Do spurious wakeups in Java actually happen?

java

concurrency

synchronization

thread-scheduler

byAlex Kataev·Feb 6, 2025

Yes, spurious wakeups do occur in Java. They can cause a thread to wake up spontaneously without any notifying signal or a specified timeout condition. Here's how to protect against them:

synchronized(lock) {
    while (!conditionMet) { //condition check, because spurious wakeups dislike logic
        lock.wait();
    }
}

An additional check following wait() should be done to ensure safety from these unpredictable wakeup calls.

Grasping spurious wakeups

Spurious wakeups are an intriguing beast in concurrency, causing a thread to wake from waiting without any specific request. Bizarre as it may sound, yes, such monsters do exist, and, yes, you need to be ready to defeat them.

Taming the spurious wakeup beast

To subdue this phenomenon, always engage the wait condition within a loop while managing synchronization. This makes sure the thread keeps the midnight oil burning (i.e., keeps waiting) if the wakeup was just a prank caller.

Core count effect

Like in movies, the more characters involved, the more complex it gets. Systems with a higher core count experience more spurious wakeups. So, don't be surprised if your 24-core machine acts a bit spookier.

Keeping an eye on the scheduler

The Java's thread scheduler might also have had a hard day and cause spurious wakeups during some abnormal events. In such scenarios, ensure you have a disaster recovery plan, i.e., code for graceful recovery.

Delving into the 'why'

To fully comprehend spurious wakeups, you need to descend into the rabbit hole of operating system signals and system calls. In the world of Linux, futex system calls and POSIX signals are often the culprits causing these interruptions.

Be a strategic coder

As Java soldiers, we always need to be prepared for some unexpected conduct from our multi-threaded programming allies. This includes being ready to slide past spurious behaviors and embrace the unexpected by anticipating the unpredictable.

Tackling myths

Contrary to popular belief, spurious wakeups aren't Java folklore — they are real. Unwrap the myth to level up your concurrent code game.

Exploring further

Need more brain food? Head over to Wikipedia or delve into the world of Cameron Purdy's blog for insightful narratives diving deep into spurious wakeups.

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