How to break out of a loop when no more threads are active

Question

I've got a program where I initialize and start a variable number of threads. I need the main program to interact with the threads so I put those inside an infinite loop. After all the threads have died though I would like a way to break the loop. Here goes part of my code:

public static void main(String[] args) {
//some random initializing

for(int i = 1; i <= num; i++){     //p1 is a Runnable
        Thread t = new Thread(p1, "t"+ Integer.toString(i));
        t.start();
} 
while(true){
//do some stuff while threads are active
//not sure what if statement to put here to break loop
}

I can post more of my code if I need to, but I think it would be largely irrelevant.

you can check the answer here. http://stackoverflow.com/questions/5066160/java-threads-and-main-thread — Raghu, Dec 05 '13 at 06:18
http://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html — Franz Ebner, Dec 05 '13 at 06:19
How about creating threadgroup and using method getActiveCount() ?? — Suresh Atta, Dec 05 '13 at 06:23
The reason I didn't use a thread group is this is for a homework assignment on multi-threading. I am only suppose to use the basic .start(), .wait(), and .notify(). To organize my threads. — Peter T., Dec 05 '13 at 06:36

Subhrajyoti Majumder · Answer 1 · 2013-12-05T06:24:19.753

3

you can use CountDownLatch(since java 1.5) here, tt is a synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

Snippet:

CountDownLatch signal = new CountDownLatch(num);
...
for(int i = 1; i <= num; i++){     //p1 is a Runnable
        Thread t = new Thread(p1, "t"+ Integer.toString(i),signal);
        t.start();
} 
signal.await();           // wait for all to finish

... In run() Method of your Thread...

public void run(){
       ...                // do stuff
       signal.countDown(); // sending signal that work has done.
}

edited Dec 05 '13 at 06:24

answered Dec 05 '13 at 06:19

Subhrajyoti Majumder

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+1. Also, if you want to do something while waiting (as OP wants), you could go. `while (latch.getCount() > 0) { do stuff }` – user949300 Dec 05 '13 at 06:34

score 1 · Answer 2 · answered Dec 05 '13 at 06:18

1

Instead of stalling with a while loop, just join all the threads. The join method essentially just waits for the thread to finish.

Thread[] threads = new Thread[num];
for(int i = 1; i <= num; i++){
    Thread t = new Thread(p1, "t"+ Integer.toString(i));
    threads[i] = t;
    t.start();
} 

for(int i = 1; i <= num; i++){
    threads[i].join();
}

For more info consult the docs

answered Dec 05 '13 at 06:18

Sinkingpoint

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If you want to do something in the while loop, wouldn't the join block the main thread? – Vineet Kosaraju Dec 05 '13 at 06:19
@Bucco Indeed. I made the assumption that the wile loop was just for stalling purposes. – Sinkingpoint Dec 05 '13 at 06:24
The loop in the main method actually interacts with the running threads so I don't think this will help me. – Peter T. Dec 05 '13 at 06:40

score 1 · Accepted Answer · edited May 23 '17 at 12:13

I would make an array of Threads, to check them later on if they are done, then go through the same for loop but also set the values in the threads array. Finally, in the while loop, I would check if all the threads are not done using the state method of the thread - How to check if thread is terminated?

Thread[] threads = new Thread[num];

for(int i = 1; i <= num; i++){     //p1 is a runnable.
        Thread t = new Thread(p1, "t"+ Integer.toString(i));
        threads[i-1] = t;
        t.start();
} 

while(!allThreadsCompleted(threads)) {
 // do some of your code here
}

public static boolean allThreadsCompleted(Thread[] threads) {
    for(int i = 0; i < threads.length; i++) {
        if(threads[i].getState()!=Thread.State.TERMINATED)
            return false;
    }
    return true;
}

I realize now that this would be useful if you actually want to do something in the while loop, not just for stalling purposes. Hope this helps.

Thanks this looks like I should be able to get it to work now. — Peter T., Dec 05 '13 at 06:42

How to break out of a loop when no more threads are active

3 Answers3