CountDownLatch

应用场景：1.多线程任务汇总。2.多线程任务阻塞住，等待发令枪响，一起执行。

减法计数器

每次有线程调用，数量-1，当计数器归零，countDownLatch.await()就会被唤醒向下执行。

import java.util.concurrent.CountDownLatch;

// 计数器
public class CountDownLatchDemo {
    public static void main(String[] args) throws InterruptedException {
        //总数是6  必须要执行任务的时候再使用
        CountDownLatch countDownLatch = new CountDownLatch(6);
        for (int i = 0; i < 6; i++) {
            new Thread(()->{
                System.out.println(Thread.currentThread().getName() + " Go out");
                countDownLatch.countDown();//数量-1
            },String.valueOf(i)).start();
        }
        countDownLatch.await();
        System.out.println("Close Door");
    }

}

原理：

countDownLatch.countDown(); //数量-1

countDownLatch.await();//等待计数器归零，然后再往下执行

每次线程调用countDown()数量-1，假设计数器变成0，countDownLatch.await()就会被唤醒，继续执行！

CyclicBarrier

应用场景：比如LOL类游戏，满10人一组，开始游戏。

加法计算器

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;

public class CyclicBarrierDemo {
    public static void main(String[] args) {
         //集齐七颗龙珠召唤神龙
        CyclicBarrier cyclicBarrier = new CyclicBarrier(7,()->{
            System.out.println("召唤神龙成功！");
        });

        for (int i = 1; i <= 7; i++) {
            final int temp = i;
            new Thread(()->{
                System.out.println(Thread.currentThread().getName() + "收集了第" + temp + "颗龙珠");
                try {
                    cyclicBarrier.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (BrokenBarrierException e) {
                    e.printStackTrace();
                }
            }).start();
        }
    }
}

Semaphore

Semaphore：信号量

抢车位(6辆车，3个停车位)

import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;

public class SemaphoreDemo {
    public static void main(String[] args) {
        //线程数量 比作 停车位
        Semaphore semaphore = new Semaphore(3);

        for (int i = 1; i <= 6; i++) {
            new Thread(()->{
                //acquire() 得到
                try {
                    semaphore.acquire();
                    System.out.println(Thread.currentThread().getName() + "抢到车位");
                    TimeUnit.SECONDS.sleep(2);
                    System.out.println(Thread.currentThread().getName() + "离开车位");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }finally {
                    //release() 释放
                    semaphore.release();
                }
            },String.valueOf(i)).start();
        }
    }
}