AQS为什么采用双向链表

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
}

public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
    /**
         * Link to predecessor node that current node/thread relies on
         * for checking waitStatus. Assigned during enqueuing, and nulled
         * out (for sake of GC) only upon dequeuing.  Also, upon
         * cancellation of a predecessor, we short-circuit while
         * finding a non-cancelled one, which will always exist
         * because the head node is never cancelled: A node becomes
         * head only as a result of successful acquire. A
         * cancelled thread never succeeds in acquiring, and a thread only
         * cancels itself, not any other node.
         */
    volatile Node prev;

    /**
         * Link to the successor node that the current node/thread
         * unparks upon release. Assigned during enqueuing, adjusted
         * when bypassing cancelled predecessors, and nulled out (for
         * sake of GC) when dequeued.  The enq operation does not
         * assign next field of a predecessor until after attachment,
         * so seeing a null next field does not necessarily mean that
         * node is at end of queue. However, if a next field appears
         * to be null, we can scan prev's from the tail to
         * double-check.  The next field of cancelled nodes is set to
         * point to the node itself instead of null, to make life
         * easier for isOnSyncQueue.
         */
    volatile Node next;
}

public final int getQueueLength() {
    int n = 0;
    for (Node p = tail; p != null; p = p.prev) {
        if (p.thread != null)
            ++n;
    }
    return n;
}

/**
     * Returns a collection containing threads that may be waiting to
     * acquire.  Because the actual set of threads may change
     * dynamically while constructing this result, the returned
     * collection is only a best-effort estimate.  The elements of the
     * returned collection are in no particular order.  This method is
     * designed to facilitate construction of subclasses that provide
     * more extensive monitoring facilities.
     *
     * @return the collection of threads
     */
public final Collection<Thread> getQueuedThreads() {
    ArrayList<Thread> list = new ArrayList<Thread>();
    for (Node p = tail; p != null; p = p.prev) {
        Thread t = p.thread;
        if (t != null)
            list.add(t);
    }
    return list;
}

/**
     * Returns a collection containing threads that may be waiting to
     * acquire in exclusive mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to an exclusive acquire.
     *
     * @return the collection of threads
     */
public final Collection<Thread> getExclusiveQueuedThreads() {
    ArrayList<Thread> list = new ArrayList<Thread>();
    for (Node p = tail; p != null; p = p.prev) {
        if (!p.isShared()) {
            Thread t = p.thread;
            if (t != null)
                list.add(t);
        }
    }
    return list;
}

/**
     * Returns a collection containing threads that may be waiting to
     * acquire in shared mode. This has the same properties
     * as {@link #getQueuedThreads} except that it only returns
     * those threads waiting due to a shared acquire.
     *
     * @return the collection of threads
     */
public final Collection<Thread> getSharedQueuedThreads() {
    ArrayList<Thread> list = new ArrayList<Thread>();
    for (Node p = tail; p != null; p = p.prev) {
        if (p.isShared()) {
            Thread t = p.thread;
            if (t != null)
                list.add(t);
        }
    }
    return list;
}

/**
 * Acquires in exclusive mode, ignoring interrupts.  Implemented
 * by invoking at least once {@link #tryAcquire},
 * returning on success.  Otherwise the thread is queued, possibly
 * repeatedly blocking and unblocking, invoking {@link
 * #tryAcquire} until success.  This method can be used
 * to implement method {@link Lock#lock}.
 *
 * @param arg the acquire argument.  This value is conveyed to
 *        {@link #tryAcquire} but is otherwise uninterpreted and
 *        can represent anything you like.
 */
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
    acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
    selfInterrupt();
}

/**
 * Acquires in exclusive mode, aborting if interrupted.
 * Implemented by first checking interrupt status, then invoking
 * at least once {@link #tryAcquire}, returning on
 * success.  Otherwise the thread is queued, possibly repeatedly
 * blocking and unblocking, invoking {@link #tryAcquire}
 * until success or the thread is interrupted.  This method can be
 * used to implement method {@link Lock#lockInterruptibly}.
 *
 * @param arg the acquire argument.  This value is conveyed to
 *        {@link #tryAcquire} but is otherwise uninterpreted and
 *        can represent anything you like.
 * @throws InterruptedException if the current thread is interrupted
 */
public final void acquireInterruptibly(int arg)
throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    if (!tryAcquire(arg))
        doAcquireInterruptibly(arg);
}

/**
 * Acquires in exclusive interruptible mode.
 * @param arg the acquire argument
 */
private void doAcquireInterruptibly(int arg)
throws InterruptedException {
    final Node node = addWaiter(Node.EXCLUSIVE);
    boolean failed = true;
    try {
        for (;;) {
            final Node p = node.predecessor();
            if (p == head && tryAcquire(arg)) {
                setHead(node);
                p.next = null; // help GC
                failed = false;
                return;
            }
            if (shouldParkAfterFailedAcquire(p, node) &&
                parkAndCheckInterrupt())
                throw new InterruptedException();
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

/**
 * Returns true if the given thread is currently queued.
 *
 * <p>This implementation traverses the queue to determine
 * presence of the given thread.
 *
 * @param thread the thread
 * @return {@code true} if the given thread is on the queue
 * @throws NullPointerException if the thread is null
 */
public final boolean isQueued(Thread thread) {
if (thread == null)
    throw new NullPointerException();
for (Node p = tail; p != null; p = p.prev)
if (p.thread == thread)
    return true;
return false;
}

/**
 * Checks and updates status for a node that failed to acquire.
 * Returns true if thread should block. This is the main signal
 * control in all acquire loops.  Requires that pred == node.prev.
 *
 * @param pred node's predecessor holding status
 * @param node the node
 * @return {@code true} if thread should block
 */
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
    int ws = pred.waitStatus;
    if (ws == Node.SIGNAL)
        //如果ws为Node.SIGNAL（值为-1），
        //这意味着前驱节点已经处于等待状态，期望在释放同步状态时唤醒后继节点。
        //在这种情况下，方法可以直接返回true，指示当前线程可以安全地挂起。
        return true;
    if (ws > 0) {
        //如果ws大于0，说明前驱节点已被取消。
        //此时，循环向前遍历等待队列，跳过所有已取消的节点，直到找到一个未被取消的节点作为新的前驱节点，并更新相应的链接。
        //这个过程是为了维护等待队列的健康状态，移除其中的无效节点。
        do {
            node.prev = pred = pred.prev;
        } while (pred.waitStatus > 0);
        pred.next = node;
    } else {
        //如果前驱节点的等待状态不是SIGNAL（也即，它是0或PROPAGATE），
        //则将前驱节点的等待状态更新为SIGNAL。
        //这是通过compareAndSetWaitStatus方法完成的，它安全地修改节点的状态，以指示当前节点（node）在释放同步状态时需要被唤醒。
        //这里并不立即挂起当前线程，而是返回false，让调用者知道它需要再次尝试获取同步状态，在无法获取时再决定挂起。
        compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
    }
    return false;
}