Java CyclicBarrier

[Tiam Bezalel]

hi, I would like to understand why this program waits indefinitely while the lock is lifted by two threads.

    import java.util.concurrent.*;
     
    class Tour {
        CyclicBarrier cb;
     
        Tour(CyclicBarrier cb) {
            this.cb = cb;
            try {
                cb.await();
            } catch(InterruptedException | BrokenBarrierException e) {}
        }
    }
     
    public class Test {
        public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
            var cb = new CyclicBarrier(2, () -> System.out.println("START...")); //Line n1
            cb.await(); //Line n2
            var tour = new Tour(cb); //Line n3
            System.out.println("END..."); //Line n4
        }
    }

Variable 'cb' infers to CyclicBarrier type.

`new CyclicBarrier(2, () -> System.out.println("START..."));` means 2 threads must call await() so that Cyclic Barrier is tripped and barrier action is executed.

At Line n2, await() is invoked on 'cb' by Main-Thread and it waits until one more thread would invoke await() on the same object.

[Norm Radder]

 why this program waits indefinitely

The one thread in the program is waiting at line n2.
There needs to be another thread that calls await to release the first thread.  See the example in the API doc.

[Piet Souris]

Indeed. And you might think that switching line 17 and line 18 would solve the problem, but since Tour is still executed on the same Thread as main(), the result is the same.

As Norm says, you can see an example in the API, but this is a tad easier, me thinks

class Tour implements Runnable {
    CyclicBarrier cb;

    Tour(CyclicBarrier cb) {
        this.cb = cb;
    }
    
    @Override
    public void run() {
        try {
            cb.await();
        } 
        catch (InterruptedException | BrokenBarrierException e) {
        }
    }
}

class Test {

    public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
        var cb = new CyclicBarrier(2, () -> System.out.println("START...")); //Line n1
        var tour = new Tour(cb); //Line n3
        new Thread(tour).start();   // this will wait for tne Cyclic Barrier,
                                    // but it will not block main()'
        cb.await(); //Line n2
        // var tour = new Tour(cb); //Line n3
        System.out.println("END..."); //Line n4
    }
}

[Ron McLeod]

An alternative to creating the thread yourself would be to use an ExecutorService.

public class Test {
    public static void main(String[] args) throws InterruptedException, BrokenBarrierException {
        var executor = Executors.newSingleThreadExecutor();
        var cb = new CyclicBarrier(2, () -> System.out.println("START..."));
        executor.submit(new Tour(cb));
        cb.await();
        System.out.println("END...");
        executor.shutdown();
    }
}