多线程 闭锁CountDownLatch

　　在下面的代码中，由于多线程并行的原因，可能主线程的耗时计算早就执行完了，而上面的线程还在打印，因此不能正确计算耗时时间。

public class JavaDemo {
    public static void main(String[] args) throws ParseException {

        long begin = System.currentTimeMillis();

        LatchDemo ld = new LatchDemo();

        for (int i = 0; i < 5; i++) {
            new Thread(ld).start();
        }

        long end = System.currentTimeMillis();

        System.out.println("耗时：" + (end - begin));

    }
}

class LatchDemo implements Runnable {

    @Override
    public void run() {

        for (int i = 0; i < 50000; i++) {
            System.out.println(i);
        }

    }
}

　　引入闭锁机制，在指定个数的线程没执行完之前，不会继续执行后面的代码

public class JavaDemo {
    public static void main(String[] args) throws ParseException {

        CountDownLatch latch = new CountDownLatch(5);
        LatchDemo ld = new LatchDemo(latch);

        long begin = System.currentTimeMillis();
        for (int i = 0; i < 5; i++) {
            new Thread(ld).start();
        }

        //countdown为0之前将一直等待
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        long end = System.currentTimeMillis();

        System.out.println("耗时：" + (end - begin));

    }
}

class LatchDemo implements Runnable {

    private CountDownLatch latch;

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

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

　　正确计算耗时，控制台打印

......
49997
49998
49999
耗时：1262

Process finished with exit code 0