if main method completes the execution, what happens to any long running thread?

Question

since main() runs on a thread. and as soon as the main() finishes, main-thread should stop. So if main() has invoked a long running thread which is yet to finish even after main() has done all the task. Since main() is returned, would the other threads be terminated? i guess no. but why?

public static void main(String[] s){    
    new LongRunningThread().start();
}

The JVM only exits once all non daemon threads have completed. So yes your program would exit after your long task completes. — assylias, May 07 '13 at 18:53

score 10 · Accepted Answer · edited Jun 20 '20 at 09:12

10

The process will terminate when there are no more non-daemon threads, killing any daemon threads if necessary. However, if you do have any non-daemon threads, those will prevent the process from terminating.

From Thread.setDaemon:

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 invoked before the thread is started.

And from section 12.8 of the JLS:

A program terminates all its activity and exits when one of two things happens:

All the threads that are not daemon threads terminate.

Some thread invokes the exit method of class Runtime or class System, and the exit operation is not forbidden by the security manager.

edited Jun 20 '20 at 09:12

Community

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answered May 07 '13 at 18:52

Jon Skeet

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On a related note, in what state the main thread would be then? since it's execution is complete but other threads are preventing it from stopping. – Ankit Nov 25 '17 at 18:21
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@Ankit: No, other threads are preventing *the process from terminating*. The main thread has completed and doesn't exist any more. But the process keeps running because there are other foreground threads. – Jon Skeet Nov 25 '17 at 18:37

score 1 · Answer 2 · answered May 07 '13 at 18:58

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if your long running thread is not a daemon thread, it will not get terminated once the main thread exits. The JVM continues to run threads until the exit method of Runtime is called (and permitted to run) or all non-daemon threads have died. If your long running thread is not a daemon thread, JVM will not exit (i.e. thread will continue to be available for running).

answered May 07 '13 at 18:58

ali haider

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I just saw the post above by Jon - I would suggest that you use that as the answer. – ali haider May 07 '13 at 18:59

score 1 · Answer 3 · answered May 07 '13 at 19:03

To answer your question why, it's because making your thread a non-daemon thread means that you don't want it to be terminated abruptly, you want it to be terminated in an orderly way by running to completion or by being canceled. If your thread was killed by the JVM once main was exited that would be equivalent to making the thread a daemon.

if main method completes the execution, what happens to any long running thread?

3 Answers3

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