Singleton, cannot go in the runnable task

Question

I'm trying to run a task automatically (all the 30s).

For that, I built a singleton :

public class PortalSingleton {
    private static final Logger LOG = LoggerFactory.getLogger(PortalSingleton.class);
    private static final int INITIAL_DELAY = 0;
    private static final int DELAY = 30;

    private static volatile ScheduledExecutorService instance;
    private static HomepageView homeView = new HomepageView();

    private PortalSingleton() {}

    public static final void refreshGridHomePageAutomatically() {
        Runnable task = () -> UI.getCurrent().access(() -> {
            homeView.refreshGrid();
            LOG.info("The grid has been refreshed Automatically");
        });
        getInstance().scheduleWithFixedDelay(task, INITIAL_DELAY, DELAY, TimeUnit.SECONDS);
    }

    public final static ScheduledExecutorService getInstance() {
        if (instance == null) {
            synchronized (ScheduledExecutorService.class) {
                if (instance == null) {
                    instance = Executors.newScheduledThreadPool(1);
                }
            }
        }
        return instance;
    }
}

But, I didn't have any issue/error AND I didn't have my log msg and my grid hasn't been refreshed..

The behavior expected is :

1. my grid refresh
2. see the log msg

Even if I delete the line homeView.refreshGrid();, I don't have my log msg...

What did I do wrong?

Thanks,

EDIT : I call it by doing : PortalSingleton.refreshGridHomePageAutomatically();

EDIT2, thanks @Holger :

public class PortalSingleton {
    private static final Logger LOG = LoggerFactory.getLogger(PortalSingleton.class);
    private static final int INITIAL_DELAY = 0;
    private static final int DELAY = 30;

    private static final ScheduledExecutorService instance = Executors.newScheduledThreadPool(1);
    private static HomepageView homeView = new HomepageView();

    private PortalSingleton() {
    }

    public static final void refreshGridHomePageAutomatically() {
        Runnable task = () -> UI.getCurrent().access(() -> {
            homeView.refreshGrid();
            LOG.info("The grid has been refreshed Automatically");
        });
        try {
            getInstance().scheduleWithFixedDelay(task, INITIAL_DELAY, DELAY, TimeUnit.SECONDS);
        } catch (Exception e) {
            LOG.error("error" + e);
        }
    }

    public final static ScheduledExecutorService getInstance() {
        return instance;
    }
}

Answer 1

When you schedule an action, you do not get a feedback when an exception occurs. Instead, it will just stop executing it:

ScheduledExecutorService.scheduleWithFixedDelay(…):

…If any execution of the task encounters an exception, subsequent executions are suppressed.

Therefore, you will have to use a try … catch block in the action itself to report it, e.g. in the lambda expression defining your Runnable:

Runnable task = () -> {
    try { UI.getCurrent().access(…); }
    catch (Exception e) { LOG.error("error" + e); }
};

It looks suspicious to me that you are calling UI.getCurrent() from a non-UI thread which I suspect to return null causing a NullPointerException when trying to invoke a method on it.