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

Solution 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.

Solution 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.

Solution 3

The term "daemon thread" in Java is a bit misleading, as it really means "that thread is not supposed to keep the JVM alive". This means that the JVM will shut down as soon as the last non-daemon thread terminated (as you already stated in your question).

What you are possibly looking for is the Apache Commons Daemon project, which allows to create nice "system services", started through /etc/init.d/ entries and all. This works on Windows and *nix systems.

Comments

[Costi Ciudatu]

@M.J.: The JavaDoc for Thread.setDaemon() says: "The Java Virtual Machine exits when the only threads running are all daemon threads". So if your thread is a daemon, the JVM will exit. What you need is a non-daemon thread, if you want that to keep the JVM running.

[oberlies]

How about removing the parts of the answer which turned out to be incorrect after the question was clarified?