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

The Object.wait() method temporarily cedes possession of a lock so that another thread that is requesting the lock can proceed. Object.wait() must always be called from a synchronized block or method. To resume the waiting thread, the requesting thread must invoke the notify() method to notify it. Furthermore, the wait() method should be invoked in a loop that checks if 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.

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

    // Consume when condition holds
  }
}

The notification mechanism notifies the waiting thread and lets it check its condition predicate. The invocation of notify() or notifyAll() in another thread cannot precisely determine which waiting thread is resumed. A condition predicate statement is provided so that only the correct thread will resume upon receiving the notification. A condition predicate also helps when a thread is required to block until a condition becomes true, such as reading data from an input stream before proceeding.

Safety and liveness are both concerns when using the wait/notify mechanism. Safety requires that all objects maintain consistent states in a multithreaded environment [[Lea 2000]]. Liveness requires that every operation or method invocation execute to completion without interruption.

To guarantee liveness, the while loop condition must be tested before the wait() method is invoked. This is done in case 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, the while loop condition must be tested even after the wait() method is invoked. While wait() is meant 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 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—There is no guarantee that a random notification will not be received when the condition predicate is false. This means that the invocation of wait() could be nullified by the notification.
• misdelivered 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. 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, the condition predicate must be checked after the wait() method is invoked. A while loop is the best choice for checking the condition predicate 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 concurrency utilities instead of the wait/notify mechanism. However, legacy code may 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. If the notification is accidental or malicious, the thread can 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 before and after wait() is called.

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

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

Risk Assessment

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

Automated Detection

TODO

Related Vulnerabilities

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

Bibliography

[[API 2006]] Class Object
[[Bloch 2001]] Item 50: Never invoke wait outside a loop
[[Lea 2000]] 3.2.2 Monitor Mechanics, 1.3.2 Liveness
[[Goetz 2006]] Section 14.2, Using Condition Queues

THI02-J. Do not invoke Thread.run()      Thread APIs (THI)      THI04-J. Notify all waiting threads instead of a single thread