Thread-doubt on wait

[Sebanti Sanyal]

These classes are defined in the same file. What is the output? (1 correct answer)

  class Job extends Thread {
    private Integer number = 0;
    public void run() {
        for (int i = 1; i < 1000000; i++) {
            number++;
        }
    }
    public Integer getNumber() {
        return number;
    }
}
public class Test {
    public static void main(String[] args)
    throws InterruptedException {
        Job thread = new Job();
        thread.start();
        synchronized (thread) {
            thread.wait();
        }
        System.out.println(thread.getNumber());
    }
}

1.It prints 0.
2.It prints 999999.
3.The output is not guaranteed to be any of the above.

Ref:Nikos' Java blog-SCJP 6 Mock exam for Threads(Q 66).
Correct Answer:2

Could you please explain the output?I expected the main thread to wait indefinitely in absence of notify()/notifyAll().

[Stephan van Hulst]

Personally I believe the answer should be option 3. The call to wait() may return at any moment, because of spurious wakeups. So the program may print some arbitrary value between 0 and 999999, or it may not terminate at all.

[Sebanti Sanyal]

Thanks for your reply. Is there any correlation between the end of run() method and release of locks or notification?

[Stephan van Hulst]

There is no correlation between the finishing of the run() method and the release of locks or notification. However, there may be a correlation between the finishing of a run() method and spurious wakeups.

I tested the program myself, and it indeed prints 999999, but I don't think this can be guaranteed. If I remove the thread.start() statement, the program runs indefinitely, so I believe the termination of a thread may cause other wait() statements to return spuriously. Again, this is an artifact of the implementation, and the results may differ on different operating systems or different virtual machines.

[Sebanti Sanyal]

Thank you very much Stephan for the clear explanation.

[Anayonkar Shivalkar]

Hi Stephan,

The call to wait() may return at any moment, because of spurious wakeups.

I'm not sure what is meant by this. In given code, nobody is explicitly calling notify/notifyAll method on thread, so it is very very unlikely (rather I would say impossible) to get out of wait method before thread execution is finished. Once run method is executed, 'wait' will be over.

If I remove the thread.start() statement, the program runs indefinitely

This happened because, thread was not running, nobody's gonna ever call notify/notifyAll on that thread, and hence 'wait' will never end.

Hence, in my opinion, output should (and would) be 999999.