Page History

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.

private Vector vector;
// ...

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

    // Resume when condition holds
  }
}

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 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 still must still be encased within a loop to prevent the following vulnerabilities [Bloch 2001]:

• Thread in the middle ����������‚�š�š�š�š����������€š�š�š�š�? : 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 ����������‚�š�š�š�š����������€š�š�š�š�? : A random or malicious notification can be received when the condition predicate is false. Such a notification would cancel the wait() method.
• Misdelivered notification ����������‚�š�š�š�š����������€š�š�š�š�? : 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 despite being required to remain dormant.
• Spurious wakeups ����������‚�š�š�š�š����������€š�š�š�š�? Certain JVM : Certain Java Virtual Machine (JVM) implementations are vulnerable to spurious wakeups that result in waiting threads waking up even without a notification [API 20062014].

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 also be invoked inside a loop. According to the Java API [API 20062014], 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.

...

This noncompliant code example invokes the wait() method inside a traditional if block and fails to check the postcondition after the notification is received. If the notification were accidental or malicious, the thread could wake up prematurely.

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

...

This compliant solution calls the wait() method from within a while loop to check the condition both before and after the call to wait().:

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

Invocations of the java.util.concurrent.locks.Condition.await() method also must also be enclosed in a similar loop.

...

Failure to encase the wait() or await() methods inside a while loop can lead to indefinite blocking and denial of service (DoS).

Automated Detection

Bibliography

...

Image Added Image Added THI02-J. Notify all waiting threads rather than a single thread      09. Thread APIs (THI)