目录

一、CountDownLatch测试demo

二、CountDownLatch源码分析

三、cyclebarrier測試demo

四、CountDownLatch源码分析

兩者區別：

一、CountDownLatch测试demo

注：這裏調用countDownLatch.await()的綫程稱之爲主綫程

public class CountDownLatchTest {
    private static  final int NUM=3 ;

    public static void main(String[] args) throws InterruptedException {
        CountDownLatch countDownLatch = new CountDownLatch(NUM);
        for(int i = 1; i <= NUM; i++){
            new Thread(() -> {
                System.out.println(String.format("%s已经准备好了",Thread.currentThread().getName()));
                countDownLatch.countDown();
                System.out.println(String.format("还有%d位玩家未准备好",countDownLatch.getCount()));
            }, "玩家"+i).start();
        }
        System.out.println("主线程正在等待所有玩家准备好");
        countDownLatch.await();
        TimeUnit.SECONDS.sleep(1);
        System.out.println("主线程被唤醒,开始主线程逻辑");
    }

主线程正在等待所有玩家准备好
玩家2已经准备好了
玩家3已经准备好了
玩家1已经准备好了
还有2位玩家未准备好
还有1位玩家未准备好
还有0位玩家未准备好
主线程被唤醒,开始主线程逻辑

二、CountDownLatch源码分析

核心类AQS——抽象队列同步器AbstractQueuedSynchronizer

①主线程countDownLatch.await() ->doAcquireSharedInterruptibly死循环->第二次循环进入“标记1”的parkAndCheckInterrupt(),里面LockSupport.park(this),主线程进入等待状态；

②子线程countDownLatch.countDown()，每执行一次检查并修改AQS的state，state是否已countDown到0，如果到0则unparkSuccessor(h),里面调用LockSupport.unpark(s.thread),唤醒调用countDownLatch.await的主线程，主线程唤醒后继续上图的死循环，到“标记2”时tryAcquireShared会检查AQS的state并返回r=1退出死循环，然后执行主线程countDownLatch.await后面的逻辑

三、cyclebarrier測試demo

package com.example.demo;

import java.io.IOException;
import java.util.concurrent.*;

public class CyclicBarrierTest {

    public static void main(String[] args) throws IOException, InterruptedException {
        CyclicBarrier barrier = new CyclicBarrier(3, () -> {
            System.out.println("栅栏已达到开放条件");
        });

        ExecutorService executor = Executors.newFixedThreadPool(3);
        executor.submit(new Thread(new Runner(barrier, "1号选手")));
        executor.submit(new Thread(new Runner(barrier, "2号选手")));
        executor.submit(new Thread(new Runner(barrier, "3号选手")));
        executor.submit(new Thread(new Runner(barrier, "4号选手")));
        executor.submit(new Thread(new Runner(barrier, "5号选手")));
        executor.submit(new Thread(new Runner(barrier, "6号选手")));
        executor.submit(new Thread(new Runner(barrier, "7号选手")));
        System.out.println("主线程完毕");
        executor.shutdown();
    }
}

class Runner implements Runnable {
    private CyclicBarrier barrier;
    private String name;

    public Runner(CyclicBarrier barrier, String name) {
        super();
        this.barrier = barrier;
        this.name = name;
    }

    @Override
    public void run() {
        try {
            System.out.println(name + " 准备好了...");
            barrier.await(3, TimeUnit.SECONDS);
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println(name + " 起跑！");
    }
}

四、CountDownLatch源码分析

核心类——抽象队列同步器AbstractQueuedSynchronizer+Condition接口

①子綫程每次調用barrier.await，int index = --count計數器減一，如果index！=0，進入“標記1”trip.await()。trip為Condition接口類型，實現仍然是AQS，裏面是LockSupport.park(this);

如果index=0則進入“標記2”，這之前如果cyclebarrier如果傳入了barrierAction會在這裏執行【最后一个线程到达之后栅栏触发（但在释放所有线程之前），该命令只在每个屏障点运行一次】。“標記2”nextGeneration()方法裏會執行trip.signalAll()喚醒柵欄上所有綫程並重置計數器count。

兩者區別：

1.countDownLatch.countdown()只是在countdown到0时对"主"线程进行唤醒（如果多个线程调用await()方法，则都会被唤醒），子线程不会阻塞且会执行countDownLatch.countdown()后的逻辑，子线程会优先主线程结束;

2.circlebarrier阻塞的是barrier.await()的线程，计数器count递减到0时重置计数器，唤醒所有阻塞的线程，然后继续执行barrier.await()后的逻辑；

3.circlebarrier计数器可重置；

4.两者都是基于park/unpark等待唤醒机制实现，两者的await()都支持等待超时\中断，规避死锁

其它：任何一个await阻塞的线程被中断，那么栅栏就认为是打破了，所有阻塞的await将继续运行并抛出BrokenBarrierException；await超时同样如此

package com.smtc.cloud.msg.mqtt;

import java.io.IOException;
import java.util.concurrent.*;

public class CyclicBarrierTest {

    public static void main(String[] args) throws IOException, InterruptedException {
        CyclicBarrier barrier = new CyclicBarrier(3, () -> {
            System.out.println("栅栏已达到开放条件");
        });

        ExecutorService executor = Executors.newFixedThreadPool(3);
        executor.submit(new Thread(new Runner(barrier, "1号选手")));
        executor.submit(new Thread(new Runner(barrier, "2号选手")));
        System.out.println("主线程完毕");
        executor.shutdown();
    }
}

class Runner implements Runnable {
    private CyclicBarrier barrier;
    private String name;

    public Runner(CyclicBarrier barrier, String name) {
        super();
        this.barrier = barrier;
        this.name = name;
    }

    @Override
    public void run() {
        try {
            System.out.println(name + " 准备好了...");
            if (Thread.currentThread().getName().contains("2")){
                Thread.currentThread().interrupt();
            }
            barrier.await();
        } catch (Exception e) {
            e.printStackTrace();
        }
        System.out.println(name + " 起跑！");
    }
}

其它测试

public class ConcurrentTest  {

    // 请求总数
    public static int clientTotal = 5000;
    // 同时并发执行的线程数
    public static int threadTotal = 200;
    // public static int count=0;有线程安全问题，结果可能不为5000
    public static AtomicInteger count=new AtomicInteger(0);
    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();//5000次循环完成，每次到这里获取令牌并-1当没有令牌就阻塞，一个线程执行完业务后释放令牌计数+1，阻塞的4800队列来抢这个信号执行
                    Thread.sleep(1111);
                    count.addAndGet(1);
                    //count++;
                    semaphore.release();
                } catch (Exception e) {
                }
                countDownLatch.countDown();//所有线程执行到这阻塞到countdown到0时一起执行
                System.out.println(countDownLatch.getCount());

            });
        }
        countDownLatch.await();//
        executorService.shutdown();
        System.out.println(count);
    }

}