What causes a synchronization lock to get stuck?

Question

I have this simple code, it's simply a testing case

        try
    {
        synchronized(this) {
        while(number != 4)
        {
            System.out.println("Waiting...");
            this.wait();
        }
        number = 4;
        this.notifyAll();
        }
        

    }
    catch(InterruptedException e)
    {}

Of what I know regarding use of the wait() method, once wait is invoked, what comes after should be done. However I can't see to get the wait to end in this case. I have attempted to place a second synchronized block but that doesn't seem to work.

Do you know what could be causing the wait to hang? I looked up deadlocking but that seems to be an entirely different matter.

You need another thread calling `notify` to let `wait` continue. — Burak Serdar, Mar 03 '21 at 02:40
where are you incrementing/decrementig the "number" such that number!=4. You havent provided any details of the number ?? — akshaya pandey, Mar 03 '21 at 02:58

score 0 · Answer 1 · answered Mar 03 '21 at 02:42

Think u better get some idea from here

then modify your code accordingly, as long there is no another thread is running.

The wait() method causes the current thread to wait indefinitely until another thread >either invokes notify() for this object or notifyAll().

user15187356 · Answer 2 · 2021-03-03T02:51:10.127

Do you know what could be causing the wait to hang?

You have a loop that's waiting for number to be set to 4. No code is executed that sets number to 4. Therefore the loop waits forever.

    while(number != 4)
    {
        System.out.println("Waiting...");
        this.wait();
    }

    /* you can't get here until some other thread sets number to 4. */
    number = 4;
    this.notifyAll();
    }

The wait call does not terminate until some other code calls notify, and then when you wake up out of the wait, you'll loop back again unless number is now 4.

You can't have a thread simultaneously be in wait and executing the code that issues the notify. And if wait didn't wait, it would not be useful.

Not intending to be rude, but the problem here seems to be a lack of understanding of sequential execution of program code.

What causes a synchronization lock to get stuck?

2 Answers2