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
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.
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.
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