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The Object.wait() method is used to temporarily cede cedes possession of a lock so that another thread that is other threads that may be requesting the lock can proceed. Object. It wait() must always be used inside called from a synchronized block or method. To make the The waiting thread resume, the requesting thread must notify() it. Furthermore, the 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 should must be invoked in from a loop that checks if whether a condition predicate holds. Note that a condition predicate is not the same as 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(). The Following is the correct way to invoke the wait() method when the vector is empty is shown below.

private Vector vector;
// ...

public void consumeElement() throws InterruptedException {
  synchronized (vector) {
    while (vector.isEmpty()) {
      vector.wait(); 
    }

    // ConsumeResume when condition holds
  }
}

The notification mechanism notifies the waiting thread and lets 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. A condition predicate statement is provided so that only the correct thread will Condition predicate statements allow notified threads to determine whether they should resume upon receiving the notification. A condition predicate also helps Condition predicates are also useful when a thread is required to block until a condition becomes true, for instanceexample, when it should not proceed without reading some data from waiting for data to arrive on an input stream before reading the data.

When Both safety and liveness  are concerns when using the wait/notify mechanism, two properties come into the picture:

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Safety: Its main goal is to ensure that all objects maintain consistent states in a multi-threaded environment. \[[Lea 00|AA. Java References#Lea 00]\]

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. 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.

To guarantee liveness, programs must test the while loop condition should be tested before invoking the wait() method. This is because early test checks whether another thread has already satisfied the condition predicate might have already been made true by some other thread with a good chance that the same thread also sent out a notify signaland sent a notification. Invoking the wait() method after the notify signal notification has been sent is futile and results in an indefinite blocking.

Wiki MarkupTo guarantee _ safety_, the {{while}} loop condition must be tested even after invoking {{wait()}}. While {{wait()}} is meant to block indefinitely until a notification is received, it should still be encased within a loop \[[Bloch 01|AA. Java References#Bloch 01]\], to prevent the following vulnerabilitiesprograms 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 still must 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 time-of-check, time-of-use (TOCTOU) race condition.
• Malicious notification: There is no guarantee that a random notification will not A random or malicious notification can be received when the condition predicate is false. This means that the invocation of Such a notification would cancel the wait() may be nullified by the notification method.
• Mis-delivered Misdelivered notification: Sometimes on The order in which threads execute after receipt of a notifyAll() signal is unspecified. Consequently, an unrelated thread can could start executing and it is possible for discover that its condition predicate to be trueis satisfied. Consequently, it may could resume execution whilst it was despite being 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 \[[API 06|AA. Java References#API 06]\].Java Virtual Machine (JVM) implementations are vulnerable to spurious wakeups that result in waiting threads waking up even without a notification [API 2014].

For these reasons, programs must Because of these reasons, it is indispensable to check the condition predicate after the wait() is invoked 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 interface {{Condition}} must also be invoked inside a loop. According to the Java API \[[API 06|AA. Java References#API 06]\], Interface {{Condition}}:the Condition interface also must be invoked inside a loop. According to the Java API [API 2014], 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.

Newer New code should use the java.util.concurrent.locks concurrency utilities as opposed to in place of the wait/notify mechanism, however. However, legacy code may require that complies with the other requirements of this rule is permitted to depend on the wait/notify mechanism.

Noncompliant Code Example

This noncompliant code example invokes the wait() method inside a traditional if block and fails to check the post condition postcondition after the notification (is received. If the notification were accidental or malicious) is received. This means that , the thread can could wake up when it is not supposed to do soprematurely.

synchronized (object) {
  if (<condition does not hold>) {
    object.wait();
  }
  // Proceed when condition holds
}

Compliant Solution

This compliant solution encloses calls the wait() method in from within a while loop and as a result checks to check the condition during both pre and post before and after the call to wait() invocation times.:

synchronized (object) {
  while (<condition does not hold>) {
    object.wait(); 
  }
  // Proceed when condition holds
}

Similarly, invocations of the await() method Invocations of the java.util.concurrent.locks.Condition interface should always .await() method also must be enclosed in a similar loop.

Risk Assessment

To guarantee liveness and safety, Failure to encase the wait() and or await() methods should always be invoked inside a while loop can lead to indefinite blocking and denial of service (DoS).

Automated Detection

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Tool	Version	Checker	Description
Parasoft Jtest	Include Page Parasoft_V Parasoft_V	CERT.THI03.UWIL	Call 'wait()' and 'await()' only inside a loop that tests the liveness condition
SonarQube	Include Page SonarQube_V SonarQube_V	S2274	"Object.wait(...)" and "Condition.await(...)" should be called inside a "while" loop

Bibliography

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TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[API 06|AA. Java References#API 06]\] [Class Object|http://java.sun.com/javase/6/docs/api/java/lang/Object.html]
\[[Bloch 01|AA. Java References#Bloch 01]\] Item 50: Never invoke wait outside a loop
\[[Lea 00|AA. Java References#Lea 00]\] 3.2.2 Monitor Mechanics, 1.3.2 Liveness
\[[Goetz 06|AA. Java References#Goetz 06]\] Section 14.2, Using Condition Queues

CON17-J. Do not invoke ThreadGroup methods      11. Concurrency (CON)      CON19-J. Notify all waiting threads instead of a single thread