main()

[Monica Dharwad]

Hi, Can you please explain this statement preferable with an example code.

"When main() ends, that may or may not be the end of the program. The JVM will run until the only remaining threads are daemon threads. "

[marc weber]

Welcome to JavaRanch!

Is your question asking what a daemon thread is? Or is your question asking how main could spawn a separate thread? (Or both?)

[Monica Dharwad]

Thanks a lot. I would like to know both.

[marc weber]

Without multithreading, when a method is called, the next line of code does not execute until the method returns.

doThis(); //then when method returns...
doThis(); //then when method returns...
doThis(); //then when method returns...
finish();

But with multithreading, a separate process can start in its own thread, allowing the calling thread to continue on with what it's doing.

startNewThread(); //then as soon as start call returns...
finish();

In this case, even after finish() returns, the new thread's run method can continue executing separately. As long as the new thread is not a daemon, this would keep the program alive.

Basically a daemon thread is a background process whose completion is not required for the program to end.

[Monica Dharwad]

Thanks a lot for the reply. It explains the concept very well.

[Nishita Jain]

When main() ends, that may or may not be the end of the program
i didnt get this line..
Because i think when main() ends it will surely terminate your program as it is main thread.
please clear my doubt

thank you.

[Vijay Kiran]

Main is just a starting point of your program. Check out the following program:

   
public class MainClass {
public static void main(String[] args) {
System.out.println("Start of method");
SomeThread thread = new SomeThread();
thread.start();
System.out.println("End of main Method");
}
}
 
public class SomeThread extends Thread{
 
public void run(){
while(true){
System.out.println("SomeThread running");
try {
Thread.sleep(Long.MAX_VALUE);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
 

Even after it prints out "End of main Method" the program will continue to run.

[Ulf Dittmer]

Because i think when main() ends it will surely terminate your program as it is main thread.

The program will terminate if all non-daemon threads have terminated. If a GUI is visible then there will be several non-daemon threads, e.g. the event handling thread. In many applications the main method terminates very quickly, right after constructing the GUI.
[ August 16, 2006: Message edited by: Ulf Dittmer ]

[Nishita Jain]

hello Vijay and Dittmer ,
thank you very much for your replies....i debug that code and cleared my doubt..
Nishita