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Ensure Object.wait(...) and Condition.await(...) are Used Inside a "while" Loop

High
correctnesssynchronization

What is it?​

In a multithreaded environment, using Object.wait(...) or Condition.await(...) methods inappropriately can lead to faulty thread execution due to "spurious wakeups".

Why apply it?​

If wait() or await() is not called inside a loop that checks the condition, spurious wakeups could cause incorrect behavior by assuming the condition is met when it isn't.

How to fix it?​

Ensure wait() or await() calls are made inside a while loop that checks the condition. This ensures that logic is executed only when the condition is confirmed to be true.

Examples​

Example 1:​

Negative​

This noncompliant example uses if, which can result in code execution despite spurious wakeups.

synchronized (lock) {
  if (!conditionMet()) {
    lock.wait();  // Noncompliant; `wait` is not in a loop
  }
  // Risky execution as condition might not be true.
  processTask();
}

Example 2:​

Positive​

This compliant example uses a while loop to ensure that the condition is checked continuously even after spurious wakeups.

synchronized (lock) {
  while (!conditionMet()) {
    lock.wait();  // Compliant; `wait` is in a loop and re-checks the condition
  }
  // Execute logic safely assuming the condition is met
  processTask();
}

Negative​

A noncompliant example that uses an if statement, vulnerable to spurious wakeups.

synchronized (lock) {
  if (sharedQueue.isEmpty()) {
    lock.wait();  // Noncompliant; `wait` is not used inside a loop
  }
  // May fail if queue is empty due to wakeup
  Object item = sharedQueue.poll();
  handleItem(item);
}

Example 3:​

Positive​

The compliant example handles concurrency correctly with a while loop that ensures the condition is met.

synchronized (lock) {
  while (sharedQueue.isEmpty()) {
    lock.wait();  // Compliant; waits inside a loop, guarding the condition
  }
  // Process an item from the queue
  Object item = sharedQueue.poll();
  handleItem(item);
}