LinkedBlockingQueue调用await时的锁状态疑问——结合take()源码解析

Great question—this is a super common point of confusion with Condition objects in Java's concurrency framework! Let's break down exactly what's happening here to clear up your confusion.

Let's walk through the lock flow step by step: #

1. First, the thread grabs the lock: When takeLock.lockInterruptibly() runs, the thread successfully acquires the takeLock before entering the try block. At this point, it definitely holds the lock.
2. await() releases the lock temporarily: When notEmpty.await() is called, the method does two critical things under the hood:
   • It releases the takeLock the thread currently holds (so other threads—like one calling put()—can access the queue).
   • It puts the thread into a waiting state until another thread signals the notEmpty condition.

The key part: What happens when the thread is awakened? #

When another thread triggers notEmpty.signal() (say, after adding an element to the queue), the waiting thread doesn't just jump back into the loop right away. Instead:

• The thread first re-acquires the takeLock (this logic is built into the Condition's await() implementation). It will block here until it can get the lock again.
• Only after successfully re-grabbing the lock does the await() method return, and the thread moves back to the while loop check.

Why no IllegalMonitorStateException? #

The Condition rule ("you must hold the lock when calling await()") is always satisfied here:

• We only call notEmpty.await() when we already hold takeLock (right after entering the try block, and every time we loop back to it, we've re-acquired the lock from the wake-up step).
• When we wake up and exit await(), we have the lock again, so any subsequent checks or re-calls to await() are valid.

Quick real-world example #

Let's say Thread A calls take() on an empty queue:

1. Thread A takes takeLock, sees count is 0, calls notEmpty.await()—releases the lock and waits.
2. Thread B calls put(), adds an element, then triggers notEmpty.signal().
3. Thread A is awakened, waits to re-acquire takeLock (if Thread B still holds related locks, it pauses here).
4. Once Thread A has takeLock again, it exits await() and checks the while loop. Now count > 0, so it dequeues the element.

This guarantees that every time we interact with the notEmpty Condition or check the queue count, we're holding the necessary lock—no exceptions thrown.

内容的提问来源于stack exchange，提问作者Litchy