AbstractQueuedSynchronizer 源码分析（共享锁）

class Driver2 { // ...
  void main() throws InterruptedException {
    CountDownLatch doneSignal = new CountDownLatch(N);
    Executor e = ...

    for (int i = 0; i < N; ++i) // create and start threads
      e.execute(new WorkerRunnable(doneSignal, i));

    doneSignal.await();           // wait for all to finish
  }
}

class WorkerRunnable implements Runnable {
  private final CountDownLatch doneSignal;
  private final int i;
  WorkerRunnable(CountDownLatch doneSignal, int i) {
    this.doneSignal = doneSignal;
    this.i = i;
  }
  public void run() {
    try {
      doWork(i);
      doneSignal.countDown();
    } catch (InterruptedException ex) {} // return;
  }

  void doWork() { ... }
}

private static final class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = 4982264981922014374L;

    Sync(int count) {
        setState(count);
    }

    int getCount() {
        return getState();
    }
    /**
      * 返回值>=0 表示获取锁成功，
      * >0 表示需要向后传播 =0不向后传播
      * <0 表示获取锁失败
     */
    protected int tryAcquireShared(int acquires) {
        return (getState() == 0) ? 1 : -1;
    }

    protected boolean tryReleaseShared(int releases) {
        // Decrement count; signal when transition to zero
        for (;;) {
            int c = getState();
            if (c == 0)
                return false;
            int nextc = c-1;
            if (compareAndSetState(c, nextc))
                return nextc == 0;
        }
    }
}

public void await() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}
public void countDown() {
    sync.releaseShared(1);
}
public boolean await(long timeout, TimeUnit unit)
    throws InterruptedException {
    return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}

public final boolean releaseShared(int arg) {
    if (tryReleaseShared(arg)) {    //1
        doReleaseShared();
        return true;
    }
    return false;
}
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) {
            int ws = h.waitStatus;
            if (ws == Node.SIGNAL) {
                if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))   //2
                    continue;            // loop to recheck cases
                unparkSuccessor(h);
            }
            else if (ws == 0 &&
                     !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))   //3
                continue;                // loop on failed CAS
        }
        if (h == head)                   // loop if head changed   //4
            break;
    }
}

public final void acquireSharedInterruptibly(int arg)
        throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    if (tryAcquireShared(arg) < 0)               // 1
        doAcquireSharedInterruptibly(arg);
}
private void doAcquireSharedInterruptibly(int arg)
    throws InterruptedException {
    final Node node = addWaiter(Node.SHARED);     // 2
    boolean failed = true;
    try {
        for (;;) {
            final Node p = node.predecessor();  // 3
            if (p == head) {
                int r = tryAcquireShared(arg);
                if (r >= 0) {
                    setHeadAndPropagate(node, r);   
                    p.next = null; // help GC
                    failed = false;
                    return;
                }
            }
            if (shouldParkAfterFailedAcquire(p, node) &&   // 4
                parkAndCheckInterrupt())                   //5
                throw new InterruptedException();
        }
    } finally {
        if (failed)                          // 6
            cancelAcquire(node);
    }
}

private Node addWaiter(Node mode) {
    Node node = new Node(Thread.currentThread(), mode);
    // Try the fast path of enq; backup to full enq on failure
    Node pred = tail;
    if (pred != null) {
        node.prev = pred;
        if (compareAndSetTail(pred, node)) {
            pred.next = node;
            return node;
        }
    }
    enq(node);
    return node;
}
private Node enq(final Node node) {
    for (;;) {
        Node t = tail;
        if (t == null) { // Must initialize
            if (compareAndSetHead(new Node()))
                tail = head;
        } else {
            node.prev = t;
            if (compareAndSetTail(t, node)) {
                t.next = node;
                return t;
            }
        }
    }
}

private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
    int ws = pred.waitStatus;
    if (ws == Node.SIGNAL)      //1
        /*
         * This node has already set status asking a release
         * to signal it, so it can safely park.
         */
        return true;
    if (ws > 0) {              //2
        /*
         * Predecessor was cancelled. Skip over predecessors and
         * indicate retry.
         */
        do {                    
            node.prev = pred = pred.prev;
        } while (pred.waitStatus > 0);     
        pred.next = node;
    } else {                   //3
        /*
         * 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;
}

private void setHeadAndPropagate(Node node, int propagate) {
    Node h = head; // Record old head for check below
    setHead(node);
    /*
     * Try to signal next queued node if:
     *   Propagation was indicated by caller,
     *     or was recorded (as h.waitStatus either before
     *     or after setHead) by a previous operation
     *     (note: this uses sign-check of waitStatus because
     *      PROPAGATE status may transition to SIGNAL.)
     * and
     *   The next node is waiting in shared mode,
     *     or we don't know, because it appears null
     *
     * The conservatism in both of these checks may cause
     * unnecessary wake-ups, but only when there are multiple
     * racing acquires/releases, so most need signals now or soon
     * anyway.
     */
    if (propagate > 0 || h == null || h.waitStatus < 0 ||
        (h = head) == null || h.waitStatus < 0) {
        Node s = node.next;
        if (s == null || s.isShared())
            doReleaseShared();
    }
}
//头结点对应的线程已经获得了锁，
//相当于于出队，这个节点已经不再竞争锁了
//再竞争锁会再加入到队列中
private void setHead(Node node) {
    head = node;
    node.thread = null;
    node.prev = null;
}