public class AsyncDemo {

  private static void doSomeTask() {
    System.out.println("Hello World");
  }

  private static void onCompletion() {
    System.out.println("All tasks finished");
  }

  public static void main(String[] args) {
    ExecutorService executor = Executors.newCachedThreadPool();
    final CountDownLatch latch = new CountDownLatch(2);

    executor.execute(new Task(latch));
    executor.execute(new Task(latch));

    executor.execute(() -> {
      try {
        latch.await();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      onCompletion();
    });
    executor.shutdown();
  }

  private static class Task implements Runnable {

    /**
     * CountDownLatch 是JDK提供的一个简单的线程监测工具
     * 基于简单的计数，调用countDown()方法表明当前线程已经终止
     * 在监测线程中调用await()方法,该方法会一直挂起直到所有其它线程终止
     */
    private final CountDownLatch latch;

    public Task(CountDownLatch latch) {
      this.latch = latch;
    }

    @Override
    public void run() {
      try {
        doSomeTask();
      } catch (Exception e) {
        e.printStackTrace();
      } finally {
        latch.countDown();
      }
    }
  }
}