"Since the notEmpty.await() method wakes up and returns, then this thread must have obtained the lock" This sentence is correct.

Suppose the code you write is:

if (count == 0) notEmpty.await();

One thing that is clear is that we need to ensure that it is inreturn dequeue()之前需要满足的一个条件是count != 0。我们假设 线程A 此时拿到了lock，那么 线程A 的notEmpty.await()此时便会停止阻塞，准备向下执行return dequeue()。但是假设在竞争激烈的条件下，线程A 拿到 lock 之后，准备执行下一条 JVM 指令的时候，线程B 此时抢占了 lock，然后继续向下执行return dequeue()，刚好使得 count 变为了 0；而此时因为写的只是if(count == 0)，那么线程 A 在拿到 lock 之后，还是会继续向下执行return dequeue(), which leads to the error.

As to whywait()、await()this method of blocking when the conditions are not metmust beused in a while loop, you can refer to Article 69 (page 244) of the second edition of "Effective Java".

public ArrayBlockingQueue(int capacity, boolean fair) { if (capacity <= 0) throw new IllegalArgumentException(); this.items = new Object[capacity]; lock = new ReentrantLock(fair); notEmpty = lock.newCondition(); //notEmpty 就是这个锁的条件 notFull = lock.newCondition(); }

So when the current queue in your code is empty (count == 0), the callnotEmpty.await()，这段代码对锁是有影响的，实际上底层上已经释放了锁，只是这个方法保证了被唤醒时一定又能够拿回锁（当有元素放入队列会调用notEmpty.signal()进行唤醒），那为什么需要使用while呢？因为insert后lock.unlock，未必notEmpty.await()will wake you up immediately. You may insert a thread to run the remove method before