;i<4;i++){s=getRandomCode(0, 4, null);System.out.println(s);}/*** 生成随机字符串* @param type 类型 0：大小写+数字 1：大小写 2：数字* @param length长度* @param exChars排除的字符* @return*/public static String getRandomCode(int type,int length,String exChars){StringBuffer sb=new StringBuffer();Random random=new Random();int i=0;switch (type) {case 0:while(i<length){int t=random.nextInt(123);if((t>=97||(t>=65&&t<=90)||(t>=48&&t<=57))&&(exChars==null||exChars.indexOf((char)t)<0)){sb.append((char)t);i++;}}break;case 1:while(i<length){int t=random.nextInt(123);if((t>=97||(t>=65&&t<=90))&&(exChars==null||exChars.indexOf((char)t)<0)){sb.append((char)t);i++;}}break;case 2:while(i<length){int t=random.nextInt(123);if(((t>=48&&t<=57))&&(exChars==null||exChars.indexOf((char)t)<0)){sb.append((char)t);i++;}}break;default:break;}return sb.toString();}
3.设置背景颜色、字体以及边框
// 设定图像背景色(因为是做背景，所以偏淡)Color color=getRandomColor(200, 250);g.setColor(color);g.fillRect(0, 0, width, height);// 创建字体，字体的大小应该根据图片的高度来定。Font font = new Font("Times New Roman", Font.HANGING_BASELINE, 28);// 设置字体。g.setFont(font);
// 画边框。g.setColor(Color.BLACK);g.drawRect(0, 0, width - 1, height - 1);
4.随机生成多条干扰线，加大攻击难度。
// 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到。g.setColor(getRandomColor(160, 200));for (int i = 0; i < 155; i++) {int x = random.nextInt(width);int y = random.nextInt(height);int xl = random.nextInt(12);int yl = random.nextInt(12);g.drawLine(x, y, x + xl, y + yl);}
其中getRandomColor():
// 给定范围获得随机颜色public static Color getRandomColor(int fc, int bc) {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);}
5.按位写入验证码，分顺序分布和随机分布。
// 写入验证码。for (int i = 0; i < s.length(); i++) {String strRand = s.charAt(i)+"";// 生成随机颜色g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));//等间隔出现 //g.drawString(strRand, 15*i + 6 , 24);//随机位置出现g.drawString(strRand, (int) (15*i+Math.random()*10) , (int) (Math.random()*10+20));}
6.再进行扭曲图片。
//扭曲图片shearX(g, width, height, color);shearY(g, width, height, color);
//延水平方向扭曲图片private static void shearX(Graphics g, int w1, int h1, Color color) {Random random=new Random();int period = 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 + (2.2831853071795862D * (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);}}}//延垂直方向扭曲图片private static void shearY(Graphics g, int w1, int h1, Color color) {Random random=new Random();int period = random.nextInt(5) + 10; // 35;boolean borderGap = true;int frames = 20;int phase = random.nextInt(2);for (int i = 0; i < w1; i++) {double d = (double) (period >> 1)* Math.sin((double) i / (double) period+ (2.2831853071795862D * (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);}}}
6.图像生效，并以文件形式输出。
// 图象生效g.dispose();output(image);
其中 output(image) 输出jpg文件 :
//将图片输出成文件public static void output(BufferedImage image) {//String randomCode = getRandomCode(0, 4, null);try {File file = new File("d:/test_captcha.jpg");ImageIO.write(image, "jpg", file);} catch (IOException e) {// TODO: handle exceptionSystem.out.println("保存失敗");e.printStackTrace();}}
文章多处参考自：/maixiaodou/p/7305729.html
