Java - While loop with embedded threads bug

Question

I have this code below, that evaluates if three threads are done, and if yes, it continues with the code. The problem is that when I include some sort of print statement before the if statement, it works as usual. However, when I don't include the print, it continues forever. Here it is:

while (!are_we_done) {
    System.out.println(are_we_done);
    if (thread_arr[0].are_we_done==true && thread_arr[1].are_we_done==true && thread_arr[2].are_we_done==true) {
        are_we_done=true;
    }
}

Any clue as to what's going on? Thanks in advance for any help/advice.

See: Loop doesn't see changed value without a print statement — Boann
– Boann, Commented Nov 29, 2014 at 16:42

Erwin Bolwidt · Accepted Answer · 2014-11-29 04:38:42Z

3

The problem was that I had to specify the are_we_done variable in the thread class as volatile.

edited Nov 29, 2014 at 4:38

Erwin Bolwidt

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answered Nov 29, 2014 at 4:30

SalmonKiller

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Comments

ursa · Accepted Answer · 2014-11-29 17:16:25Z

0

Your work with threads is awesome - google for 'busy waiting'.

in main thread introduce 'latch = new CountDownLatch(<number of threads>)' variable
pass it into all your threads
on finish of the thread call 'latch.countDown()'
in main thread wait for all spawned threads complete with 'latch.await(...)'

Example:

public static void main(String... args) throws Exception {
    Thread[] threads = new Thread[3];
    CountDownLatch latch = new CountDownLatch(threads.length);

    for (int i = 0; i < threads.length; i++) {
        threads[i] = new Thread(new YourRunnable(latch));
        threads[i].start();
    }

    while (!latch.await(1000)) {
        System.out.println("Not complete yet");
    }

    System.out.println("Complete!");
}

public class YourRunndable implements Runnable {
    ... // fields + constructor

    public void run() {
        try {
            ... // do your staff
        } finally {
            latch.countDown();
        }
    }
}

edited Nov 29, 2014 at 17:16

answered Nov 29, 2014 at 17:09

ursa

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2 Comments

SalmonKiller Over a year ago

In that case, how do I transfer information from the threads in the array to the main thread?

ursa Over a year ago

in short - use shared data structures (e.g. collections, queues, fields). read this tutorial - it is the best guide for multi-threading. an hour of reading and testing, but much simpler to live further.

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