同步辅助类：

    CountDownLatch是一个同步辅助类，在jdk5中引入，它允许一个或多个线程等待其他线程操作完成之后才执行。    

实现原理 ：

    CountDownLatch是通过计数器的方式来实现，计数器的初始值为线程的数量。每当一个线程完成了自己的任务之后，就会对计数器减1，当计数器的值为0时，表示所有线程完成了任务，此时等待在闭锁上的线程才继续执行，从而达到等待其他线程完成任务之后才继续执行的目的。

 

CountDownLatch主要方法：

    CountDownLatch具体是通过同步器来实现的，使用AQS状态来表示计数：

    /**
     * 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();
        }

        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;
            }
        }
    }

    private final Sync sync;

　　

1、构造函数 

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

通过传入一个数值来创建一个CountDownLatch，数值表示线程可以从等待状态恢复，countDown方法必须被调用的次数

 

2、countDown方法

public void countDown() {
        sync.releaseShared(1);
    }　　

线程调用此方法对count进行减1。当count本来就为0，此方法不做任何操作，当count比0大，调用此方法进行减1，当new count为0，释放所有等待当线程。

 

3、await方法

（1）不带参数

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

调用此方法时，当count为0，直接返回true，当count比0大，线程会一直等待，直到count的值变为0，或者线程被中断（interepted，此时会抛出中断异常）。

（2）带参数

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

调用此方法时，当count为0，直接返回true，当count比0大，线程会等待一段时间，等待时间内如果count的值变为0，返回true；当超出等待时间，返回false；或者等待时间内线程被中断，此时会抛出中断异常。 

 

CountDownLatch实践

司机和工人，工人必须等到司机来了才能装货上车，司机必须得等到所有工人把货物装上车了之后才能把车开走。

工人类：

public class Worker implements Runnable {

    private String workerCode;

    private CountDownLatch startLatch;
    private CountDownLatch latch;

    Worker(CountDownLatch startLatch, CountDownLatch latch, String workerCode) {
        this.startLatch = startLatch;
        this.latch = latch;
        this.workerCode = workerCode;
    }

    public void run() {
        try {
            startLatch.await();
            doWork();
            latch.countDown();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void doWork() {
        System.out.println("Worker " + workerCode + " is loading goods...");
    }
}

司机类：

public class Driver {
    public static void main(String[] args) throws InterruptedException {
        CountDownLatch startLatch = new CountDownLatch(1);
        CountDownLatch latch = new CountDownLatch(10);
        ExecutorService executor = Executors.newFixedThreadPool(10);

        for(int i=0; i<10; i++) {
            executor.execute(new Worker(startLatch, latch, "worker" + i));
        }

        System.out.println("Driver is here.");

        startLatch.countDown();

        System.out.println("Workers get to work.");

        latch.await();

        System.out.println("Driver is ready to go.");

        executor.shutdown();
    }
}　　

运行结果：

Driver is here.
Workers get to work.
Worker worker0 is loading goods...
Worker worker1 is loading goods...
Worker worker2 is loading goods...