Communication between threads in Java without lock

Question

So what i want to achieve is two threads taking turn in executing their task. I originally had just one question;

1. How can i achieve two threads taking turns in executing their task without using a lock? Reason why i don't want the lock is because I'm feeling stupid, using a lock when theres no common resource the two threads are trying to access.

So I was going to make a small example of the code, and then i discovered that i wasn't able to make it work, even with locks. So my second question is; how do i make the code work as expected? The way i see it, it should work, but that's just me :)

1. Thread1 prints message
2. Thread1 signals that Thread2 can print message
3. Thread2 prints message
4. Thread2 signals that Thread1 can start all over

---

public class App {

    Lock lock = new ReentrantLock();
    Condition cond1 = lock.newCondition();
    Condition cond2 = lock.newCondition();

    public App() {
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    while (true) {
                        lock.lock();
                        System.out.println("Thread 1");
                        cond2.signalAll();
                        cond1.await();
                        lock.unlock();
                    }
                } catch (InterruptedException e) {
                }
            }
        });
        thread1.start();

        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    while (true) {
                        lock.lock();
                        cond2.await();
                        System.out.println("           Thread 2");
                        cond1.signalAll();
                        lock.unlock();
                    }
                } catch (InterruptedException e) {
                }
            }
        });
        thread2.start();
    }

    public static void main(String[] args) {
        new App();
    }

}

Answer 1

(1) await() is usually used in a loop; not doing so is a sign of bug.

while( some condition not met )
    cond.await();

(2) unlock() should be in a finally block

(3) signal() only signals currently waiting threads; the signal is lost if there's no thread waiting.

    lock.lock();
    cond.signal(); // lost
    cond.await();  // won't wake up

(4) nothing wrong to use good old synchronized for simple cases like this. Actually you better understand it first before using more "advanced" stuff.

(5) a solution:

    Lock lock = new ReentrantLock();
    Condition cond = lock.newCondition();
    int turn=1; // 1 or 2


    // thread1

            lock.lock();
            try
            {
                while (true)
                {
                    while(turn!=1)
                        cond.await();
                    System.out.println("Thread 1");
                    turn=2;
                    cond.signal();
                }
            }
            finally
            {
                lock.unlock();
            }

    // thread2
    // switch `1` and `2`

(6) a ring of threads, each waking up the next

    int N = 9;
    Thread[] ring = new Thread[N];
    for(int i=0; i<N; i++)
    {
        final int ii = i+1;
        ring[i] = new Thread(()->
        {
            while(true)
            {
                LockSupport.park();   // suspend this thread
                System.out.printf("%"+ii+"d%n", ii);
                LockSupport.unpark(ring[ii%N]);  // wake up next thread
                // bug: spurious wakeup not handled 
            }
        });
    }

    for(Thread thread : ring)
        thread.start();

    LockSupport.unpark(ring[0]);  // wake up 1st thread

Answer 2

You could use thread notify() and wait(): Oracle Documentation