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Assume I have a main thead. In this main thread I create and start 5 threads. Then my main thread has to wait until one of those previous created threads has finished. When such a thread is finished, my main thread has to create and start another thread, so that there are always 5 (or less) threads alive. So, my question is: Is there something like CountDownLatch that enables me wait when an upper threshold has been reached?

Joseph Müller
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  • Does the main thread have to do this? Just add a callback/listener to your runnables. – Sotirios Delimanolis Dec 16 '14 at 18:22
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    possible duplicate of [Java Wait for thread to finish](http://stackoverflow.com/questions/4691533/java-wait-for-thread-to-finish) – Matias Cicero Dec 16 '14 at 18:22
  • You may check if any of the 5 threads have finished, if not then sleep your main thread for a certain period of time. May be add a listener that would notify your main thread to wake up and spawn a new thread. – Syed Mauze Rehan Dec 16 '14 at 18:25
  • Well, I dont rly want to use join because I dont want to keep track of all threads alive. And yes, the main thread has to do this, so cant rly use a listener either. – Joseph Müller Dec 16 '14 at 18:26
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    see in the list of answers in the link given by @MatiCicero .. CountDownLatch latch = new CountDownLatch(3); ... latch.await(); // Wait for countdown – LeTex Dec 16 '14 at 18:27
  • Then the only alternative I can think of is to poll and check if a thread is alive or not. – Sotirios Delimanolis Dec 16 '14 at 18:28
  • Or checkout this one: http://stackoverflow.com/questions/1323408/get-a-list-of-all-threads-currently-running-in-java – LeTex Dec 16 '14 at 18:30
  • Is it possible that what you actually want is a [thread pool of fixed size](http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ThreadPoolExecutor.html) 5? – 5gon12eder Dec 16 '14 at 18:31

1 Answers1

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Consider using thread pool. Anyway, the code below is doing what you are asking for.

import java.util.concurrent.atomic.AtomicInteger;

public class Main {
    public static void main(String[] args) throws InterruptedException {
        final AtomicInteger atomicInteger = new AtomicInteger(0);
        for (;;) {
            while (atomicInteger.get() < 5) {
                atomicInteger.getAndIncrement();
                Thread t = new Thread() {
                    public void run() {
                        try {
                            Thread.sleep(1000);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        atomicInteger.getAndDecrement();
                    };
                };
                t.start();
            }
            Thread.sleep(100);
        }
    }
}