coutdownlatch的使用

package com.example.demo;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Random;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

/**
 * 关于计数器（countdownlathch）的使用
 * 当计数器为0时，唤醒使用计数器锁住的线程，注意一个线程可能会执行俩次，可能需要sychronized去控制
 */
public class CountdownTest {


    public static void main(String[] args) {
        CountDownLatch countDownLatch = new CountDownLatch(3);
        // 构造一个线程池
        ThreadPoolExecutor threadPool = new ThreadPoolExecutor(2, 4, 3, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(3),
                new ThreadPoolExecutor.DiscardOldestPolicy());

        Thread t1 = new Thread(new Customer(countDownLatch, "t1"));

        Thread t2 = new Thread(new Customer(countDownLatch, "t2"));

        Thread t3 = new Thread(new Customer(countDownLatch, "t3"));

        Thread t4 = new Thread(new Waitress(countDownLatch, "t4"));
        threadPool.execute(t1);
        threadPool.execute(t2);
        threadPool.execute(t3);
        threadPool.execute(t4);
    }


}

class Customer implements Runnable {
    private CountDownLatch latch;
    private String name;

    public Customer(CountDownLatch latch, String name) {
        this.latch = latch;
        this.name = name;
    }

    @Override
    public void run() {
        try {
            SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss.SSS");
            Random random = new Random();

            System.out.println(sdf.format(new Date()) + " " + name + "出发去饭店");
            Thread.sleep((long) (random.nextDouble() * 3000) + 1000);
            System.out.println(sdf.format(new Date()) + " " + name + "到了饭店");
            latch.countDown();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

class Waitress implements Runnable {
    private CountDownLatch latch;
    private String name;

    public Waitress(CountDownLatch latch, String name) {
        this.latch = latch;
        this.name = name;
    }

    @Override
    public void run() {
        try {
            SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss.SSS");
            System.out.println(sdf.format(new Date()) + " " + name + "等待顾客");
            latch.await();
            System.out.println(sdf.format(new Date()) + " " + name + "开始上菜");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

latch.countDown();
//=====================
 public void countDown() {
        sync.releaseShared(1);
    }
//=====================
  public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
//=====================
private void doReleaseShared() {
        /*
         * Ensure that a release propagates, even if there are other
         * in-progress acquires/releases.  This proceeds in the usual
         * way of trying to unparkSuccessor of head if it needs
         * signal. But if it does not, status is set to PROPAGATE to
         * ensure that upon release, propagation continues.
         * Additionally, we must loop in case a new node is added
         * while we are doing this. Also, unlike other uses of
         * unparkSuccessor, we need to know if CAS to reset status
         * fails, if so rechecking.
         */
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            // loop to recheck cases
                    unparkSuccessor(h);
                }
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                // loop on failed CAS
            }
            if (h == head)                   // loop if head changed
                break;
        }
    }

 latch.await();
//=======================
    public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }
//=======================
    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }
//=======================
private void doAcquireSharedInterruptibly(int arg)
        throws InterruptedException {
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            for (;;) {
                final Node p = node.predecessor();
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                }
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }