Calls to Object.wait() should always be placed inside while loops to handle two critical concurrency scenarios
properly.
First, the Java Virtual Machine can produce spurious wakeups. This means that a thread waiting on a condition can be awakened even
when the condition has not actually been satisfied. If the wait() call is inside an if statement, the thread will proceed
after the spurious wakeup without rechecking the condition, potentially leading to incorrect behavior.
Second, race conditions can occur when multiple threads are involved. Between the time a condition is checked and the
wait() method is called, another thread might change the condition. If the condition was satisfied by another thread before
wait() is invoked, the current thread could wait indefinitely for a condition that has already been met.
Using a while loop ensures that the condition is rechecked immediately after the thread wakes up, providing protection against both
spurious wakeups and race conditions.
What is the potential impact?
When wait() calls are not properly guarded by while loops, applications can experience:
- Deadlocks: Threads may wait indefinitely for conditions that have already been satisfied
- Data corruption: Threads may proceed with operations when the required conditions are not actually met
- Unpredictable behavior: Spurious wakeups can cause threads to execute code at inappropriate times
- Race conditions: Multiple threads may interfere with each other’s execution in unexpected ways
These issues are particularly difficult to debug because they may occur intermittently and depend on timing and thread scheduling.
