图形验证码

public class CaptchaServlet extends HttpServlet {

    // 验证码宽度
    private static int width = 80;

    // 验证码高度
    private static int height = 28;
    
    // 验证码字符长度
    private static int captchaCharLength = 4;

    // 验证码的字符集
    private static char[] captchaChars = "ABCEHKN23456789PRSTXYZ".toCharArray();

    // 验证码的字符集大小
    private static int captchaCharsSize = captchaChars.length;
    
    // 验证码存储在 session 的键名
    private static final String CAPTCHA_KEY = "_CAPTCHA_SESSION_KEY_";

    private static final long serialVersionUID = 5438961220479548231L;

    @Override
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
        
        response.setHeader("Cache-Control", "no-store");
        response.setHeader("Pragma", "no-cache");
        response.setDateHeader("Expires", 0);
        response.setContentType("image/jpeg");
        
        String captchaText = generateRandomCode();
        
        BufferedImage image = generateImage(captchaText);
        
        HttpSession session = request.getSession();
        session.setAttribute(CAPTCHA_KEY, captchaText);
        
        try (OutputStream out = response.getOutputStream()) {
            ImageIO.write(image, "jpg", out);
        }
        
    }
    
    /**
     * 生成随机码字符串
     */
    private String generateRandomCode() {
        Random random = ThreadLocalRandom.current();
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < captchaCharLength; i++) {
            builder.append(captchaChars[random.nextInt(captchaCharsSize)]);
        }
        return builder.toString();
    }
    
    /**
     * 生成验证码图片
     * 
     * @param captchaText
     *            验证码字符串
     * @return BufferedImage
     */
    private BufferedImage generateImage(String captchaText) {
        
        BufferedImage image = new BufferedImage(width, height,
                BufferedImage.TYPE_INT_RGB);
        
        Graphics2D graphics = (Graphics2D) image.getGraphics();
        
        // 设置背景颜色
        graphics.setColor(Color.WHITE);
        graphics.fillRect(0, 0, width, height);
        
        // 设置边框
        graphics.setColor(Color.GRAY);
        graphics.drawRect(0, 0, width - 1, height - 1);
        
        // 设置字体
        graphics.setColor(Color.BLACK);
        graphics.setFont(new Font("Calibri", Font.BOLD, height - 6));
        
        // 平滑不锯齿
        graphics.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
        
        int length = captchaText.length();
        
        char[] chars = captchaText.toCharArray();
        
        // 绘制验证码字符
        for (int i = 0; i < length; i++) {
            graphics.drawChars(chars, i, 1, ((width - 8) / length) * i + 7,
                    height / 2 + length / 2 + 6);
        }
        
        graphics.dispose();
        
        return image;
        
    }

}

web.xml

<servlet>
    <servlet-name>CaptchaServlet</servlet-name>
    <servlet-class>org.fanlychie.servlet.CaptchaServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>CaptchaServlet</servlet-name>
    <url-pattern>/captcha</url-pattern>
</servlet-mapping>

JSP

<img src="captcha" onclick="this.src+=''" style="cursor:pointer;" title="看不清？换一个">

干扰线

在 graphics.dispose(); 前调用 drawLine(graphics);

private void drawLine(Graphics2D graphics) {
    Point2D[] point2ds = getPoints(.1f, .3f, .5f, .9f);
    graphics.setRenderingHints(new RenderingHints(
            RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON));
    graphics.setColor(Color.BLACK);
    graphics.setStroke(new BasicStroke(2));
    for (int i = 0; i < point2ds.length - 1; i++) {
        graphics.drawLine(
                (int) point2ds[i].getX(),
                (int) point2ds[i].getY(),
                (int) point2ds[i + 1].getX(), 
                (int) point2ds[i + 1].getY());
    }
}

private Point2D[] getPoints(float f1, float f2, float f3, float f4) {
    Random rand = ThreadLocalRandom.current();
    CubicCurve2D curve2d = new CubicCurve2D.Float(
            width * f1, (height * (rand.nextInt(8) + 2)) * .1f, 
            width * f2, (height * (rand.nextInt(8) + 2)) * .1f,
            width * f3, (height * (rand.nextInt(8) + 2)) * .1f, 
            width * f4, (height * (rand.nextInt(8) + 2)) * .1f);
    PathIterator iterator = curve2d.getPathIterator(null, 3);
    Point2D point2ds[] = new Point2D[200];
    int count = 0;
    float[] coords = new float[6];
    while (!iterator.isDone()) {
        switch (iterator.currentSegment(coords)) {
        case PathIterator.SEG_MOVETO:
        case PathIterator.SEG_LINETO:
            point2ds[count] = new Point2D.Float(coords[0], coords[1]);
        }
        count++;
        iterator.next();
    }
    Point2D[] points = new Point2D[count];
    System.arraycopy(point2ds, 0, points, 0, count);
    return points;
}