public Semaphore(int permits) {------permits 表示能同时有多少个线程访问我们的资源
        sync = new NonfairSync(permits); -------------默认创建的是非公平锁。
    }

abstract static class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = 1192457210091910933L;

    Sync(int permits) {
        setState(permits);-------传入的permits做i为了state的值，作为资源总数

    }

 public void acquire() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);---------每次申请一次资源
    }

public final void acquireSharedInterruptibly(int arg)
        throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();-----------------线程无效直接抛异常
    if (tryAcquireShared(arg) < 0)  --------------------拿不到资源，需要进行入队操作
        doAcquireSharedInterruptibly(arg); ---------入队操作
}

final int nonfairTryAcquireShared(int acquires) {  --------获取资源的操作
    for (;;) {
        int available = getState();    --------------拿到现有的资源
        int remaining = available - acquires; 
        if (remaining < 0 ||
            compareAndSetState(available, remaining)) -----------原子操作，多线程情况下会可能失败，所以无线循环自旋下去，直到成功；
            return remaining;---------------------如果大于等于0那么就是拿到了资源，如果小于0，那么线程就要进入等待队列
    }
}
为什么要用死循环----compareAndSetState这个是cas原子操作，失败之后要循环重复继续操作，直到成功。死循环也就结束了。

private void doAcquireSharedInterruptibly(int arg)-------------线程入队操作
    throws InterruptedException {
    final Node node = addWaiter(Node.SHARED);---------------注意这里是以共享的方式入队
    boolean failed = true;
    try {
        for (;;) {
            final Node p = node.predecessor();
            if (p == head) {    --------新入队的节点会判断他上一个节点是不是头节点，如果是头节点会再次尝试获取资源，
                int r = tryAcquireShared(arg);
                if (r >= 0) {    -------------如果获取到资源，那么这个阻塞队列就要清空了，里面没有在等待的线程了。
                    setHeadAndPropagate(node, r);
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
            }
            if (shouldParkAfterFailedAcquire(p, node) && ----------如果获取不到资源，那么就要线程阻塞了
                parkAndCheckInterrupt())       -----------parkAndCheckInterrupt这个方法会将线程阻塞（挂起），线程都阻塞了，这个死循环就不会执行了，这也就是为什么juc源码写了很多
                                                          死循环都没问题地原因，我们可以借鉴。当线程被唤醒之后又开始这个死循环，尝试拿资源（非公平锁有可能拿不到），
                                                          拿不到再次被阻塞挂起。

                throw new InterruptedException();
        }
    } finally {
        if (failed)
            cancelAcquire(node);
    }
}

private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {  ---------------判断线程能否被正常阻塞
    int ws = pred.waitStatus;
    if (ws == Node.SIGNAL)     -----------------如果上一个节点是有效的在等待的线程，那么该线程就可以插入到队列后面
        /*
         * This node has already set status asking a release
         * to signal it, so it can safely park.
         */
        return true;
    if (ws > 0) {     -----------如果上一个节点是无效的，那就查找上上个节点是不是有效的，直到找到那个有效的节点，然后将该节点插入到那个有效节点后面，中间的无效节点从链表中删除，后面的节点要找前面
                                的节点这也就说明了为什么我们地等待队列要设计成双链表，不光有next。next这种找后驱节点地操作还有pre .pre这样前驱节点。所以需要双链表。
        /*
         * Predecessor was cancelled. Skip over predecessors and
         * indicate retry.
         */
        do {
            node.prev = pred = pred.prev;
        } while (pred.waitStatus > 0);
        pred.next = node;
    } else {
        /*
         * waitStatus must be 0 or PROPAGATE.  Indicate that we
         * need a signal, but don‘t park yet.  Caller will need to
         * retry to make sure it cannot acquire before parking.
         */
        compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
    }
    return false;
}

protected final boolean tryReleaseShared(int releases) {
            for (;;) {
                int current = getState();
                int next = current + releases;    -----------获取当前的资源然后给资源加回去
                if (next < current) // overflow
                    throw new Error("Maximum permit count exceeded");
                if (compareAndSetState(current, next))  -----------------CAS算法还资源，死循环，直到成功还回去，死循环结束。
                    return true;
            }
        }

资源还回去之后执行doReleaseShared方法唤醒其他线程抢资源

private void doReleaseShared() {
    /*
     * Ensure that a release propagates, even if there are other
     * in-progress acquires/releases.  This proceeds in the usual
     * way of trying to unparkSuccessor of head if it needs
     * signal. But if it does not, status is set to PROPAGATE to
     * ensure that upon release, propagation continues.
     * Additionally, we must loop in case a new node is added
     * while we are doing this. Also, unlike other uses of
     * unparkSuccessor, we need to know if CAS to reset status
     * fails, if so rechecking.
     */
    for (;;) {
        Node h = head;
        if (h != null && h != tail) {   --------发现阻塞队列有阻塞线程
            int ws = h.waitStatus;
            if (ws == Node.SIGNAL) {
                if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                    continue;            // loop to recheck cases
                unparkSuccessor(h);    ---------跳过头节点，唤醒下一个节点
            }
            else if (ws == 0 &&     
                     !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                continue;                // loop on failed CAS
        }
        if (h == head)                   // loop if head changed
            break;
    }
}

private void unparkSuccessor(Node node) {
    /*
     * If status is negative (i.e., possibly needing signal) try
     * to clear in anticipation of signalling.  It is OK if this
     * fails or if status is changed by waiting thread.
     */
    int ws = node.waitStatus;
    if (ws < 0)
        compareAndSetWaitStatus(node, ws, 0);

    /*
     * Thread to unpark is held in successor, which is normally
     * just the next node.  But if cancelled or apparently null,
     * traverse backwards from tail to find the actual
     * non-cancelled successor.
     */
    Node s = node.next;
    if (s == null || s.waitStatus > 0) {
        s = null;
        for (Node t = tail; t != null && t != node; t = t.prev)
            if (t.waitStatus <= 0)
                s = t;             ------循环找到waitStatus<0能唤醒的节点调用unpark方法唤醒线程。

    }
    if (s != null)
        LockSupport.unpark(s.thread);
}