Question

I want to write a command line daemon that runs forever. I understand that if I want the JVM to be able to shutdown gracefully in linux, one needs to wrap the bootstrap via some C code. I think I’ll be ok with a shutdown hook for now.

On to my questions:

1. My main(String[]) block will fire off a separate Superdaemon.
2. The Superdaemon will poll and loop forever.

So normally I would do:

class Superdaemon extends Thread { ... }  class Bootstrap {     public static void main( String[] args )     {         Thread t = new Superdaemon();         t.start();          t.join();     } } 

Now I figured that if I started Superdaemon via an Executor, I can do

Future<?> f = exec.submit( new Superdaemon() );  f.get(); 

Is Future.get() implemented with Thread.join() ? If not, does it behave equivalently ?

Regards,

ashitaka

Answer 1

Yes, the way you’ve written these is equivalent.

However, you don’t really need to wait for the Superdaemon thread to complete. When the main thread finishes executing main(), that thread exits, but the JVM will not. The JVM will keep running until the last non-daemon thread exits its run method.

For example,

public class KeepRunning {   public static void main(String[] args) {     Superdaemon d = new Superdaemon();     d.start();     System.out.println(Thread.currentThread().getName() + ': leaving main()');   } }  class Superdaemon extends Thread {   public void run() {     System.out.println(Thread.currentThread().getName() + ': starting');     try { Thread.sleep(2000); } catch(InterruptedException e) {}     System.out.println(Thread.currentThread().getName() + ': completing');   } } 

You’ll see the output:

main: leaving main() Thread-0: starting Thread-0: completing 

In other words, the main thread finishes first, then the secondary thread completes and the JVM exits.