THI03-J. Always invoke wait() and await() methods inside a loop

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 due to 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(). 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();
    }

    // Consume 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 predicate are also useful when a thread is required to block until a condition becomes true, for example 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 before invoking the wait() method. This early test checks whether another thread has already satisfied the condition predicate and sent a notification. Invoking the wait() method after the notification has been sent results in indefinite blocking.

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]]:

• 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-to-time-of-use (TOCTOU) 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.

• spurious wake-ups — Certain JVM implementations are vulnerable to spurious wake-ups that result in waiting threads waking up even without a notification [[API 2006]].

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 must also be invoked inside a loop. According to the Java API [[API 2006]], 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.

New code should use the java.util.concurrent.locks concurrency utilities in place of the wait/notify mechanism. However, legacy code that complies with the other requirements of this rule is permitted to depend upon 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 the notification is received. When the notification was accidental or malicious, the thread could wake up prematurely.

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

Compliant Solution

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 must also be enclosed in a similar loop.

Risk Assessment

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

Bibliography

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