ScheduledExecutorService exiting before executing virtual thread

Question

I'm facing an unusual behavior when trying to experiment with java 19 virtual thread, the code below works perfectly with platform thread. but it exits suddenly when i switch to virtual thread. adding while(true); at the end solves the problem and the program keeps printing "hello". is it possible to use virtual thread with ScheduledExecutorService?

        ThreadFactory factory = Thread.ofVirtual().factory();
        ScheduledExecutorService scheduledExecutorService = Executors.newSingleThreadScheduledExecutor(factory);
            scheduledExecutorService.scheduleAtFixedRate(() -> {
                System.out.println("hello");
            }, 0, 5000, TimeUnit.MILLISECONDS);
    }
   //while(true);

From [the Javadoc](https://download.java.net/java/early_access/loom/docs/api/java.base/java/lang/Thread.html): "_Virtual threads are daemon threads and so do not prevent the Java virtual machine from terminating_". Seems similar to, but not exactly the same as, [this problem](https://stackoverflow.com/questions/51879659/completablefuture-is-not-getting-executed-if-i-use-the-executorservice-pool-it) (though you can't apply the same fix, as it looks like virtual threads are _always_ daemon threads, and you can't change that). — Slaw, Jun 03 '22 at 16:30

Slaw · Accepted Answer · 2022-06-03T18:18:52.847

From the documentation:

Virtual threads are daemon threads and so do not prevent the Java virtual machine from terminating.

This appears to be absolute; there's no way to make a virtual thread non-daemon.

So, if you want to use a thread pool (scheduled or not) which uses virtual threads, then you'll need to make sure at least one non-daemon platform thread remains alive for as long as you need. Your while (true); loop currently does this, but in a way I strongly discourage; even sleeping the thread would be better, though still not great. In a more complex application, like a server or desktop application, you'll likely have one or more non-daemon threads doing work anyway (e.g., the "event loop" in a desktop application) that will keep the JVM alive.

ScheduledExecutorService exiting before executing virtual thread

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