How to Build a Thread-Safe BlockingQueue in Java From Scratch

Implementing a custom BlockingQueue is a common Low-Level Design interview challenge at companies like Apple and Amazon, testing a candidate's grasp of thread coordination and JVM internals. A naive implementation using synchronized blocks with notifyAll() causes the 'thundering herd' problem, while using if instead of while leaves the queue vulnerable to spurious wakeups. The recommended approach uses a single ReentrantLock paired with two separate Condition instances — notFull and notEmpty — to isolate producer and consumer wait states. This design ensures that only the relevant thread is woken via signal(), reducing unnecessary context switching. Key implementation rules include always guarding await() calls with a while loop and using try-finally blocks to guarantee lock release even when exceptions occur.