Java - 如何知道线程何时等待？

Thread t1 = new Thread(new Runnable() {
    @Override
    public void run() {
        // 执行任务
    }
});
t1.start();

Thread t2 = new Thread(new Runnable() {
    @Override
    public void run() {
        try {
            t1.join(); // 等待 t1 线程执行完毕
            // 执行任务
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
});
t2.start();

CountDownLatch latch = new CountDownLatch(1);

Thread t1 = new Thread(new Runnable() {
    @Override
    public void run() {
        // 执行任务
        latch.countDown(); // 减少计数器
    }
});
t1.start();

Thread t2 = new Thread(new Runnable() {
    @Override
    public void run() {
        try {
            latch.await(); // 等待计数器变为 0
            // 执行任务
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
});
t2.start();

CyclicBarrier barrier = new CyclicBarrier(3);

for (int i = 0; i < 3; i++) {
    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                // 执行任务
                barrier.await(); // 等待其他线程
                // 执行任务
            } catch (InterruptedException | BrokenBarrierException e) {
                e.printStackTrace();
            }
        }
    }).start();
}

Semaphore semaphore = new Semaphore(2);

for (int i = 0; i < 5; i++) {
    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                semaphore.acquire(); // 获取许可
                // 执行任务
                semaphore.release(); // 释放许可
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }).start();
}