大厂面试必会：AQS LockSupport Unsafe之间的关系

/**
* CAS head field. Used only by enq.
* 入队时 CAS 修改队列 head
*/
private final boolean compareAndSetHead(Node update) {
	return unsafe.compareAndSwapObject(this, headOffset, null, update);
}

/**
* CAS tail field. Used only by enq.
* 入队时 CAS 修改队列 tail
*/
private final boolean compareAndSetTail(Node expect, Node update) {
	return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
}

// 未获取到锁的线程被包装成 Node，然后加入队列中
// 进入队列的 Node 如果处在 head，则尝试抢锁一次
// 若不在 head 位置或在 head 抢锁不成功，则进入 park
final boolean acquireQueued(final Node node, int arg) {
	boolean failed = true;
	try {
		boolean interrupted = false;
		for (;;) {
			final Node p = node.predecessor(); // 返回前一个节点
			if (p == head && tryAcquire(arg)) { // 前一个节点是head，则尝试抢锁一次
				setHead(node); // 抢锁成功，自己变为 head
				p.next = null; // help GC
				failed = false;
				return interrupted;
			}
			if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) // 若 Lock 的逻辑是抢锁失败后 park，则这里让线程进入 park
				interrupted = true;
		}
	} finally {
		if (failed)
			cancelAcquire(node);
	}
}

// 入队的线程 park
private final boolean parkAndCheckInterrupt() {
	LockSupport.park(this);
	return Thread.interrupted();
}

// LockSupport 的 park 方法直接调用 Unsafe#park
public static void park() {
	UNSAFE.park(false, 0L);
}
// LockSupport 的 unpark 方法直接调用 Unsafe#unpark
public static void unpark(Thread thread) {
	if (thread != null)
		UNSAFE.unpark(thread);
}