JVM exits once main method exits
This is NOT TRUE. The reason is , the thread which runs the main(String[] args) method,( I assume here 'main' means the entry-point method for any Java appln.) can create 1 or more threads . Assume that the main method's thread creates 2 more threads and starts them. Also know that apart from these 3, (1 main's thread+2 created threads by main's thread) there are other system thread (so called daemon threads) which are in fact service threads which help runnning the application. For example the GUI thread, GC thread etc.

So all the thread are running in this application. We CAN NOT gurantee that all threads will finish up at the same time OR in fact the child thread created by the main's thread will finish at all. What if one of the child threads is suspended ? What if one of the child threads is waiting for something to happen? What if the child thread is in fact doing a time-consuming work and not yet finished? The main's thread could have finished earlier. Does it means JVM abrubts the child thread's work and terminates? Not at all. The JVM waits until all the user thread are done and happy. If there is no gurantee that a thread may be terminated by JVM then what's the use of creating a thread itself , afraid of may be stopped at any time? Then there is no reliable functionality at any time right? One thread's functionalty depens on other if it were the case.

The JVM exits when all the users threads are finished successfully and left over are ONLY the servicing threads from JVM.
See the concept in action now.

class Test {
	public static void main(String[] args) throws Exception{
		UserThread thread1 = new UserThread();
		UserThread thread2 = new UserThread();
		thread1.start();
		thread2.start();
		System.out.println("Hello all. Main(String[]) end in 1 sec. :-)");
		Thread.sleep(1000);
	}

}
class UserThread extends Thread {
	public void run() {
	     for (;; ) {
		System.out.println(Thread.currentThread().getName()+" : I am alive");
		try {
		     sleep(1000);
		}catch(InterruptedException e) {
			}
	    }
	}
}

Are you saying that the mail() thread dies after
1000 millisecs. I ran your code and it looks like
the two threads which are created are alive and kicking
(go into an infinite loop).

Am I right about the main thread ?

Regds.

FYI, JVM also dies when the System.exit or exit method of Runtime is called.

Regards
Gunjan

It is quite interesting to see the results. You all also enjoy it. The bottom line is If a parent thread creates some child threads and starts, them eventhoug the parent thread finishes its work ,[NOTE Here carefully. executing the main(String[] srgs) method alone is the work of the main(..) method's thread. (the parent thread ] it WAITS for its children threads to finish. I am showing this with the foll. code. I am giving the output also for your reference.

So the bottom line is JVM DOES NOT exit when the main()method exits. It exits when ALL THE NON-DAEMON (user) threads finish.

/* Maha's Bench Mark of the results
//All 3 user threads switches

main		//Total 5 (3 user +2 daemon)
thread 0	//Total 5 (3 user +2 daemon)	
thread 1	//Total 5 (3 user +2 daemon)	
main		//Total 5 (3 user +2 daemon)
thread 0	//Total 5 (3 user +2 daemon)	
thread 1	//Total 5 (3 user +2 daemon)

//look carefully here. main(..)'s Thread finished it's work.
//But still it waits for it's children to finish
//Evidence is the Total val remains SAME
//See here.Now, only the other 2 user threads swithes
thread 0	//Total 5 (3 user +2 daemon)
thread 1	//Total 5 (3 user +2 daemon)
thread 0	//Total 5 (3 user +2 daemon)
//See here. Thread 0 finished (it slept for 4 secs..
//So Now the Toatal = 4 . Only 4 are left in 
//Thread 1's group. 2 daemon + 1 parent 
(main) + 1 user (thread 1)
thread 1	//Total 4 (2 user +2 daemon)
*/
ThreadName =main ActiveCount = 5
Thread[main,5,main] is  NOT Daemon Thread 
Thread[SymcJIT-LazyCompilation-0,1,main] is  Daemon Thread 
Thread[SymcJIT-LazyCompilation-PA,10,main] is  Daemon Thread 
Thread[Thread-0,5,main] is  NOT Daemon Thread 
Thread[Thread-1,5,main] is  NOT Daemon Thread 
ThreadName =Thread-0 ActiveCount = 5
Thread[main,5,main] is  NOT Daemon Thread 
Thread[SymcJIT-LazyCompilation-0,1,main] is  Daemon Thread 
Thread[SymcJIT-LazyCompilation-PA,10,main] is  Daemon Thread 
Thread[Thread-0,5,main] is  NOT Daemon Thread 
Thread[Thread-1,5,main] is  NOT Daemon Thread 
ThreadName =Thread-1 ActiveCount = 5
Thread[main,5,main] is  NOT Daemon Thread 
Thread[SymcJIT-LazyCompilation-0,1,main] is  Daemon Thread 
Thread[SymcJIT-LazyCompilation-PA,10,main] is  Daemon Thread 
Thread[Thread-0,5,main] is  NOT Daemon Thread 
Thread[Thread-1,5,main] is  NOT Daemon Thread 
ThreadName =main ActiveCount = 5
Thread[main,5,main] is  NOT Daemon Thread 
Thread[SymcJIT-LazyCompilation-0,1,main] is  Daemon Thread 
Thread[SymcJIT-LazyCompilation-PA,10,main] is  Daemon Thread 
Thread[Thread-0,5,main] is  NOT Daemon Thread 
Thread[Thread-1,5,main] is  NOT Daemon Thread 
ThreadName =Thread-0 ActiveCount = 5
Thread[main,5,main] is  NOT Daemon Thread 
Thread[SymcJIT-LazyCompilation-0,1,main] is  Daemon Thread 
Thread[SymcJIT-LazyCompilation-PA,10,main] is  Daemon Thread 
Thread[Thread-0,5,main] is  NOT Daemon Thread 
Thread[Thread-1,5,main] is  NOT Daemon Thread 
ThreadName =Thread-1 ActiveCount = 5
Thread[main,5,main] is  NOT Daemon Thread 
Thread[SymcJIT-LazyCompilation-0,1,main] is  Daemon Thread 
Thread[SymcJIT-LazyCompilation-PA,10,main] is  Daemon Thread 
Thread[Thread-0,5,main] is  NOT Daemon Thread 
Thread[Thread-1,5,main] is  NOT Daemon Thread 
ThreadName =Thread-0 ActiveCount = 5
Thread[SymcJIT-LazyCompilation-0,1,main] is  Daemon Thread 
Thread[SymcJIT-LazyCompilation-PA,10,main] is  Daemon Thread 
Thread[Thread-0,5,main] is  NOT Daemon Thread 
Thread[Thread-1,5,main] is  NOT Daemon Thread 
Thread[Thread-2,5,main] is  NOT Daemon Thread 
ThreadName =Thread-1 ActiveCount = 5
Thread[SymcJIT-LazyCompilation-0,1,main] is  Daemon Thread 
Thread[SymcJIT-LazyCompilation-PA,10,main] is  Daemon Thread 
Thread[Thread-0,5,main] is  NOT Daemon Thread 
Thread[Thread-1,5,main] is  NOT Daemon Thread 
Thread[Thread-2,5,main] is  NOT Daemon Thread 
ThreadName =Thread-0 ActiveCount = 5
Thread[SymcJIT-LazyCompilation-0,1,main] is  Daemon Thread 
Thread[SymcJIT-LazyCompilation-PA,10,main] is  Daemon Thread 
Thread[Thread-0,5,main] is  NOT Daemon Thread 
Thread[Thread-1,5,main] is  NOT Daemon Thread 
Thread[Thread-2,5,main] is  NOT Daemon Thread 
ThreadName =Thread-1 ActiveCount = 4
Thread[SymcJIT-LazyCompilation-0,1,main] is  Daemon Thread 
Thread[SymcJIT-LazyCompilation-PA,10,main] is  Daemon Thread 
Thread[Thread-1,5,main] is  NOT Daemon Thread 
Thread[Thread-2,5,main] is  NOT Daemon Thread 

