CountDownLatch源码解析之await()

end.await(); 
  ↓
public void await() throws InterruptedException {
  sync.acquireSharedInterruptibly(1);
}

private static final class Sync extends AbstractQueuedSynchronizer {
　　...
}

sync.acquireSharedInterruptibly(1); 
  ↓
public final void acquireSharedInterruptibly(int arg) //AbstractQueuedSynchronizer
      throws InterruptedException {
  if (Thread.interrupted())
    throw new InterruptedException();
  if (tryAcquireShared(arg) < 0)
    doAcquireSharedInterruptibly(arg);
}

protected int tryAcquireShared(int acquires) {
  return (getState() == 0) ? 1 : -1;
}

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

CountDownLatch end = new CountDownLatch(2);
  ↓
public CountDownLatch(int count) {
  if (count < 0) throw new IllegalArgumentException("count < 0");
  this.sync = new Sync(count);
}
  ↓
Sync(int count) {
  setState(count);
}

doAcquireSharedInterruptibly(arg);
  ↓
private void doAcquireSharedInterruptibly(int arg)
    throws InterruptedException {
    final Node node = addWaiter(Node.SHARED);
    boolean failed = true;
    try {
      for (;;) {
        final Node p = node.predecessor();
        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) &&
          parkAndCheckInterrupt())
          throw new InterruptedException();
      }
    } finally {
      if (failed)
        cancelAcquire(node);
    }
  }

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

addWaiter(Node.SHARED) //Node.SHARED 表示该结点处于共享模式
  ↓
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; // private transient volatile Node tail;
  if (pred != null) {
    node.prev = pred;
    if (compareAndSetTail(pred, node)) {
      pred.next = node;
      return node;
    }
  }
  enq(node);
  return node;
}

enq(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;
      }
    }
  }
}

compareAndSetHead(new Node())
  ↓
/**
 * CAS head field. Used only by enq.
 */
private final boolean compareAndSetHead(Node update) {
  return unsafe.compareAndSwapObject(this, headOffset, null, update);
}

      +------+ prev   +----------------+
head(tail) |   | <---- node | currentThread |
      +------+      +----------------+
      
          ↓
          
    +------+ prev      +----------------+
head  |   | <---- node(tail) | currentThread |
    +------+         +----------------+

for (;;) {
  final Node p = node.predecessor();
  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) &&
    parkAndCheckInterrupt())
    throw new InterruptedException();
}

shouldParkAfterFailedAcquire(p, node)
  ↓
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
  int ws = pred.waitStatus;
  if (ws == Node.SIGNAL) //static final int SIGNAL  = -1;
    /*
     * This node has already set status asking a release
     * to signal it, so it can safely park.
     */
    return true;
  if (ws > 0) {
    /*
     * 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;
}
  ↓
/**
 * CAS waitStatus field of a node.
 */
private static final boolean compareAndSetWaitStatus(Node node,
                           int expect,
                           int update) {
  return unsafe.compareAndSwapInt(node, waitStatusOffset,
                  expect, update);
}

    +--------------------------+  prev      +------------------+
head  | waitStatus = Node.SIGNAL | <---- node(tail) | currentThread  |
    +--------------------------+         +------------------+

 private final boolean parkAndCheckInterrupt() {
    LockSupport.park(this);
    return Thread.interrupted();
  }