多線程（七、同步計數器-CountDownLatch）

CountDownLatch是一個線程的同步計數器，主要有2個使用場景。

1、作為一個開關，N個子線程準備就緒，等待主線程的放行，然后子線程一起開始執行。

Task

package com.jane;

import java.util.concurrent.CountDownLatch;

public class Task implements Runnable {
    private CountDownLatch countDownLatch;
    private String name;

    public Task(CountDownLatch countDownLatch, String name) {
        this.countDownLatch = countDownLatch;
        this.name = name;
    }

    @Override
    public void run() {
        try {
            System.out.println(name + "開始等待開關......");
            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(name + "執行任務完成");
    }
}

主線程

public class Main {

    public static void main(String[] args) throws InterruptedException {
        //主線程創建，和子線程共享
        CountDownLatch countDownLatch = new CountDownLatch(1);

        System.out.println("主線程啟動......");

        //創建3個子線程
        Thread t1 = new Thread(new Task(countDownLatch, "線程-1"));
        Thread t2 = new Thread(new Task(countDownLatch, "線程-2"));
        Thread t3 = new Thread(new Task(countDownLatch, "線程-3"));

        t1.start();
        t2.start();
        t3.start();

        Thread.sleep(2000);
        System.out.println("主線程執行任務完畢，放行......");
        countDownLatch.countDown();
        System.out.println("主線程執行完畢");

    }
}

執行結果

2、主線程等待N個子線程的處理結果，子線程都完成后，主線程繼續執行。

Task

package com.jane;

import java.util.concurrent.CountDownLatch;

public class Task1 implements Runnable {
    private CountDownLatch countDownLatch;
    private String name;

    public Task1(CountDownLatch countDownLatch, String name) {
        this.countDownLatch = countDownLatch;
        this.name = name;
    }

    @Override
    public void run() {
        try {
            System.out.println(name + "開始開始執行任務......");
            Thread.sleep(2000);
            System.out.println(name + "執行任務完成");
            countDownLatch.countDown();
        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}

主線程

package com.jane;

import java.util.concurrent.CountDownLatch;

public class Main1 {

    public static void main(String[] args) throws InterruptedException {
        //主線程創建，和子線程共享
        CountDownLatch countDownLatch = new CountDownLatch(3);

        System.out.println("主線程啟動......");

        //創建3個子線程
        Thread t1 = new Thread(new Task1(countDownLatch, "線程-1"));
        Thread t2 = new Thread(new Task1(countDownLatch, "線程-2"));
        Thread t3 = new Thread(new Task1(countDownLatch, "線程-3"));

        t1.start();
        t2.start();
        t3.start();

        System.out.println("主線程等待子線程執行結果......");
        countDownLatch.await();
        System.out.println("主線程執行完畢");

    }
}

執行結果

注意：CountDownLatch的計數只能使用一次，不會自動重新開始。