Java - the thread execution order

Question

I am trying to start 10 threads strictly in turn using a semaphore. That is, after the execution of thread-0, thread-1 should be executed, but not thread-2.

But the problem is that the threads is arrives to the semaphore.acquire()-method in out of order, and therefore the execution of the threads is out of order. How can I solve this problem with semaphore but without using thread.join()?

public class Main {

    private Semaphore semaphore = new Semaphore(1, true);

    public static void main(String[] args) {
        new Main().start();
    }

    private void start() {
        for (int i = 0; i < 10; i++) {
            Runnable runnable = () -> {
                try {
                    semaphore.acquire();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                System.out.println("In run method " + Thread.currentThread().getName());
                semaphore.release();
            };
            Thread thread = new Thread(runnable);
            thread.start();
        }
    }
    
}

Output:

In run method Thread-0
In run method Thread-1
In run method Thread-4
In run method Thread-5
In run method Thread-3
In run method Thread-2
In run method Thread-6
In run method Thread-7
In run method Thread-9
In run method Thread-8

user_13784117 · Answer

You need a synchronization object with some sort of concept of ordering. If you're familiar with US grocery stores, consider the "take a number" device at the deli counter that tells you whose turn it is.

Rough sketch of code:

class SyncThing {
   int turn = 0; 
   synchronized void waitForTurn(int me) {
       while (turn != me)
           wait();
   }
   synchronized void nextTurn() {
        turn++;
        notifyAll();
   }
}

then declare SyncThing syncThing = new SyncThing();

and run the i'th thread thus:

        Runnable runnable = () -> {
            syncThing.waitForTurn(i);
            System.out.println("In run method " + Thread.currentThread().getName());
            syncThing.nextTurn();
        };

This is typed in off the top of my head and is not offered as complete code, but it should show the way.

user_3461397 · Answer

private void start() {
    final AtomicInteger counter = new AtomicInteger();

    for (int i = 0; i < 10; i++) {
        final int num = i;
        new Thread(() -> {
            while (counter.get() != num) {
            }
            System.out.println("In run method " + Thread.currentThread().getName());
            counter.incrementAndGet();
        }).start();
    }
}

Java - the thread execution order

2 Answers