Why Does One Thread Always Reacquire the Object Lock After notify() or notifyAll()?

The Subtle Distinction Between notify() and notifyAll()

While the primary difference between notify() and notifyAll() lies in the number of waiting threads they awaken (one versus all), this raises another question:

Why does one thread always re-acquire the object lock?

In the general case, both notify() and notifyAll() do not specify which waiting thread will be selected to re-acquire the lock. The JVM or system thread scheduler makes this selection, which can be non-deterministic.

The Need for notifyAll()

However, using notify() in certain scenarios can lead to deadlock, as illustrated by the following example:

Producer/Consumer Class with notify()

public class ProducerConsumer {

    private final int MAX_SIZE = 1;  // Buffer size

    private List<Object> buf = new ArrayList<>();

    public synchronized void put(Object o) {
        while (buf.size() == MAX_SIZE) {
            wait();
        }
        buf.add(o);
        notify();
    }

    public synchronized Object get() {
        while (buf.size() == 0) {
            wait();
        }
        Object o = buf.remove(0);
        notify();
        return o;
    }
}

Deadlock Scenario:

1. Producer P1 puts an object into the buffer.
2. Producer P2 and P3 attempt to put objects but are blocked because the buffer is full.
3. Consumer C1 attempts to get an object from the buffer.
4. C1 is executing and gets the object, then notifies a waiting thread.
5. P2 or C2 can be awakened by the notification, but they are both blocked trying to re-acquire the lock.
6. C3 is also blocked trying to acquire the lock.

As a result, all three threads are waiting indefinitely, leading to deadlock.

The Solution: notifyAll()

To resolve this deadlock, one must use notifyAll() instead of notify() in the producer/consumer code. This ensures that all waiting threads are awakened, preventing deadlocks.

Recommendation:

For most scenarios, notifyAll() is the preferred method as it avoids potential deadlocks. If the specific scenario requires waking up only one specific waiting thread, then notify() can be used with caution.

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