When Should You Use `notifyAll()` Instead of `notify()` in Java?

Notify() vs. NotifyAll(): A Refresher and Practical Differences

While you've likely encountered explanations on the number of threads woken by notify() and notifyAll(), let's further delve into their practical differences.

Why Use NotifyAll() Most of the Time?

As suggested in the answer, it's generally recommended to use notifyAll() in most cases. This ensures that all waiting threads are woken, preventing deadlock situations.

Understanding the Producer-Consumer Example

The provided producer-consumer example demonstrates why using notify() can lead to deadlock.

Why the While Loop Around Wait is Necessary?

The while loop around wait() ensures that the thread rechecks the condition after waking, preventing it from accessing a non-existent element if another thread has already completed the operation.

Deadlock Prevention with NotifyAll()

Using notifyAll() prevents the deadlock situation illustrated in the example, where multiple threads are waiting and none are being notified.

Practical Considerations

• Mutex Exclusion: wait() loops use mutually exclusive guards, ensuring threads are not waiting simultaneously in both the put() and get() methods.
• Non-Deterministic Thread Selection: Waking a thread with notify() allows the JVM to choose any waiting thread non-deterministically.
• Order of Lock Acquisition: When multiple threads attempt to acquire a lock, the order of acquisition can be non-deterministic.
• Synchronized Method Exclusivity: Only one thread can execute a synchronized method in a class at a time.

Conclusion

In summary, notifyAll() is the preferred choice for waking waiting threads, preventing deadlock conditions and ensuring fairness in resource allocation. Understanding the differences between notify() and notifyAll() is crucial in multi-threaded programming for effective coordination and avoiding potential issues.

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