The Object.wait() method temporarily cedes possession of a lock so that other threads that may be requesting the lock can proceed. Object.wait() must always be called from a synchronized block or method. The waiting thread resumes execution only after it has been notified, generally as the result of the invocation of the notify() or notifyAll() method by some other thread. The wait() method must be invoked from a loop that checks whether a condition predicate holds. Note that a condition predicate is the negation of the condition expression in the loop. For example, the condition predicate for removing an element from a vector is !isEmpty(), whereas the condition expression for the while loop condition is isEmpty(). Following is the correct way to invoke the wait() method when the vector is empty.
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Both 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]. The liveness property requires that every operation or method invocation execute to completion without interruption.
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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]:
- 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. - Spurious wakeups — Certain JVM implementations are vulnerable to spurious wakeups that result in waiting threads waking up even without a notification [API 2006].
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().
Similarly, the await() method of the Condition interface must also be invoked inside a loop. According to the Java API [API 2006], 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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Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
THI03-J | low | unlikely | medium | P2 | L3 |
Bibliography
[API 2006] | |
Item 50. Never invoke | |
[Lea 2000] | 3.2.2, Monitor Mechanics; 1.3.2, Liveness |
Section 14.2, Using Condition Queues |
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THI02-J. Notify all waiting threads rather than a single thread Rule 10: Thread APIs (THI)