Are Spurious Wakeups in Java Real or Just a Myth?

Spurious Wakeups in Java: Reality or Myth?

Spurious wakeups, where a thread awakens from blocking operations without any apparent trigger, are a persistent concern in locking discussions. However, their actual occurrence remains a mystery.

Possible Causes of Spurious Wakeups

The term "spurious" suggests an event without an obvious cause. However, several factors can potentially lead to this phenomenon:

• Linux Implementation: As outlined in the Wikipedia article, Linux's pthread_cond_wait() function employs the futex system call. When a process receives a signal, all its blocking system calls, including pthread_cond_wait(), abruptly return with an EINTR error. If the signal occurs while the thread is outside the futex system call, the thread may miss a legitimate wakeup, resulting in a spurious wakeup.

Example Code and Forced Spurious Wakeups

Consider the following code snippet:

<code class="java">public class Spurious {
    public static void main(String[] args) {
        Lock lock = new ReentrantLock();
        Condition cond = lock.newCondition();
        lock.lock();
        try {
            try {
                cond.await();
                System.out.println("Spurious wakeup!");
            } catch (InterruptedException ex) {
                System.out.println("Just a regular interrupt.");
            }
        } finally {
            lock.unlock();
        }
    }
}</code>

Normally, this code will block on the cond.await() call until it is awakened by another thread signaling the condition. However, you can force a spurious wakeup by sending a signal to the running process.

Conclusion

While the concept of spurious wakeups is well-documented, its actual occurrence in practical scenarios remains elusive. The explanations provided in the Wikipedia article, particularly for Linux-based environments, offer plausible reasons for their potential existence. However, empirical evidence of their prevalence under real-world conditions is still lacking.

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