private MysqlConnectionPool() throws DaoConnectException{
        if (DB_MAX_CONNECTIONS == null || DB_MAX_CONNECTIONS <= 0) {
            throw new DaoConnectException("Error configuration db.maxconnect.");
        }

        queue = new ArrayBlockingQueue<>(DB_MAX_CONNECTIONS);
        for (int i = 0; i < DB_MAX_CONNECTIONS; i++) {
            queue.offer(createNewConnection());
        }
    }