When Should You Use Java's notifyAll() Over notify()?

Revisited: Java's notify() vs. notifyAll()

While both notify() and notifyAll() serve to awaken waiting threads, their underlying behaviors differ in crucial ways.

notify() Wakes a Single Thread

As expected, notify() selects a single waiting thread to be awakened. This selection is non-deterministic, making it impossible to determine which thread will resume execution.

notifyAll() Awakes All Waiting Threads

In contrast, notifyAll() wakes up all threads that are waiting on the same lock. This ensures that all interested threads have an opportunity to resume execution and compete for further lock acquisition.

Why Use notifyAll() Most of the Time?

The primary reason to prefer notifyAll() is to avoid potential deadlocks. The example provided illustrates a scenario where using notify() can lead to a deadlock situation. Without notifyAll(), it's possible that only a subset of threads will be awakened, leaving others indefinitely waiting.

Guard Conditions and Non-deterministic Lock Acquisition

The example also highlights the importance of guard conditions surrounding wait loops. This ensures that threads are re-evaluating the condition before performing any operations, avoiding race conditions and incorrect state modifications.

Additionally, it's crucial to remember that lock acquisition after waiting can be non-deterministic. Therefore, using notifyAll() ensures that all waiting threads have an equal chance of acquiring the lock and continuing execution.

Conclusion

Understanding the subtle differences between notify() and notifyAll() is crucial for avoiding potential concurrency issues and deadlocks. When in doubt, it's advisable to use notifyAll() to ensure fair and efficient thread awakening.

The above is the detailed content of When Should You Use Java's notifyAll() Over notify()?. For more information, please follow other related articles on the PHP Chinese website!