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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 \[[Lea 2000|AA. Bibliography#LeaReferences#Lea 00]\]. 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 \[[Bloch 2001|AA. Bibliography#BlochReferences#Bloch 01]\]: |
- 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 Markup Spurious wakeups --- Certain JVM implementations are vulnerable to spurious wakeups that result in waiting threads waking up even without a notification \[[API 2006|AA. Bibliography#APIReferences#API 06]\].
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().
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Similarly, the {{await()}} method of the {{Condition}} interface must also be invoked inside a loop. According to the Java API \[[API 2006|AA. Bibliography#APIReferences#API 06]\], Interface {{Condition}} |
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<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="382b6afda3a2d1ee-a92799d1-49a84eea-b722ac4f-a2a15b6285e56ae38fdb3fd0"><ac:plain-text-body><![CDATA[ | [[Bloch 2001 | AA. Bibliography#Bloch References#Bloch 01]] | Item 50. Never invoke | ]]></ac:plain-text-body></ac:structured-macro> | |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="71a17f2cbf732815-18aa4d1e-48794cfa-a438bb2c-2f4721f3e3796aa39d2895e8"><ac:plain-text-body><![CDATA[ | [[Lea 2000 | AA. Bibliography#Lea References#Lea 00]] | 3.2.2, Monitor Mechanics; 1.3.2, Liveness | ]]></ac:plain-text-body></ac:structured-macro> | |
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="b0f77f54f7cd7e53-0e7e7062-4306425f-a330aa00-8e6752e8b64126b6b67ea06f"><ac:plain-text-body><![CDATA[ | [[Goetz 2006 | AA. Bibliography#Goetz References#Goetz 06]] | Section 14.2, Using Condition Queues | ]]></ac:plain-text-body></ac:structured-macro> |
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