CountDownLatch概念

CountDownLatch又被稱為倒計(jì)時(shí)鎖，它是一個(gè)同步輔助類，它允許一個(gè)或多個(gè)線程一直等待直到其他線程執(zhí)行完畢才開(kāi)始執(zhí)行。用給定的計(jì)數(shù)初始化CountDownLatch，其含義是要被等待執(zhí)行完的線程個(gè)數(shù)。每次調(diào)用CountDown()，計(jì)數(shù)減1。
主程序執(zhí)行到await()函數(shù)會(huì)阻塞等待線程的執(zhí)行，直到計(jì)數(shù)為0
我們來(lái)看下它的方法。

public CountDownLatch(int count); //指定計(jì)數(shù)的次數(shù)，只能被設(shè)置1次

public void countDown();          //調(diào)用此方法則計(jì)數(shù)減1

public void await() throws InterruptedException   //調(diào)用此方法會(huì)一直阻塞當(dāng)前線程，直到計(jì)時(shí)器的值為0，除非線程被中斷。

Public Long getCount();           //得到當(dāng)前的計(jì)數(shù)

Public boolean await(long timeout, TimeUnit unit) //調(diào)用此方法會(huì)一直阻塞當(dāng)前線程，直到計(jì)時(shí)器的值為0，除非線程被中斷或者計(jì)數(shù)器超時(shí)，返回false代表計(jì)數(shù)器超時(shí)。

CountDownLatch實(shí)例

模擬10人賽跑。10人跑完后才喊"Game Over."

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
public class CountDownLatchTest {
    private static final int RUNNER_COUNT = 10;
    public static void main(String[] args) throws InterruptedException {
        final CountDownLatch begin = new CountDownLatch(1);
        final CountDownLatch end = new CountDownLatch(10);
        final ExecutorService exec = Executors.newFixedThreadPool(10);
        for (int i = 0; i < RUNNER_COUNT; i++) {
            final int NO = i + 1;
            //從1號(hào)到10號(hào)線程都在這里被建立
            Runnable run = new Runnable() {
                @Override
                public void run() {
                    try {
                        //所有線程阻塞在這里，等待槍響
                        begin.await();
                        //每個(gè)人跑完的時(shí)間
                        Thread.sleep((long)(Math.random() * 10000));
                        System.out.println("No." + NO + " arrived");
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    } finally {
                        //每跑完一個(gè)人，倒計(jì)時(shí)鎖減一
                        end.countDown();//（倒計(jì)時(shí)鎖被減一）
                    }
                }
            };
            exec.submit(run);
        }

        System.out.println("Game Start ...");
        //槍響
        begin.countDown();//倒計(jì)時(shí)鎖被解除減一（begin解除）
        //等待所有人跑完
        end.await();//倒計(jì)時(shí)鎖沒(méi)有被解除之前，將被阻塞在這里
        end.await(30, TimeUnit.SECONDS);
        System.out.println("Game Over.");

        exec.shutdown();
    }

}

輸出

Game Start ...
No.6 arrived
No.4 arrived
No.10 arrived
No.3 arrived
No.9 arrived
No.5 arrived
No.8 arrived
No.7 arrived
No.1 arrived
No.2 arrived
Game Over.

CountDownLatch與join的區(qū)別

調(diào)用join方法需要等待thread執(zhí)行完畢才能繼續(xù)向下執(zhí)行,而CountDownLatch只需要檢查計(jì)數(shù)器的值為零就可以繼續(xù)向下執(zhí)行，相比之下，CountDownLatch更加靈活一些，可以實(shí)現(xiàn)一些更加復(fù)雜的業(yè)務(wù)場(chǎng)景。

面試題

（1）解釋一下CountDownLatch概念?
（2）CountDownLatch 和CyclicBarrier的不同之處?
（3）給出一些CountDownLatch使用的例子?
（4） CountDownLatch 類中主要的方法?