Spurious Wakeups in Java: Myth or Reality?

Spurious Wakeups in Java: A Myth or Reality?

The concept of spurious wakeups, where a thread is unexpectedly awakened from a wait state without an apparent reason, has been a topic of discussion in Java programming. However, the prevalence of these events in practice has been questioned.

According to the Wikipedia article on spurious wakeups, the Linux implementation of pthread_cond_wait(), which is utilized by Java, experiences this behavior. Specifically, signals received by the process can lead to abrupt returns from blocking system calls with EINTR. Due to potential race conditions, the system may fail to detect legitimate wakeups, resulting in spurious wakeups.

To illustrate this, consider the following Java code:

<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>

To artificially induce a spurious wakeup in this code without relying on random events, simply send a signal to the process running the code. On Linux, this can be achieved using the kill -CONT command, where is the process ID.

In conclusion, while the theoretical possibility of spurious wakeups exists in Linux due to the implementation of pthread_cond_wait(), their occurrence in a decent hardware/software environment appears to be sporadic. As such, the concern over spurious wakeups should not deter developers from utilizing appropriate synchronization techniques such as locking and condition variables.

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