An application exits when there are no non-daemon threads running. : Daemon Thread « Thread « Java Tutorial

1. Java Tutorial
2. Thread
3. Daemon Thread

class MyThread extends Thread {
  MyThread() {
    setDaemon(true);
  }

  public void run() {
    boolean isDaemon = isDaemon();
    System.out.println("isDaemon:" + isDaemon);
  }
}

public class Main {
  public static void main(String[] argv) throws Exception {
    Thread thread = new MyThread();
    thread.setDaemon(true);
    thread.start();
  }
}