AQS --- 渐入佳境

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
   
    static final class Node {
       ……
       volatile int waitStatus;
       volatile Node prev;
       volatile Node next;
       volatile Thread thread;
       ……
    }

    private transient volatile Node head;
    private transient volatile Node tail;

    /**
     * The synchronization state.
     */
    private volatile int state;
}

           +------+  prev +-----+       +-----+
      head |      | <---- |     | <---- |     |  tail
           +------+       +-----+       +-----+

/**
 * Creates an instance of {@code ReentrantLock}.
 * This is equivalent to using {@code ReentrantLock(false)}.
 */
 public ReentrantLock() {
     sync = new NonfairSync();
 }

/**
 * Creates an instance of {@code ReentrantLock} with the
 * given fairness policy.
 *
 * @param fair {@code true} if this lock should use a fair ordering policy
 */
 public ReentrantLock(boolean fair) {
     sync = fair ? new FairSync() : new NonfairSync();
 }

final boolean nonfairTryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0) // overflow
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

 protected final boolean tryAcquire(int acquires) {
    final Thread current = Thread.currentThread();
    int c = getState();
    if (c == 0) {
        if (!hasQueuedPredecessors() &&
            compareAndSetState(0, acquires)) {
            setExclusiveOwnerThread(current);
            return true;
        }
    }
    else if (current == getExclusiveOwnerThread()) {
        int nextc = c + acquires;
        if (nextc < 0)
            throw new Error("Maximum lock count exceeded");
        setState(nextc);
        return true;
    }
    return false;
}

public final boolean hasQueuedPredecessors() {
    // The correctness of this depends on head being initialized
    // before tail and on head.next being accurate if the current
    // thread is first in queue.
    Node t = tail; // Read fields in reverse initialization order
    Node h = head;
    Node s;
    return h != t &&
        ((s = h.next) == null || s.thread != Thread.currentThread());
}