java多線程對CountDownLatch的使用實例

介紹

CountDownLatch是一個同步輔助類，它允許一個或多個線程一直等待直到其他線程執行完畢才開始執行。

用給定的計數初始化CountDownLatch，其含義是要被等待執行完的線程個數。

每次調用CountDown()，計數減1

主程序執行到await()函數會阻塞等待線程的執行，直到計數為0

實現原理

計數器通過使用鎖（共享鎖、排它鎖）實現

實例1

場景：模擬10人賽跑。10人跑完后才喊"Game Over."

package com.jihite;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

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(RUNNER_COUNT);
        final ExecutorService exec = Executors.newFixedThreadPool(10);

        for (int i = 0; i < RUNNER_COUNT; i++) {
            final int NO = i + 1;
            Runnable run = new Runnable() {
                @Override
                public void run() {
                    try {
                        begin.await();
                        Thread.sleep((long)(Math.random() * 10000));
                        System.out.println("No." + NO + " arrived");
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    } finally {
                        end.countDown();
                    }
                }
            };
            exec.submit(run);
        }

        System.out.println("Game Start ...");
        begin.countDown();
        end.await();
//        end.await(30, TimeUnit.SECONDS);
        System.out.println("Game Over.");

        exec.shutdown();
    }
}

分析：代碼中定義了2個計數器，個數分別為1和10。

如果不執行begin.countDown()，進程會一致阻塞在begin.await()

主進程執行到end.awit()阻塞等待end計數器清0，進程中每執行一次CountDown()減1，所有執行完后主進程繼續往下執行

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.

注：countDown()一定要執行到（考慮異常及線程與開始計數設置不一致），否則會一直卡在await()（可以設置時間，超過一定時間就不等了）

實例2（和join的相似處）

場景：流水線上有3個worker: worker1、worker2、worker3,只有當worker1和worker2執行完時才可以執行worker3

WorkerCount.java

package com.jihite;

import java.util.concurrent.CountDownLatch;

public class WorkerCount extends Thread {
    private String name;
    private long time;
    private CountDownLatch countDownLatch;

    public WorkerCount(String name, long time, CountDownLatch countDownLatch) {
        this.name = name;
        this.time = time;
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        try {
            System.out.println(name + "開始工作");
            Thread.sleep(time);
            System.out.println(name + "工作完成, 耗時："+ time);
            countDownLatch.countDown();
            System.out.println("countDownLatch.getCount():" + countDownLatch.getCount());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

CountDownLatch實現：

    @Test
    public void CountDownLatchTest() throws InterruptedException {
        int COUNT = 2;
        final CountDownLatch countDownLatch = new CountDownLatch(COUNT);
        WorkerCount worker0 = new WorkerCount("lilei-0", (long)(Math.random() * 10000), countDownLatch);
        WorkerCount worker1 = new WorkerCount("lilei-1", (long)(Math.random() * 10000), countDownLatch);
        worker0.start();
        worker1.start();
        countDownLatch.await();
        System.out.println("准備工作就緒");

        WorkerCount worker2 = new WorkerCount("lilei-2", (long)(Math.random() * 10000), countDownLatch);
        worker2.start();
        Thread.sleep(10000);
    }

輸出：

lilei-0開始工作
lilei-1開始工作
lilei-1工作完成, 耗時：4039
countDownLatch.getCount():1
lilei-0工作完成, 耗時：9933
countDownLatch.getCount():0
准備工作就緒
lilei-2開始工作
lilei-2工作完成, 耗時：6402
countDownLatch.getCount():0

 

該場景join也可以完成

Worker.java

package com.jihite;
public class Worker extends Thread{
    private String name;
    private long time;

    public Worker(String name, long time) {
        this.name = name;
        this.time = time;
    }

    @Override
    public void run() {
        try {
            System.out.println(name + "開始工作");
            Thread.sleep(time);
            System.out.println(name + "工作完成, 耗時："+ time);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

join實現

 @Test
    public void JoinTest() throws InterruptedException {
        Worker worker0 = new Worker("lilei-0", (long)(Math.random() * 10000));
        Worker worker1 = new Worker("lilei-1", (long)(Math.random() * 10000));
        Worker worker2 = new Worker("lilei-2", (long)(Math.random() * 10000));
        worker0.start();
        worker1.start();

        worker0.join();
        worker1.join();
        System.out.println("准備工作就緒");

        worker2.start();
        Thread.sleep(10000);
    }

輸出

lilei-0開始工作
lilei-1開始工作
lilei-1工作完成, 耗時：4483
lilei-0工作完成, 耗時：6301
准備工作就緒
lilei-2開始工作
lilei-2工作完成, 耗時：6126

既然這樣，那CountDownLatch和join的區別在哪？通過下面的場景三就可以看出

實例3（和join的不同處）

場景：流水線上有3個worker: worker1、worker2、worker3,只有當worker1和worker2兩者的階段一都執行完后才可以執行worker3

WorkerCount2.java

package com.jihite;

import java.util.concurrent.CountDownLatch;

public class WorkerCount2 extends Thread {
    private String name;
    private long time;
    private CountDownLatch countDownLatch;

    public WorkerCount2(String name, long time, CountDownLatch countDownLatch) {
        this.name = name;
        this.time = time;
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        try {
            System.out.println(name + "開始階段1工作");
            Thread.sleep(time);
            System.out.println(name + "階段1完成, 耗時："+ time);
            countDownLatch.countDown();

            System.out.println(name + "開始階段2工作");
            Thread.sleep(time);
            System.out.println(name + "階段2完成, 耗時："+ time);

        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

此時用join無法實現，只能用CountDownLatch

 @Test
    public void CountDownLatchTest2() throws InterruptedException {
        int COUNT = 2;
        final CountDownLatch countDownLatch = new CountDownLatch(COUNT);
        WorkerCount2 worker0 = new WorkerCount2("lilei-0", (long)(Math.random() * 10000), countDownLatch);
        WorkerCount2 worker1 = new WorkerCount2("lilei-1", (long)(Math.random() * 10000), countDownLatch);
        worker0.start();
        worker1.start();
        countDownLatch.await();
        System.out.println("准備工作就緒");

        WorkerCount2 worker2 = new WorkerCount2("lilei-2", (long)(Math.random() * 10000), countDownLatch);
        worker2.start();
        Thread.sleep(10000);
    }

輸出

lilei-0開始階段1工作
lilei-1開始階段1工作
lilei-0階段1完成, 耗時：3938
lilei-0開始階段2工作
lilei-1階段1完成, 耗時：6259
lilei-1開始階段2工作
准備工作就緒
lilei-2開始階段1工作
lilei-0階段2完成, 耗時：3938
lilei-1階段2完成, 耗時：6259
lilei-2階段1完成, 耗時：7775
lilei-2開始階段2工作