The daemon thread's life cycle is same with the life cycle of the application which starts this daemon thread. If the application finishes, daemon threads are terminated at the same time. An example for a daemon thread is the garbage collection.

"Daemon Thread" is also called "Service Thread"; normal threads are called "User Thread".

Additionally, please do not use daemon threads to do I/O operations.

The following example is that one application starts a user thread which runs a daemon thread with infinity loop.

The user thread dies first, but the daemon thread continues.

If the application dies, the daemon thread is dead immediately.

public class DaemonThread extends Thread {
    public DaemonThread() {
        this.setDaemon(true);
    }
    @Override
    public void run() {
        while (true) {
            System.out.println("Begin to sleep.");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Wake up.");
        }
    }
}

public class MainApp implements Runnable{
    
    @Override
    public void run(){
        DaemonThread dt = new DaemonThread();
        dt.start();
        System.out.println("The user thread is terminated.");
    }

public static void main(String[] args) {
        Thread thread = new Thread(new MainApp());
        thread.start();
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("The main thread is terminated.");
    }

}

Output:

The user thread is terminated.
Begin to sleep.
Wake up.
Begin to sleep.
Wake up.
Begin to sleep.
The main thread is terminated.