Threads wait until all threads return output using java?

Question

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How to wait for a set of threads to complete?

I want to run three threads, each thread calls different classes. These classes do some processing and return a Boolean value, I want to wait until all three threads return their output. I want to know how it's possible to implement this using Java.

Thanks

score 4 · Accepted Answer · edited Nov 23 '11 at 15:22

You can use Thread.join() to do that:

Thread[] threads = new Thread[numOfThreads];
for (int i = 0; i < threads.length; i++) {
    threads[i] = new Thread(new Runnable() {
        public void run() {
            System.out.println("xxx");
        }
    });
    threads[i].start();
}

for (int i = 0; i < threads.length; i++) {
    try {
        threads[i].join();
    } catch (InterruptedException e) {
    }
}

For your solution

Thread[] threads = new Thread[3];
threads[i] = new Thread(new Runnable() {
        ...
}).start();
threads[i] = new Thread(new Runnable() {
        ...
}).start();
threads[i] = new Thread(new Runnable() {
        ...
}).start();

for (int i = 0; i < threads.length; i++) {
    try {
        threads[i].join();
    } catch (InterruptedException e) {
    }
}

Peter Lawrey · Answer 2 · 2011-07-05T05:48:29.157

2

If you are using an ExecutorService you can do

 ExecutorService es = /* create a reusable thread pool */

 List<Future> futures = new ArrayList<Future>();
 futures.add(es.submit(myRunnable1));
 futures.add(es.submit(myRunnable2));
 futures.add(es.submit(myRunnable3));
 for(Future f: futures) f.get(); // wait for them to finish.

If you want to return a boolean, you should use a Callable instead. You can also use invokeAll

 ExecutorService es = /* create a reusable thread pool */

 List<Future<Boolean>> futures = new ArrayList<Future<Boolean>>();
 es.invokeAll(Arrays.asList(
    new Callable<Boolean>() {
        public Boolean call() {
             return true;
        }
    },
    new Callable<Boolean>() {
        public Boolean call() {
             return false;
        }
    },
    new Callable<Boolean>() {
        public Boolean call() {
             return true;
        }
    }
 ));

 for(Future<Boolean> f: futures) {
      Boolean result = f.get();
 }

edited Jul 05 '11 at 05:48

answered Jul 04 '11 at 10:07

Peter Lawrey

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+1 for `ExecutorService` but `invokeAll` might be a better bet. – Qwerky Jul 04 '11 at 10:20
I thought of that but invokeAll takes a collection of Callable. ;) – Peter Lawrey Jul 04 '11 at 10:23
Actually a Callable is what may be called for (pun intended) because he wants them to return a boolean value. I second the use of invokeAll(). There is always Executors.callable() that wraps a Runnable. – sjlee Jul 04 '11 at 17:12
@Qwerty, sjlee, I have added an example using Callable and invokeAll. – Peter Lawrey Jul 05 '11 at 05:48

score 0 · Answer 3 · answered Jul 04 '11 at 10:01

0

A thread has a join method. If your main thread calls that method on a thread t, the main thread will wait until t has finished.

See http://download.oracle.com/javase/6/docs/api/java/lang/Thread.html#join()

answered Jul 04 '11 at 10:01

Mathias Schwarz

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score 0 · Answer 4 · answered Jul 04 '11 at 10:01

You can use a "barrier" (see CyclicBarrier for an implementation in the Java API). Alternatively, if you just want to get the output, but don't need them all to finish before handling the output, you could represent your computations using Future which will block if needed when getting the result.

score 0 · Answer 5 · answered Jul 04 '11 at 10:03

You may use Thread.join() as the following: -

Thread t1 = new Thread(myRunnable1);
Thread t2 = new Thread(myRunnable2);
Thread t3 = new Thread(myRunnable3);

t1.start();
t2.start();
t3.start();

t1.join();
t2.join();
t3.join();

//do something when all 3 threads finish

I hope this may help to achieve your requirement.

Regards,

Charlee Ch.

score 0 · Answer 6 · answered Jul 04 '11 at 10:07

0

Please take a look at the examples in ExecutorCompletionService

answered Jul 04 '11 at 10:07

denis.solonenko

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Threads wait until all threads return output using java?

6 Answers6