Why Your Program Will Not Exit

main reaches its last line and prints the completion message. The process does not end, and nothing in the logs says why.

java

Thread poller = new Thread(() -> {
    while (true) {
        checkForWork();
        try {
            Thread.sleep(1_000);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            return;
        }
    }
});

poller.start();
System.out.println("started");

The virtual machine exits when the last non-daemon thread finishes. main is one, the poller is another, and the poller never finishes — so the process stays alive with nothing useful left to do.

What the daemon flag changes
Compared on	Non-daemon, the default	Daemon
Keeps the process alive	Yes	No
Killed at exit	No — the machine waits	Yes, wherever it had got to
Suits	Work that must finish	Background polling, monitoring, cleanup

java

poller.setDaemon(true);
poller.start();

must be set before start(), or it throws

For work that must finish, keep it non-daemon and give it a way to stop — a volatile boolean the loop checks, or an interrupt. A thread you cannot ask to stop is a process you cannot shut down.