/**
 * Synchronization control For CountDownLatch.
 * Uses AQS state to represent count.
 */
private static final class Sync extends AbstractQueuedSynchronizer {
    private static final long serialVersionUID = 4982264981922014374L;

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

    int getCount() {
        return getState();
    }

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

    // 2
    public 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;
        }
    }
}

private final Sync sync;

   /**
    * Constructs a {@code CountDownLatch} initialized with the given count.
    *
    * @param count the number of times {@link #countDown} must be invoked
    *        before threads can pass through {@link #await}
    * @throws IllegalArgumentException if {@code count} is negative
    */
   public CountDownLatch(int count) {
       if (count < 0) throw new IllegalArgumentException("count < 0");
       this.sync = new Sync(count);
   }

   /**
    * Causes the current thread to wait until the latch has counted down to
    * zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
    *
    * <p>If the current count is zero then this method returns immediately.
    *
    * <p>If the current count is greater than zero then the current
    * thread becomes disabled for thread scheduling purposes and lies
    * dormant until one of two things happen:
    * <ul>
    * <li>The count reaches zero due to invocations of the
    * {@link #countDown} method; or
    * <li>Some other thread {@linkplain Thread#interrupt interrupts}
    * the current thread.
    * </ul>
    *
    * <p>If the current thread:
    * <ul>
    * <li>has its interrupted status set on entry to this method; or
    * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
    * </ul>
    * then {@link InterruptedException} is thrown and the current thread's
    * interrupted status is cleared.
    *
    * @throws InterruptedException if the current thread is interrupted
    *         while waiting
    */
   // 1
   public void await() throws InterruptedException {
       sync.acquireSharedInterruptibly(1);
   }

   /**
    * Causes the current thread to wait until the latch has counted down to
    * zero, unless the thread is {@linkplain Thread#interrupt interrupted},
    * or the specified waiting time elapses.
    *
    * <p>If the current count is zero then this method returns immediately
    * with the value {@code true}.
    *
    * <p>If the current count is greater than zero then the current
    * thread becomes disabled for thread scheduling purposes and lies
    * dormant until one of three things happen:
    * <ul>
    * <li>The count reaches zero due to invocations of the
    * {@link #countDown} method; or
    * <li>Some other thread {@linkplain Thread#interrupt interrupts}
    * the current thread; or
    * <li>The specified waiting time elapses.
    * </ul>
    *
    * <p>If the count reaches zero then the method returns with the
    * value {@code true}.
    *
    * <p>If the current thread:
    * <ul>
    * <li>has its interrupted status set on entry to this method; or
    * <li>is {@linkplain Thread#interrupt interrupted} while waiting,
    * </ul>
    * then {@link InterruptedException} is thrown and the current thread's
    * interrupted status is cleared.
    *
    * <p>If the specified waiting time elapses then the value {@code false}
    * is returned.  If the time is less than or equal to zero, the method
    * will not wait at all.
    *
    * @param timeout the maximum time to wait
    * @param unit the time unit of the {@code timeout} argument
    * @return {@code true} if the count reached zero and {@code false}
    *         if the waiting time elapsed before the count reached zero
    * @throws InterruptedException if the current thread is interrupted
    *         while waiting
    */
   // 2
   public boolean await(long timeout, TimeUnit unit)
       throws InterruptedException {
       return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
   }

   /**
    * Decrements the count of the latch, releasing all waiting threads if
    * the count reaches zero.
    *
    * <p>If the current count is greater than zero then it is decremented.
    * If the new count is zero then all waiting threads are re-enabled for
    * thread scheduling purposes.
    *
    * <p>If the current count equals zero then nothing happens.
    */
   // 3
   public void countDown() {
       sync.releaseShared(1);
   }

   /**
    * Returns the current count.
    *
    * <p>This method is typically used for debugging and testing purposes.
    *
    * @return the current count
    */
   public long getCount() {
       return sync.getCount();
   }

   /**
    * Returns a string identifying this latch, as well as its state.
    * The state, in brackets, includes the String {@code "Count ="}
    * followed by the current count.
    *
    * @return a string identifying this latch, as well as its state
    */
   public String toString() {
       return super.toString() + "[Count = " + sync.getCount() + "]";
   }