Thread Jobs in Java

Question

I want to spawn 200 threads simultaneously in Java. What I'm doing right now is running into a loop and creating 200 threads and starting them. After these 200 gets completed, I want to spawn another 200 set of threads and so on.

The gist here is that the first 200 threads I spawned need to be FINISHED before spawning the next set. I tried the code below, but its not working

for(int i=0;i<200;i++){
    Thread myThread = new Thread(runnableInstance);
    myThread.start();
}
for(int i=0;i<200;i++){
    Thread myThread = new Thread(runnableInstance);
    myThread.start();
}

Note: I have intentionally put the for loop Twice, but the desired effect I intend is not happening simply because the second for loop is executed before the first set of threads end their execution.

Please advise

Answer 1

You should keep a list of the threads you have created. Then once you have started all of them, you can loop over the list and perform a join on each one. When the join loop finishes, all of your threads will have run to completion.

List<Thread> threads = new List<Thread>();
for(int i=0;i<200;i++){
    Thread myThread = new Thread(runnableInstance);
    myThread.start();
    threads.add(myThread);
}
//All threads are running
for(Thread t : threads) {
    t.join();
}
//All threads are done