How to keep my program alive for as long a daemon thread is running?

Question

I have a requirement, that I want to start a poller once which will run foreever until the machine is restarted or the process is being killed. Now, I tried to start the poller from a main method using a shell script, but the problem is that as soon as the main method completed its execution, the poller also stoped working, as i am not using any servers to achieve so.

I heard something about daemon threads, but I am wondering how to create a daemon thread, which will run forever, and help my poller to run also.

UPDATE:

public class SomeThread extends Thread {

    @Override
    public void run() {
        UnitPoller unitPoller = new UnitPoller();
        unitPoller.doPolling();
    }

    public static void main(String[] args) {
        SomeThread someThread = new SomeThread();
        someThread.setDaemon(true);
        someThread.start();
    }
}

Above is my updated class, now whenever I execute this thread from the main method, it creates a thread but as soon as the execution of main method completes, my poller stops working, as the JVM shuts down.

With this problem, what should i do.

Thanks

Answer 1

You just create a thread and call th.setDaemon(true) before calling th.start().

Edit:

The above answers the question "how to create a daemon thread", but (as the scope of the question has changed), a proper answer would be: don't create a daemon thread if you want your thread to keep the JVM from exiting once the main thread completed.

Answer 2

1) You need someThread.setDaemon(false) instead of 'true'. A daemon thread actualy does NOT stop java from shutting down.

From the javadoc:

void java.lang.Thread.setDaemon(boolean on)

Marks this thread as either a daemon thread or a user thread. The Java Virtual Machine exits when the only threads running are all daemon threads.

This method must be called before the thread is started.

2) I think it's not your main, but your run() method that finishes to soon. Try to put a while (true) loop around your doPolling method.

@Override
public void run() {
    UnitPoller unitPoller = new UnitPoller();
    while (true)
       unitPoller.doPolling();
}

3) It's cleaner to call join() inside the main then to rely on daemon thread behavior.

    try {
        someThread.join();
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

4) If you need a clean way to shut down the deamonthread. Consider implementing InterruptedException to exit the polling task. You can also use the shutdown hook.