java中CountDownLatch

CountDownLatch：它是一个辅助类，在完成一组正在其他线程执行的操作之前，它允许线程一直等待。这个类使用一个整数进行初始化，这个线程就是等待完成的操作的数目。当一个线程要等待某些操作先执行完成时，需要调用await()方法，这个方法让线程进入到休眠直到等待的所有操作都完成，当某一个操作完成后，它将调用countDown()方法将CountDownLatch类的内部计数器减1,当计数器变成0的时候，CountDownLatch类将唤醒所有调用await()的方法而进入休眠的线程。

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

public class Student implements  Runnable {

    private CountDownLatch countDownLatch ;
    
    
    public Student(CountDownLatch countDownLatch) {
        super();
        this.countDownLatch = countDownLatch;
    }

    @Override
    public void run() {
        try {
            TimeUnit.SECONDS.sleep(1);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName()+"来了");
        //减少数量
        countDownLatch.countDown();
        
    }
  
  

}

import java.util.concurrent.CountDownLatch;

public class Teacher  implements  Runnable{

  private CountDownLatch   countDownLatch;
    
    
    public Teacher(CountDownLatch countDownLatch) {
        super();
        this.countDownLatch = countDownLatch;
    }
    
    @Override
    public void run() {
        System.out.println("人没齐先等人");
        try {
            countDownLatch.await();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println("人齐了开始上课");
        
    }
    
    public static void main(String[] args) {
         CountDownLatch countDownLatch = new CountDownLatch(30);
        Thread teacher = new Thread(new Teacher(countDownLatch));
        teacher.start();
        for(int i=1;i<31;i++){
            new Thread(new Student(countDownLatch),"学生"+i).start();
        }
    }

}

    public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {  //获取锁 并把数量减一
            doReleaseShared();// 如果数量小于0 就唤醒等待的线程
            return true;
        }
        return false;
    }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }