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    <p>To protect against spurious wake ups. There is no guarantee made to you by the JVM that the only possible reason the thread will ever start running again is because you called signal in the way you intended. Sometimes it will just get started accidentally and go (Spurious wake up). So you have to keep waiting again if the condition you want to run on isn't actually true. </p> <p>This is explained in the javadoc for the wait method: <a href="http://java.sun.com/javase/6/docs/api/java/lang/Object.html#wait%28long%29" rel="noreferrer">http://java.sun.com/javase/6/docs/api/java/lang/Object.html#wait%28long%29</a></p> <p>And mentioned in the docs for await: <a href="http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/Condition.html#await%28%29" rel="noreferrer">http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/Condition.html#await%28%29</a></p> <blockquote> <p>The lock associated with this Condition is atomically released and the current thread becomes disabled for thread scheduling purposes and lies dormant until one of four things happens:</p> <ul> <li><p>Some other thread invokes the signal() method for this Condition and the current thread happens to be chosen as the thread to be awakened; or</p></li> <li><p>Some other thread invokes the signalAll() method for this Condition; or</p></li> <li><p>Some other thread interrupts the current thread, and interruption of thread suspension is supported; or</p></li> </ul> <p><strong>* A "spurious wakeup" occurs.</strong></p> </blockquote> <p>Some implementations of the Condition interface may suppress spurious wakeups, but relying on that would hence be relying on an implementation detail and makes your code unportable.</p>
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    1. POWhy does java.util.concurrent.ArrayBlockingQueue use 'while' loops instead of 'if' around calls to await()?
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    1. COPerfect, thanks! Not sure if the Java 5 ReentrantLock.newCondition() implementation suppresses them or not, but I absolutely see your point about portability.
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    2. COI know this is a popular answer but I feel strongly it is not the right one. Spurious wakeups I'm sure do happen but the reason why whiles are used is because of race conditions _not_ spurious wakeups. See my answer below or the page I put up about this: http://256.com/gray/docs/misc/producer_consumer_race_conditions/
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    3. COAgreed that a loop is also the solution to two threads interleaving on lock acquisition at the entrance to put() and take(). The would be concurrent Java programmer still needs to understand the JVM's inability to guarantee the semantics of wait(). Even purpose built custom implementations where one internally managed thread is the consumer, still need loops even though such a race condition is impossible. Likewise alternate ways to prevent interleaving on the lock acquisition will not be acceptable in Java without also having the loop anyway.
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