public class MessageServiceQueueListener<T> implements MessageListener {
    
    private final int flushDBTimer;  
    private List<T> objectList;
    private ScheduledExecutorService executorFlushSchedule;

    public MessageServiceQueueListener(
            int flushDBTimer,
            ScheduledExecutorService executorFlushSchedule) {
        this.flushDBTimer = flushDBTimer;
        this.objectList = new ArrayList<>();
        this.executorFlushSchedule = executorFlushSchedule;
        
        this.executorFlushSchedule.scheduleAtFixedRate(
                    () -> {
                        if (objectList.size() > 0) {
                            ...
                        }
                    },
                    0,
                    flushDBTimer,
                    TimeUnit.SECONDS);
    }

Collections.synchronizedList(...)