When Should You Use `notifyAll()` in Java?

Java: notify() vs. notifyAll() Revisited

While it's often stated that notify() wakes only one waiting thread and notifyAll() wakes all waiting threads, understanding their practical implications is crucial.

Core Difference

Both methods awaken waiting threads, but only one thread is granted monitor acquisition at a time, regardless of the method used. This selection process is non-deterministic and depends on the VM or underlying system thread scheduler.

The Value of notifyAll()

In most scenarios, notifyAll() should be employed over notify(). When notify() is used, only one thread may be selected to execute. This can lead to deadlocks, as demonstrated in the following example:

public class ProducerConsumerBroken {

    private Object[] buf;

    public synchronized void put(Object o) {
        while (buf.length == MAX_SIZE) wait();
        buf[next] = o;
        notify();
    }

    public synchronized Object get() {
        while (buf.length == 0) wait();
        return buf[next--];
    }
}

In this example, notify() can result in deadlock because the system thread scheduler may continuously favor one thread, leaving other threads indefinitely suspended.

Deadlock Avoidance with notifyAll()

By using notifyAll(), all waiting threads (if any) are awoken, giving each thread an impartial chance to acquire the lock and proceed. This eliminates the possibility of deadlocks.

When to Use notifyAll()

Use notifyAll() whenever you want to ensure that all waiting threads are informed that the object they're waiting on has changed. This is especially important in cases where you cannot predict which thread needs to be notified or when multiple threads may be waiting on the same lock.

Recommendation

As a general rule of thumb, always use notifyAll(). This will prevent potential deadlocks and ensure fairness in thread scheduling.

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