java - Java AQS如何支持不公平锁定？

Question

在AQS的文档中，第一行说

Provides a framework for implementing blocking locks and related synchronizers (semaphores, events, etc) that rely on first-in-first-out (FIFO) wait queues.

由于AQS是基于FIFO等待队列的，因此我认为自然是公平的。但是在实现 Samephore和 ReentrantLock等许多地方，AQS既有公平版本，也有不公平版本。
所以我想知道AQS如何支持不公平锁定。
另外，我在文档中找到了这些行。但是我不明白。线程如何比已经在队列中的其他线程优先插入？

Even though this class is based on an internal FIFO queue, it does not automatically enforce >FIFO acquisition policies. The core of exclusive synchronization takes the form:

Acquire:while (!tryAcquire(arg)) {enqueue thread if it is not already queued;possibly block current thread;}

Release:if (tryRelease(arg))unblock the first queued thread;

(Shared mode is similar but may involve cascading signals.)Because checks in acquire are invoked before enqueuing, a newly acquiring thread may bargeahead of others that are blocked and queued.

Answer 1

How does a thread barge ahead of others which are already in the queue?在不公平模式下，新线程将尝试首先获取锁定。如果失败，则将其插入队列。
例子:

Thread1持有锁；

Thread2在队列中等待；

Thread1释放锁定；

几乎同时，Thread3尝试获取锁。在不公平模式下运行时，允许Thread3在通知Thread2之前获取锁。但是在公平模式下，将不允许使用Thread3。

Thread3释放锁定；

Thread2获得了锁；