Implementing stack using wait and notify

[Naresh Chaurasia]

Consider following code

class StackImpl2 {
    private Object[] stackArray;
    private volatile int topOfStack;

    StackImpl2 (int capacity) {
        stackArray = new Object[capacity];
        topOfStack = -1;
    }

    public synchronized Object pop() {
        System.out.println(Thread.currentThread() + ": popping");
        while (isEmpty())
            try {
                System.out.println(Thread.currentThread() + ": waiting to pop");
                wait();                             // (1)
            } catch (InterruptedException e) { }
        Object obj = stackArray[topOfStack];
        stackArray[topOfStack--] = null;
        System.out.println(Thread.currentThread() + ": notifying after pop");
        notify();                                   // (2)
        return obj;
    }

    public synchronized void push(Object element) {
        System.out.println(Thread.currentThread() + ": pushing");
        while (isFull())
            try {
                System.out.println(Thread.currentThread() + ": waiting to push");
                wait();                             // (3)
            } catch (InterruptedException e) { }
        stackArray[++topOfStack] = element;
        System.out.println(Thread.currentThread() + ": notifying after push");
        notify();                                   // (4)
    }

    public boolean isFull() { return topOfStack >= stackArray.length -1; }
    public boolean isEmpty() { return topOfStack < 0; }
}

abstract class StackUser extends Thread {           // (5) Stack user

    protected StackImpl2 stack;                      // (6)

    StackUser(String threadName, StackImpl2 stack) {
        super(threadName);
        this.stack = stack;
        System.out.println(this);
        setDaemon(true);                            // (7) Daemon thread
        start();                                    // (8) Start this thread
    }
}

class StackPopper extends StackUser {               // (9) Popper
    StackPopper(String threadName, StackImpl2 stack) {
        super(threadName, stack);
    }
    public void run() { while (true) stack.pop(); }
}

class StackPusher extends StackUser {               // (10) Pusher
    StackPusher(String threadName, StackImpl2 stack) {
        super(threadName, stack);
    }
    public void run() { while (true) stack.push(new Integer(1)); }
}

public class WaitAndNotifyClient {
    public static void main(String[] args)
           throws InterruptedException {            // (11)

        StackImpl2 stack = new StackImpl2(5);

        new StackPusher("A", stack);
        new StackPusher("B", stack);
        new StackPopper("C", stack);
        System.out.println("Main Thread sleeping.");
        Thread.sleep(1000);
        System.out.println("Exit from Main Thread.");
    }
}

When the wait is called at (1), does the thread give up the lock immediately and the execution of push begins or the lock is given up when the synchronized method pop() comes to end.

[Trivikram Kamat]

Naresh Chaurasia wrote:
When the wait is called at (1), does the thread give up the lock immediately and the execution of push begins or the lock is given up when the synchronized method pop() comes to end.

The lock (on which method is synchronized) is relinquished as soon as you call wait()
The execution of push() will begin only when it acquires the lock. Remember that the lock is relinquished by pop() by calling wait() doesn't mean it'll be acquired by push(). It may be acquired by some other method also - say by some other method pop() or method push() being run in different thread.

In this case when notify() is invoked in method push(), it'll wake up single thread which may be running pop() - depending on thread policies implemented by JVM (in case many threads are waiting for that lock)
Note that the object lock is not relinquished when the notifying thread invokes the notify() method. The notifying thread relinquishes the lock
at its own discretion, only then awakened thread can acquire the lock.