详解如何判断Java线程池任务已执行完

private static void isCompletedByTaskCount(ThreadPoolExecutor threadPool) {
    while (threadPool.getTaskCount() != threadPool.getCompletedTaskCount()) {
    }
}

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.FutureTask;
/**
 * 使用 FutrueTask 等待线程池执行完全部任务
 */
public class FutureTaskDemo {
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        // 创建一个固定大小的线程池
        ExecutorService executor = Executors.newFixedThreadPool(3);
        // 创建任务
        FutureTask<Integer> task1 = new FutureTask<>(() -> {
            System.out.println("Task 1 start");
            Thread.sleep(2000);
            System.out.println("Task 1 end");
            return 1;
        });
        FutureTask<Integer> task2 = new FutureTask<>(() -> {
            System.out.println("Task 2 start");
            Thread.sleep(3000);
            System.out.println("Task 2 end");
            return 2;
        });
        FutureTask<Integer> task3 = new FutureTask<>(() -> {
            System.out.println("Task 3 start");
            Thread.sleep(1500);
            System.out.println("Task 3 end");
            return 3;
        });
        // 提交三个任务给线程池
        executor.submit(task1);
        executor.submit(task2);
        executor.submit(task3);
        // 等待所有任务执行完毕并获取结果
        int result1 = task1.get();
        int result2 = task2.get();
        int result3 = task3.get();
        System.out.println("Do main thread.");
    }
}

public static void main(String[] args) throws InterruptedException {
    // 创建线程池
    ThreadPoolExecutor threadPool = new ThreadPoolExecutor(10, 20,
     0, TimeUnit.SECONDS, new LinkedBlockingDeque<>(1024));
    final int taskCount = 5;    // 任务总数
    // 单次计数器
    CountDownLatch countDownLatch = new CountDownLatch(taskCount); // ①
    // 添加任务
    for (int i = 0; i < taskCount; i++) {
        final int finalI = i;
        threadPool.submit(new Runnable() {
            @Override
            public void run() {
                try {
                    // 随机休眠 0-4s
                    int sleepTime = new Random().nextInt(5);
                    TimeUnit.SECONDS.sleep(sleepTime);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(String.format("任务%d执行完成", finalI));
                // 线程执行完，计数器 -1
                countDownLatch.countDown();  // ②
            }
        });
    }
    // 阻塞等待线程池任务执行完
    countDownLatch.await();  // ③
    // 线程池执行完
    System.out.println();
    System.out.println("线程池任务执行完成！");
}

public static void main(String[] args) throws InterruptedException {
    // 创建线程池
    ThreadPoolExecutor threadPool = new ThreadPoolExecutor(10, 20,
     0, TimeUnit.SECONDS, new LinkedBlockingDeque<>(1024));
    final int taskCount = 5;    // 任务总数
    // 循环计数器 ①
    CyclicBarrier cyclicBarrier = new CyclicBarrier(taskCount, new Runnable() {
        @Override
        public void run() {
            // 线程池执行完
            System.out.println();
            System.out.println("线程池所有任务已执行完！");
        }
    });
    // 添加任务
    for (int i = 0; i < taskCount; i++) {
        final int finalI = i;
        threadPool.submit(new Runnable() {
            @Override
            public void run() {
                try {
                    // 随机休眠 0-4s
                    int sleepTime = new Random().nextInt(5);
                    TimeUnit.SECONDS.sleep(sleepTime);
                    System.out.println(String.format("任务%d执行完成", finalI));
                    // 线程执行完
                    cyclicBarrier.await(); // ②
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                }
            }
        });
    }
}