CountDownLatch闭锁

Java并发库提供了CountDownLatch类来实现这个功能。
CountDownLatch类是一个同步工具类，在完成某些运算时，只有其他所有的运算全部完成，当前运算才继续执行。

下面是一个案例来说明CountDownLatch：

说明：当前有 5 个线程做运算处理，主线程要计算这 5 个子线程执行所消耗的时长。那么CountDownLatch在主线程中一直等待这这 5 个线程执行完后，才执行主线程。每个子线程执行完后执行latch.countDown();也就是线程-1 的操作，直到线程数为 0 的时候，主线程才会执行，主线程执行latch.await();等待着。

public class CountDownLatchDemo {
    public static final Integer  THREAD_NUM = 5;
    public static void main(String[] args) {
        CountDownLatch countDownLatch = new CountDownLatch(THREAD_NUM);
        LatchDemo latchDemo = new LatchDemo(countDownLatch);
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < THREAD_NUM; i++) {
            new Thread(latchDemo).start();
        }
        try {
            countDownLatch.await();
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }
        System.out.println("消耗时间：" + (System.currentTimeMillis() - startTime)+ "ms");
    }
}
class LatchDemo implements Runnable {
    private CountDownLatch latch;

    public LatchDemo(CountDownLatch latch) {
        this.latch = latch;
    }

    @Override
    public void run() {
        synchronized (this){
            try {
                for (int i = 0; i < 50000; i++) {
                    if (i % 2 == 0){
                        System.out.println(i+ "是偶数");
                    }
                }
            } finally {
                latch.countDown();
            }
        }
    }
}