setDaemon()

[mu gaandimara]

I did a llittle RnD on setDaemon() method of Thread and found that it does nothing.

What is the use of this method ???

[Sebastian Janisch]

Let's say your java application employs 3 Threads. None of them is a deamon.

This means, the application will not shut down until all Threads have finished their work.

Now, if Thread A is non-deamon, Thread B and C are deamons, and Thread A finishes work, Thread B and C are just killed and the application shuts down.

This is helpful for background tasks.

[mu gaandimara]

Thank you

It means it will execute unless and until the application is stopped.

[Sebastian Janisch]

No, it means the application doesn't care to wait for the Deamon Threads to finish. As soon as the regular threads are done, the deamons are killed in the middle of what they were doing.

Try this

public class DeamonTest extends Thread {
    public void run() {
        for(int i = 0; i < 10; i++) {
            System.out.println(i);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
            }
        }
    } 

    public static void main(String... args) {
        Thread t = new DeamonTest();
        t.setDeamon(true);
        t.start();
    }
}

run this, and then remove the setDeamon and run it again.

[mu gaandimara]

Is it possible to know programmetically if such thread is alive or not ???

[Sebastian Janisch]

Thread.isAlive()

[mu gaandimara]

How can I know this ???

The daemon thread will not die as long as any other threads exist. If all threads die, how can we call isAlive() ???

Can we make the thread object call any method when the thread dies ???

[Sebastian Janisch]

You should read the Thread specification.

This should help
http://java.sun.com/docs/books/tutorial/essential/concurrency/

[Himanshu Kansal]

Thread.isAlive() should be used carefully in such a scenario. It would be fruitful if it is used before the completion of the normal thread. If the non-daemon has already ended, Thread.isAlive is bound to return false.

Regards

[Himanshu Kansal]

rebius hagrid wrote: Can we make the thread object call any method when the thread dies ???

Anything that happens in your thread, or in other words, anything your thread does is there written by you in the run() method or however you pragrammed it. Before run() ends, you can do a notificatoin.

But there are other better ways to make notifications, the link marked by Sebastian is really helpful

[mu gaandimara]

I will be grateful somebody gives a program regarding this ???

[Henry Wong]

I know that I am going to catch flak for this, but... most of the time, this functionality won't be used. It is rare that an application will just have the user threads terminate, and use the daemon criteria to determine whether a JVM will exit.

Most of the time, the exit criteria is determine by a very specific event, which triggers a shutdown (saving resources, etc) and exit of the JVM (using System exit).

Interesting feature. Nice to know. But in most cases, not used...

Henry

[Ulf Dittmer]

I find daemon threads useful for background tasks that don't have any special cleanup needs. In other words, nobody gets bothered if the thread just goes away in the middle of doing something because the JVM quits, or the web app is stopped, or whatever. If those threads are daemons, they don't need to be stopped explicitly - so there's no need to keep track of them, or to add logic to terminate them gracefully.