1.背景：

• countDownLatch是在java1.5被引入，跟它一起被引入的工具類還有CyclicBarrier、Semaphore、concurrentHashMap和BlockingQueue。
• 存在于java.util.cucurrent包下。

2.概念

• countDownLatch這個(gè)類使一個(gè)線程等待其他線程各自執(zhí)行完畢后再執(zhí)行。
• 是通過(guò)一個(gè)計(jì)數(shù)器來(lái)實(shí)現(xiàn)的，計(jì)數(shù)器的初始值是線程的數(shù)量。每當(dāng)一個(gè)線程執(zhí)行完畢后，計(jì)數(shù)器的值就-1，當(dāng)計(jì)數(shù)器的值為0時(shí)，表示所有線程都執(zhí)行完畢，然后在閉鎖上等待的線程就可以恢復(fù)工作了。

3.源碼

• countDownLatch類中只提供了一個(gè)構(gòu)造器：

//參數(shù)count為計(jì)數(shù)值
public CountDownLatch(int count) {  };  

• 類中有三個(gè)方法是最重要的：

//調(diào)用await()方法的線程會(huì)被掛起，它會(huì)等待直到count值為0才繼續(xù)執(zhí)行
public void await() throws InterruptedException { };   
//和await()類似，只不過(guò)等待一定的時(shí)間后count值還沒(méi)變?yōu)?的話就會(huì)繼續(xù)執(zhí)行
public boolean await(long timeout, TimeUnit unit) throws InterruptedException { };  
//將count值減1
public void countDown() { };  

4.示例

普通示例：

public class CountDownLatchTest {

    public static void main(String[] args) {
        final CountDownLatch latch = new CountDownLatch(2);
        System.out.println("主線程開(kāi)始執(zhí)行…… ……");
        //第一個(gè)子線程執(zhí)行
        ExecutorService es1 = Executors.newSingleThreadExecutor();
        es1.execute(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                    System.out.println("子線程："+Thread.currentThread().getName()+"執(zhí)行");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                latch.countDown();
            }
        });
        es1.shutdown();

        //第二個(gè)子線程執(zhí)行
        ExecutorService es2 = Executors.newSingleThreadExecutor();
        es2.execute(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(3000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("子線程："+Thread.currentThread().getName()+"執(zhí)行");
                latch.countDown();
            }
        });
        es2.shutdown();
        System.out.println("等待兩個(gè)線程執(zhí)行完畢…… ……");
        try {
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("兩個(gè)子線程都執(zhí)行完畢，繼續(xù)執(zhí)行主線程");
    }
}

結(jié)果集：

主線程開(kāi)始執(zhí)行…… ……
等待兩個(gè)線程執(zhí)行完畢…… ……
子線程：pool-1-thread-1執(zhí)行
子線程：pool-2-thread-1執(zhí)行
兩個(gè)子線程都執(zhí)行完畢，繼續(xù)執(zhí)行主線程

模擬并發(fā)示例：

public class Parallellimit {
    public static void main(String[] args) {
        ExecutorService pool = Executors.newCachedThreadPool();
        CountDownLatch cdl = new CountDownLatch(100);
        for (int i = 0; i < 100; i++) {
            CountRunnable runnable = new CountRunnable(cdl);
            pool.execute(runnable);
        }
    }
}

 class CountRunnable implements Runnable {
    private CountDownLatch countDownLatch;
    public CountRunnable(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }
    @Override
    public void run() {
        try {
            synchronized (countDownLatch) {
                /*** 每次減少一個(gè)容量*/
                countDownLatch.countDown();
                System.out.println("thread counts = " + (countDownLatch.getCount()));
            }
            countDownLatch.await();
            System.out.println("concurrency counts = " + (100 - countDownLatch.getCount()));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

*CountDownLatch和CyclicBarrier區(qū)別：
1.countDownLatch是一個(gè)計(jì)數(shù)器，線程完成一個(gè)記錄一個(gè)，計(jì)數(shù)器遞減，只能只用一次
2.CyclicBarrier的計(jì)數(shù)器更像一個(gè)閥門(mén)，需要所有線程都到達(dá)，然后繼續(xù)執(zhí)行，計(jì)數(shù)器遞增，提供reset功能，可以多次使用