公平模式下，调用的AQS.hasQueuedPredecessors()方法
static final class FairSync extends Sync { final boolean writerShouldBlock() { return hasQueuedPredecessors(); } final boolean readerShouldBlock() { return hasQueuedPredecessors(); } } public final boolean hasQueuedPredecessors() { Node t = tail; // Read fields in reverse initialization order Node h = head; Node s; // head 头结点是当前持有锁的节点，它的下一个节点不是当前线程，返回true，表示应该要阻塞当前线程 return h != t && ((s = h.next) == null || s.thread != Thread.currentThread()); }