using join() on multiple threads.

[Carlton Hanna]

I'm wondering, if I have multiple worker threads and want the created thread to wait for them all to finish. Would this be the right way to do it:

    
Workers[] workers = new Workers[5];
for(int i = 0; i < workers.length; i++){
    workers[i].start();
}

workers[0].join();
.
.
.
workers[workers.length-1].join();

[Paul Clapham]

Yes.

Although in a real program you would more likely write a loop to join all of the threads, rather than an unrolled loop.

[Abimaran Kugathasan]

Carlton Hanna wrote:I'm wondering, if I have multiple worker threads and want the created thread to wait for them all to finish. Would this be the right way to do it:

    
Workers[] workers = new Workers[5];
for(int i = 0; i < workers.length; i++){
    workers[i].start();
}

[b]workers[0].join();
.
.
.
workers[workers.length-1].join();[/b]

You mean to say that created Thread is Main Thread, and should it wait for the worker thread to finished their jobs.

[Isuru Sampath]

Won't this do rather than looping again to join the threads again???

Workers[] workers = new Workers[5];  
for(int i = 0; i < workers.length; i++){  
    workers[i].start();  
    workers[i].join();  
}  

[Mike Simmons]

Isuru: no. That would completely neutralize the point of using multiple threads. Every time you start a new thread, you wait until it completes, doing nothing else until it completes. In that case, you might as well do all the work in one thread - all benefits of using multiple threads have been lost.

[Steven Rodeo]

You can use either a CyclicBarrier or a CountDownLatch. Its an elegant way for the main thread to wait for its worker threads to finish. Again it depends on your code wether you need to use a CyclicBarrier or a CountDownLatch.

_SM

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Main {

    public static void main(String[] args) throws BrokenBarrierException, InterruptedException {
        CyclicBarrier cdl = new CyclicBarrier(5, new Runnable() {

            public void run() {
                System.out.println( " All Threads have completed Similar to Join");
            }

        });
        Workers[] workers = new Workers[5];
        for (int i = 0; i < workers.length; i++) {
             workers[i] = new Workers(cdl);
             workers[i].start();
        }

    }
}

class Workers extends Thread {
    CyclicBarrier cbr = null;
    public Workers(CyclicBarrier cb) {
        this.cbr = cb;
    }
    @Override
    public void run() {
        System.out.println(" I am Mr.Thread : " + Thread.currentThread().getName());
        try {
            this.cbr.await();
        } catch (InterruptedException ex) {
            Logger.getLogger(Workers.class.getName()).log(Level.SEVERE, null, ex);
        } catch (BrokenBarrierException ex) {
            Logger.getLogger(Workers.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

[Isuru Sampath]

Hi Mike,

Thanks a lot for your explanation. Saved me from lots of trouble. Now I get it. The moment I call the join() method my calling thread will start waiting untill the spawned thread completes.

[Isuru Sampath]

Hi Steven,

Thanks for your suggestion. CountDownLatch is what I need. Thanks again.