Clarify Use of the join method

[d jones]

Hi,

If we have

    public static void main(String[] args) { 
 
t1 T1 = new t1(); //t1 is a class which implements Runnable
t2 T2 = new t2(); //t2 is a class which implements Runnable
 
Thread a = new Thread(T1);
Thread b = new Thread(T2);
 
a.start();
b.start();
 
try{
a.join();
}
catch(InterruptedException e){}

 
for(int i=0;i<50;i++){
System.out.println("main method");
}
 
    }

Here I would have thought that Thread b would stop running as soon as the line a.join() was executed.
But it doesn't it continues to run.

Could anyone please explain to me how the join method is working in this example? Shouldn't the a.join() line have caused any executing threads to be halted and not run again until a completed?

I understand if I have 1 thread and start it in the main method that it puts the main method on hold until the thread on which I call join completes.

But what if we wanted 1 thread to run after another, as in the above code, where I want thread b to run only when thread a has completed?

Many Thanks

[Henry Wong]

When a thread calls the join() method of a thread object -- it will wait until the thread represented by that thread object completes. Nothing else is affected.

In your example, the main thread will wait until thread A completes. Period. Thread B (or any other thread) does not wait til thread A completes. Nor does the main thread waits for any other thread to complete.

Henry

[Barry Gaunt]

Main is a controlling thread, so you can delay the starting of thread b until the main thread has joined with thread a.

   public static void main(String[] args) { 
 
t1 T1 = new t1(); //t1 is a class which implements Runnable
t2 T2 = new t2(); //t2 is a class which implements Runnable
 
Thread a = new Thread(T1);
Thread b = new Thread(T2);
 
a.start();
 
try{
a.join();
}
catch(InterruptedException e){}
 
b.start();
 
 
for(int i=0;i<50;i++){
System.out.println("main method");
}
 
    }    

You could also pass thread a to thread b and have thread b join thread a.