How to wait for a number of threads to complete?

java multithreading parallel-processing wait
DivideByHero picture DivideByHero · Aug 9, 2009 · Viewed 168.8k times · Source

What is a way to simply wait for all threaded process to finish? For example, let's say I have:

public class DoSomethingInAThread implements Runnable{

    public static void main(String[] args) {
        for (int n=0; n<1000; n++) {
            Thread t = new Thread(new DoSomethingInAThread());
            t.start();
        }
        // wait for all threads' run() methods to complete before continuing
    }

    public void run() {
        // do something here
    }


}

How do I alter this so the main() method pauses at the comment until all threads' run() methods exit? Thanks!

Answer

Martin v. L&#246;wis picture Martin v. Löwis · Aug 9, 2009

You put all threads in an array, start them all, and then have a loop

for(i = 0; i < threads.length; i++)
  threads[i].join();

Each join will block until the respective thread has completed. Threads may complete in a different order than you joining them, but that's not a problem: when the loop exits, all threads are completed.

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