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The Object.wait() method is used to temporarily cede cedes possession of a lock so that another requesting thread 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 resume activity, the requesting thread must notify the waiting thread. Moreover, the 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 should must be invoked in from a loop that checks if whether a condition predicate holds.

The invocation of notify() or notifyAll() in another thread cannot precisely determine which waiting thread must be resumed. A condition predicate statement is used so that the correct thread resumes when it receives a notification. A condition predicate also helps when a thread is required to block until a condition becomes true, for instance, when it cannot proceed without obtaining some data from an input stream.

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.

private Vector vector;
// ...

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

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

    // ProceedResume when condition holds
  }
}

Two properties come into the picture:

• Liveness: Every operation or method invocation executes to completion without interruptions, even if it goes against safety.

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

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.

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.

To guarantee liveness, programs must test the while loop condition To guarantee liveness, the while loop condition should be tested before invoking the wait() method. This is because the condition might have already been made true by some other thread which indicates that a notify signal may have already been sent from the other threadearly test checks whether another thread has already satisfied the condition predicate and sent a notification. Invoking the wait() method after the notify signal notification has been sent is futile and results in an infinitely blocked state.unmigrated-wiki-markupindefinite blocking.

To guarantee _ safety_, the {{while}} loop condition must be tested even after the call to {{wait()}}. While {{wait()}} is meant to block indefinitely until a notification is received, this practice is recommended because: \[[Bloch 01|AA. Java References#Bloch 01]\] 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 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 actually 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 notifications: There is no guarantee that a random notification will not notification: A random or malicious notification can be received when the condition does not hold. This means that the invocation of predicate is false. Such a notification would cancel the wait() is 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 blockeddormant.
• Wiki MarkupSpurious wakeups: Certain JVM implementations are vulnerable to _spurious wakeups_ that result in waiting threads waking up even without a notification \[[API 06|AA. Java References#API 06]\].

Because of these reasons, it is indispensable to check the condition using a loop, after wait() is called.

• 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 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 also must be invoked inside a loop. According to the Java API [API 2014], Interface Condition 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}}:

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 use of these methodsthat 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 after 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 toprematurely.

synchronized (object) {
  if (<condition does not hold>) {
    object.wait();
  }
  //proceed 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.:

// Condition predicate is guarded by a lock on the shared object/variable
synchronized (object) {
  while (<condition does not hold>) {
    object.wait(); 
  }

  // Proceed when condition holds
}

Likewise, if the await() method Invocations of the java.util.concurrent.locks.Condition interface is implemented, it should .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 called inside a while loop can lead to indefinite blocking and denial of service (DoS).

Automated Detection

...

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

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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Image Added Image Added Image AddedCON17-J. Avoid using ThreadGroup APIs      11. Concurrency (CON)      CON19-J. Notify all waiting threads instead of a single thread