java-并发包中的倒计数闩锁、循环栅栏、信号量

1、倒计数闩锁 CountDownLatch

/**
 * 倒计数闩锁
 * 等班里5个学生都走出教室后，老师才能走出去
 */
public class CountDownLatchDemo {

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(5);
        for (int i = 1; i <= 5; i++) {
            new Thread(() -> {
                System.out.println(Thread.currentThread().getName()+" 走出了教室 ");
                countDownLatch.countDown();  // 到这里只是给计数器减一，并不会阻塞（和CyclicBarrier区别）

                System.out.println("不阻塞");
            },String.valueOf(i)).start();
        }

        countDownLatch.await();
        System.out.println("老师走");
    }
}

2、循环栅栏

/**
 * 循环栅栏
 * 收集完7颗龙珠，才能召唤神龙
 */
public class CyclicBarrierDemo {

    private static final int NUMBER = 7;

    public static void main(String[] args) {
        CyclicBarrier cyclicBarrier = new CyclicBarrier(NUMBER, () -> {
            System.out.println("召唤神龙！");
        });

        for (int i = 1; i <= 7; i++) {
            int finalI = i;
            new Thread(() -> {
                System.out.println(Thread.currentThread().getName() + " 拿到龙珠");
                try {
                    cyclicBarrier.await(); // 到这里会阻塞，直到所有线程都达到这个点，cyclicBarrier到达阈值才往下执行（和CountDownLatch区别）
                    System.out.println(finalI);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }, String.valueOf(i)).start();
        }
    }
}

3、信号量

/**
 * 有6辆汽车，停2个车位，每辆车随机停数秒
 * (信号量的本质是保证多个线程中，只能有指定数量的线程同时工作)
 */
public class SemaphoreDemo {

    public static void main(String[] args) {

        // 设置许可变量
        Semaphore semaphore = new Semaphore(2);

        for (int i = 1; i <= 6; i++) {
            new Thread(() -> {
                try {
                    // 抢占
                    semaphore.acquire();
                    System.out.println(Thread.currentThread().getName() + " --> 抢到停车位");

                    // 设置随机停车时间
                    TimeUnit.SECONDS.sleep(new Random().nextInt(5));

                    System.out.println(Thread.currentThread().getName() + " ----------> 离开车位");
                    // 释放
                    semaphore.release();

                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }, String.valueOf(i)).start();
        }
    }
}