如何让主线程等待所有子线程执行完毕

创建线程类 ： 设置CountDownLatch成员变量，并且在run方法末尾调用CountDownLatch实例的CountDown()方法。

调用该类： 设置所需工作的线程数量，作为构造参数生成线程实例对象，启动所有线程，调用CountDownLatch的await方法。

// 创建一个线程 并设置成员变量 CountDownLatch
package multi.thread;

import java.util.concurrent.CountDownLatch;

public class ImportThread extends Thread {
    private CountDownLatch threadsSignal;
    public ImportThread(CountDownLatch threadsSignal){
        this.threadsSignal = threadsSignal;
    }
    
    @Override
    public void run(){
        System.out.println(Thread.currentThread().getName()+" :: start...");
        // do something...
        threadsSignal.countDown(); // when the thread end the number will minus 1
        System.out.println(Thread.currentThread().getName() + "结束，还有" + threadsSignal.getCount() + " 个线程");
    }
    
}

package multi.thread;

import java.util.concurrent.CountDownLatch;

public class TestEnd {
    public static void main(String[] args) {
        int threadNum = 4;
        CountDownLatch threadSignal = new CountDownLatch(threadNum); // 初始化countDown
        for (int i = 0; i < threadNum; i++) {
            Thread t = new ImportThread(threadSignal);
            t.start();
        }
        try {
            threadSignal.await(); // 等待所有子线程完成
            System.out.println(Thread.currentThread().getName() + "结束...");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}