jfinal重写图形验证码，生成纯数字验证码(jfinal-3.3)。

public class CaptchaSmallRender extends CaptchaRender {

    // 验证码随机字符数组
    protected static final char[] charArray = "0123456789".toCharArray();

    @Override
    protected String getRandomString() {
        char[] randomChars = new char[4];
        for (int i = 0; i < randomChars.length; i++) {
            randomChars[i] = charArray[random.nextInt(charArray.length)];
        }
        return String.valueOf(randomChars);
    }

}

    //@ValidateTOKEN(isValidate = false)
    public void captcha() {
        render(new CaptchaSmallRender());
    }