java生成随机数

    /**     * 生成20位的随机数,用系统当前时间的毫秒数(13)+七位随机数     * @return     */    public String generateStr() {        StringBuffer var = new StringBuffer(String.valueOf(System.currentTimeMillis()));        int len = var.length();        if (len < 20) len = 20 - len;        Random random = new Random();        String allChar = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";        for (int i = 0; i < len; i++) {            var.append(allChar.charAt(random.nextInt(allChar.length())));        }        return var.toString();    }