并发：CountDownLatch的使用

CountDownLatch：一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。与CyclicBarrier非常类似。但CountDownLatch的计数器只能使用一次，而CyclicBarrier可以循环使用。
主要方法
public CountDownLatch(int count);
public void countDown();
public void await() throws InterruptedException
示例（来自《Think In Java》，稍有改动）：注意latch.await()的位置

class TaskPortion implements Runnable{
	private static int counter = 0;
	private final int id = counter++;
	private static Random rand = new Random(47);
	private final CountDownLatch latch;
	
	TaskPortion(CountDownLatch latch){
		this.latch = latch;
	}
	@Override
	public void run() {
		try {
			doWork();
			latch.countDown();
//			latch.await();不要写在这儿，这样不能体现 CountDownLatch 的作用
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
	public void doWork() throws InterruptedException{
		TimeUnit.MILLISECONDS.sleep(rand.nextInt(2000));
		System.out.println(this + "completed!");
	}
	public String toString(){
		return String.format("%1$-3d", id);
	}
}

public class CountDownLatchDemo {
	static final int SIZE = 13;
	public static void main(String[] args) throws InterruptedException{
		ExecutorService exec = Executors.newCachedThreadPool();
		CountDownLatch latch = new CountDownLatch(SIZE);
		for(int i = 0; i < SIZE; i++){
			exec.execute(new TaskPortion(latch));
		}
		latch.await();//写这儿比较合适
		System.out.println("Launched all tasks");
		exec.shutdown();
	}
}

输出：
11 completed!
7  completed!
9  completed!
10 completed!
5  completed!
8  completed!
12 completed!
1  completed!
2  completed!
6  completed!
4  completed!
0  completed!
3  completed!
Launched all tasks