CountDownLatch第一种用法

/**
 * 看大夫任务
 */
public class SeeDoctorTask implements Runnable {
    private CountDownLatch countDownLatch;

    public SeeDoctorTask(CountDownLatch countDownLatch){
        this.countDownLatch = countDownLatch;
    }

    public void run() {
        try {
            System.out.println("开始看医生");
            Thread.sleep(3000);
            System.out.println("看医生结束，准备离开病房");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            if (countDownLatch != null)
                countDownLatch.countDown();
        }
    }

}

/**
 * 排队的任务
 */
public class QueueTask implements Runnable {

    private CountDownLatch countDownLatch;

    public QueueTask(CountDownLatch countDownLatch){
        this.countDownLatch = countDownLatch;
    }
    public void run() {
        try {
            System.out.println("开始在医院药房排队买药....");
            Thread.sleep(5000);
            System.out.println("排队成功，可以开始缴费买药");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            if (countDownLatch != null)
                countDownLatch.countDown();
        }
    }
}

/**
 * 配媳妇去看病，轮到媳妇看大夫时
 * 我就开始去排队准备交钱了。
 */
public class CountDownLaunchRunner {

    public static void main(String[] args) throws InterruptedException {
        long now = System.currentTimeMillis();
        CountDownLatch countDownLatch = new CountDownLatch(2);

        new Thread(new SeeDoctorTask(countDownLatch)).start();
        new Thread(new QueueTask(countDownLatch)).start();
        //等待线程池中的2个任务执行完毕，否则一直阻塞
        countDownLatch.await();
        System.out.println("over，回家 cost:"+(System.currentTimeMillis()-now));
    }
}

CountDownLatch第二种用法

public class CountDownLaunchRunner {

    static int sub = 0;
    static Object object = new Object();

    public static void main(String[] args) throws InterruptedException {
        long now = System.currentTimeMillis();
        CountDownLatch countDownLatch = new CountDownLatch(1);



        for(int i=0;i<2;i++){
            new Thread(new Runnable() {
                @Override
                public void run() {
                    try {
                        countDownLatch.await();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    synchronized (object){
                        for(int j=0;j<1000;j++){
                            sub++;
                        }
                    }

                    System.out.println(Thread.currentThread().getName()+"sub="+sub);
                }
            }).start();
        }
        Thread.sleep(3000);

		//⼀旦数字被减为0，则会将AQS中排队的线程(同步等待队列中从头到尾)依次唤醒
        countDownLatch.countDown();
        System.out.println("over，回家 cost:"+(System.currentTimeMillis()-now));
    }

}

/Semaphore使用

/**
 * @description :可用于流量控制，限制最大的并发访问数
 */
public class SemaphoreRunner {

    public static void main(String[] args) {
        Semaphore semaphore = new Semaphore(2);
        for (int i=0;i<10;i++){
            new Thread(new Task(semaphore,"xiaoshui+"+i)).start();
        }
    }

    static class Task extends Thread{
        Semaphore semaphore;

        public Task(Semaphore semaphore,String tname){
            super(tname);
            this.semaphore = semaphore;
            //this.setName(tname);
        }

        public void run() {
            try {
//                semaphore.acquireUninterruptibly();
                semaphore.acquire();//获取公共资源

                System.out.println(Thread.currentThread().getName()+":aquire() at time:"+System.currentTimeMillis());
                Thread.sleep(5000);

                semaphore.release();

/*                if(semaphore.tryAcquire(500,TimeUnit.MILLISECONDS)){
                    System.out.println(Thread.currentThread().getName()+":aquire() at time:"+System.currentTimeMillis());
                    Thread.sleep(5000);
                    semaphore.release();//释放公共资源
                }else{
                    fallback();
                }*/

            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        public void fallback(){
            System.out.println("降级");
        }
    }

CountDownLatch和Semaphore的区别和底层原理

CountDownLatch表示计数器，可以给CountDownLatch设置⼀个数字，⼀个线程调⽤CountDownLatch的await()将会阻塞，其他线程可以调⽤CountDownLatch的countDown()⽅法来对CountDownLatch中的数字减⼀，当数字被减成0后，所有await的线程都将被唤醒。

对应的底层原理就是，调⽤await()⽅法的线程会利⽤AQS排队，⼀旦数字被减为0，则会将AQS中排队的线程依次唤醒。

Semaphore表示信号量，可以设置许可的个数，表示同时允许最多多少个线程使⽤该信号量，通过acquire()来获取许可，如果没有许可可⽤则线程阻塞，并通过AQS来排队，可以通过release()⽅法来释放许可，当某个线程释放了某个许可后，会从AQS中正在排队的第⼀个线程开始依次唤醒，直到没有空闲许可。