Deamon Threads

[harshada patil]

public class A extends Thread {
 A() {
	 setDaemon(true);
	 }
	
	 public void run() {
		
	(new B()).start();
	 try {
	Thread.sleep(600);
	 } catch (InterruptedException x) {}
	 System.out.println("A done");
	 }
	
	 class B extends Thread {
	 public void run() {
		 
	 try {
	 Thread.sleep(600);
	 } catch (InterruptedException x) {}
	System.out.println("B done");
	 }
	 }
	
	 public static void main(String[] args) {
	(new A()).start();
	
	 }
	 }

What is the result?
A. A done
B. B done
C. A done
B done
D. B done
A done
E. There is no exception that the application will print anything.
F. The application outputs “A done” and “B done”, in no guaranteed order.


Why the answer is E?

[Keith Rainey]

Daemon threads only run as long as there are User Threads (non-daemon threads) running.

Once you start thread A in the main method, the 'main thread' (which is the only remaining user thread) ends. At that point the JVM shuts down the daemon thread. This happens before Thread A has the opportunity to print its own output or create thread B.

[Keith Rainey]

If you comment out the setDaemon() line, you will probably see it prints out the output lines for both A & B threads. In that situation, you are not creating a Daemon thread. Both A & B are user threads.

[Nomaan Butt]

call the sleep() method just after you call the start() method of Thread A, this will validate Keith's reply