The wait() method is used to temporarily cede possession of a lock so that another requesting thread can proceed. It must always be used inside a synchronized block or method. To resume activity, the requesting thread must notify the waiting thread. Moreover, the wait() method should be invoked in a loop that checks if a condition predicate holds.

The invocation of notify() or notifyAll() in another thread cannot pin point which waiting thread must be resumed. A condition predicate statement is used so that the correct thread is notified. 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.

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

  // Proceed 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.

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

To guarantee liveness, the while loop condition should be tested before invoking the wait() method. This is because the condition might already be true which indicates that a notify signal may have already been sent from the other thread. Invoking the wait() method after the notify signal has been sent is futile and results in an infinitely blocked state.

To guarantee _safety_, the {{while}} loop condition must be tested even after the call to {{wait()}}. While {{wait()}} is meant to block indefinitely till a notification is received, this practice is recommended because: \[[Bloch 01|AA. Java References#Bloch 01]\] 

• 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 actually resuming execution. This thread can change the state of the object leaving it inconsistent. This is akin to the "time of check, time of use" (TOCTOU) condition.
• Malicious notifications: There is no guarantee that a notification will not be sent when the condition does not hold. This means that the invocation of wait() is nullified by the notification.
• Mis-delivered notification: Sometimes on receipt of a notifyAll() signal, an unrelated thread can start executing and it is possible for its condition predicate to be true.
• Spurious wakeups: Certain JVM implementations are vulnerable to spurious wakeups that result in waiting threads waking up even without a notification.

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

Noncompliant Code Example

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

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

Compliant Solution

This compliant solution encloses the wait() method in a while loop and as a result checks the condition during both pre and post 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 of the java.util.concurrent.locks.Condition interface is implemented, it should be enclosed in a loop.

Risk Assessment

To guarantee liveness and safety, the wait() and await() methods should always be called inside a while loop.

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

CON17-J. Avoid using ThreadGroup APIs      11. Concurrency (CON)      CON19-J. Use notifyAll() instead of notify() to resume waiting threads