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The notification mechanism notifies the waiting thread and allows it to check its condition predicate. The invocation of notify() or notifyAll() in another thread cannot precisely determine which waiting thread will be resumed. Condition predicate statements allow notified threads to determine whether they should resume upon receiving the notification. Condition predicates are also useful when a thread is required to block until a condition becomes true, for example, when waiting for data to arrive on an input stream before reading the data.

Wiki MarkupBoth safety and liveness are concerns when using the wait/notify mechanism. The safety property requires that all objects maintain consistent states in a multithreaded environment \ [[Lea 2000|AA. References#Lea 00]\]. The liveness property requires that every operation or method invocation execute to completion without interruption.

To guarantee liveness, programs must test the while loop condition before invoking the wait() method. This early test checks whether another thread has already satisfied the condition predicate and sent a notification. Invoking the wait() method after the notification has been sent results in indefinite blocking.unmigrated-wiki-markup

To guarantee safety, programs must test the {{while}} loop condition after returning from the {{wait()}} method. Although {{wait()}} is intended to block indefinitely until a notification is received, it must still be encased within a loop to prevent the following vulnerabilities \ [[Bloch 2001|AA. References#Bloch 01]\]:

• Thread in the middle — A third thread can acquire the lock on the shared object during the interval between a notification being sent and the receiving thread resuming execution. This third thread can change the state of the object, leaving it inconsistent. This is a TOCTOU race condition.
• Malicious notification — A random or malicious notification can be received when the condition predicate is false. Such a notification would cancel the wait().
• Misdelivered notification — The order in which threads execute after receipt of a notifyAll() signal is unspecified. Consequently, an unrelated thread could start executing and discover that its condition predicate is satisfied. Consequently, it could resume execution, although it was required to remain dormant.
• Wiki MarkupSpurious wakeups --- — Certain JVM implementations are vulnerable to spurious wakeups that result in waiting threads waking up even without a notification \ [[API 2006|AA. References#API 06]\].

For these reasons, programs must check the condition predicate after the wait() method returns. A while loop is the best choice for checking the condition predicate both before and after invoking wait().

Wiki MarkupSimilarly, the {{await()}} method of the {{Condition}} interface must also be invoked inside a loop. According to the Java API \[ [API 2006|AA. References#API 06]\], Interface {{Condition}}

When waiting upon a Condition, a "spurious wakeup" is permitted to occur, in general, as a concession to the underlying platform semantics. This has little practical impact on most application programs as a Condition should always be waited upon in a loop, testing the state predicate that is being waited for. An implementation is free to remove the possibility of spurious wakeups but it is recommended that applications programmers always assume that they can occur and so always wait in a loop.

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Bibliography

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THI02-J. Notify all waiting threads rather than a single thread      09. Thread APIs (THI)