regds
maha anna

Jim,
I want your valuable feed back on this ,and the main qstn by j2user which started all this discussion.
regds
maha anna

class Test {
	public static void main(String[] args) throws Exception{
		UserThread thread1 = new UserThread();
		UserThread thread2 = new UserThread();
		thread1.start();
		thread2.start();
		for (int i = 0; i < 2; i++) {
			Test.printAllThreadsInThisThreadGroup(Thread.currentThread());
			Thread.sleep(1000);
		}
	}
	static void printAllThreadsInThisThreadGroup( Thread thread) {
		System.out.println("");
		System.out.println("ThreadName ="+thread.getName() 
		                   + " ActiveCount = " + thread.activeCount());
		System.out.println("");
		Thread[] tArray = new Thread[thread.activeCount()];
		thread.enumerate(tArray);
		for(int i = 0; i < tArray.length; i++) {
	if(((Thread)tArray[i]).isDaemon()) {
	    System.out.println((Thread)tArray[i] +" is  Daemon Thread ");
	}
	else {
 	   System.out.println((Thread)tArray[i] +" is  NOT Daemon Thread ");
	}
     }
  }
}
class UserThread extends Thread {
	public void run() {
  	  for (int i=0; i<4; i++) {
	  try {
		sleep(1000);
            Test.printAllThreadsInThisThreadGroup(Thread.currentThread());
	}catch(InterruptedException e) {
     }
   }
 }
}

[This message has been edited by maha anna (edited April 20, 2000).]

I did attempt to understand the point. But to be honest,
currently, it is a little above my head. However, I will
tune in later on and I am sure to understand it better.
Also, I will see what Jim says later on ...

Regds.

Lets begin with the main method it has brought thread t1 into a runnable state,now that does not necessarily causes its immediate execution. The program execution continues within the main method which eventually causes thread t2 to be made runnable. The execution ofcourse continues with the main thread unless you put it to sleep which hasnt been done and so it encounters a for loop and jumps into it and causes the

code:

---

printAllThreadsInThisThreadGroup(Thread.currentThread())

---

code:

---

ThreadName =main ActiveCount = 5  

---

and thus printing

code:

---

ThreadName =Thread-0 ActiveCount = 5 and
ThreadName =Thread-1 ActiveCount = 5

---

code:

---

ThreadName =main ActiveCount = 5  

---

after which it goes to sleep for 1 second following which it must exit but in the later stages we see the presence of a Thread-2 which I believe is the main Thread waiting for its exit. I would like to point out one more thing thread1 and thread2 only gets to call

code:

---

printAllThreadsInThisThreadGroup(Thread.currentThread());

---

4 times Thread-0 has already occured 4 times so it must have exited by the time Thread-1 gets to call printAllThreads...()
and there is no chance for it be present.So I think the presence of the Thread-1 and Thread-2 in the last set of statements are actually the non daemon threads and the main thread

Besides causing an infinite loop inside the run method will cause the presence of 5 activecounts as long as the program runs
How is that possible ?Please do help in this regard