Tuesday, February 14, 2006

Spurious wakeup of Java threads

Vlad Roubtsov today posted on the ADVANCED-JAVA list a message saying how he noticed that the JDK 1.5 documentation for java.lang.Object wait() method now contains this bit:

"A thread can also wake up without being notified, interrupted, or timing out, a so-called spurious wakeup. While this will rarely occur in practice, applications must guard against it by testing for the condition that should have caused the thread to be awakened, and continuing to wait if the condition is not satisfied. In other words, waits should always occur in loops, like this one:

synchronized (obj) {
while ()
... // Perform action appropriate to condition

(For more information on this topic, see Section 3.2.3 in Doug Lea's "Concurrent Programming in Java (Second Edition)" (Addison-Wesley, 2000), or Item 50 in Joshua Bloch's "Effective Java Programming Language Guide" (Addison-Wesley, 2001)."

Now, I always used while() instead of if() because even if I didn't know about this possibility for "spurious wakeups", I was always a bit paranoid about the reliability of any execution environment my code could be run in. Nevertheless, it is now a documented best practice :-)

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