Java multithreading confusing code snippet

Question

How should the code below perform if executed randomly at different time?

public class Unstoppable extends Thread {
    private int counter;

    @Override
    public void run() {
        synchronized(this) {
            for(int i = 0; i < 10; i++)
                counter++;

            this.notifyAll();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Unstoppable unStoppable = new Unstoppable();

        System.out.println("I am about to start");
        unStoppable.start();
        synchronized(unStoppable) {
            System.out.println("I was just told to wait");
            unStoppable.wait();
        }

        System.out.println(unStoppable.counter);
    }
}

At the first look at this it seemed like it would hang infinitely but quiety surprisingly every time I execute this...It completes execution.

Answer 1

Synchronization is meaningful if you have two or more threads. In your case there is just one thread.

 public static void main(String[] args) throws InterruptedException {
    Unstoppable unStoppable = new Unstoppable();
    Unstoppable unStoppable2 = new Unstoppable();
    Unstoppable unStoppable3 = new Unstoppable();
    System.out.println("I am bout to start");
    unStoppable.start();
    unStoppable2.start();
    unStoppable3.start();
    synchronized (unStoppable) {
        System.out.println("I was just told to wait");
        unStoppable.wait();
    }

    System.out.println(unStoppable.counter);
}

}

execute this at random times it might wait or try to increase the number of threads. By the way wait should be implemented like this.

 synchronized (obj) {
     while (<condition does not hold>)
         obj.wait();
     ... // Perform action appropriate to condition
  }

Answer 2

Are you ignoring exceptions somehow.... the code:

// synchronized(unstoppable) {
    System.out.println("I was just told to wait forever");
    unstoppable.wait();
// }

Should throw an IllegalMonitorStateException and immediately exit.

---

EDIT

So you changed your code, and now it works the way you expect (it waits forever), so what's the problem? (I just tested it).

Answer 3

At the first look at this it seemed like it would hang infinitely but quiety surprisingly every time I execute this...It completes execution.

It completes because when a Thread finishes, there is a behind-the-scenes call to thread.notify(). This is how thread.join() is implemented.

That said, it is not a good idea to depend on the code working in this manner -- or at least is should be labeled as a hack. You should use thread.join() instead:

unStoppable.join();

Lastly, you should consider making your Unstoppable class implement Runnable instead of extending thread. That's a better pattern:

public class Unstoppable implements Runnable {
...

Thread unStoppableThread = new Thread(new Unstoppable());
...
unStoppableThread.join();