Java并发包：CountDownLatch和CyclicBarrier

Java并发包：CountDownLatch和CyclicBarrier

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CountDownLatch

urrent.CountDownLatch是一种并发结构，它允许一个或者多个线程等待一个给定的操作集合完成。

CountDownLatch初始化时需要给定一个总数。这个总数将会随着调用countDown()方法每次递减。通过调用await()方法，线程将会等到这个总数变为0。调用await()方法会阻塞线程直到那个总数递减到0为止。

下面是一个简单的例子。Decrementer在CountDownLatch上调用了countDown()3次之后,Waiter将会释放。

CountDownLatch latch = new CountDownLatch(3);Waiter      waiter      = new Waiter(latch);
Decrementer decrementer = new Decrementer(latch);new Thread(waiter).start();
new Thread(decrementer).start();Thread.sleep(4000);

public class Waiter implements Runnable{CountDownLatch latch = null;public Waiter(CountDownLatch latch) {this.latch = latch;}public void run() {try {latch.await();} catch (InterruptedException e) {e.printStackTrace();}System.out.println("Waiter Released");}
}

public class Decrementer implements Runnable {CountDownLatch latch = null;public Decrementer(CountDownLatch latch) {this.latch = latch;}public void run() {try {Thread.sleep(1000);untDown();Thread.sleep(1000);untDown();Thread.sleep(1000);untDown();} catch (InterruptedException e) {e.printStackTrace();}}
}

添加一个客户端连接Zookeeper的例子。

/*** Created by charming on 2017/11/15.* 这里 CountDownLatch 用于停止（等待）主进程，直到客户端与ZooKeeper集合连接。* ZooKeeper集合通过监视器回调来回复连接状态。一旦客户端与ZooKeeper集合连接，监视器回调就会被调用，* 并且监视器回调函数调用CountDownLatch的countDown方法来释放锁，在主进程中await。*/
public class ZooKeeperConnection {// declare zookeeper instance to access ZooKeeper ensembleprivate ZooKeeper zoo;final CountDownLatch connectedSignal = new CountDownLatch(1);public ZooKeeper connect(String host) throws IOException,InterruptedException {zoo = new ZooKeeper(host, 5000, new Watcher() {public void process(WatchedEvent event) {if (State() == Event.KeeperState.SyncConnected) {untDown();}}});connectedSignal.await();return zoo;}// Method to disconnect from zookeeper serverpublic void close() throws InterruptedException {zoo.close();}
}

CyclicBarrier

urrent.CyclicBarrier类的同步机制可以通过某些算法实现同步线程的执行，换句话说，它就像一个屏障（或着说栏栅），在任何线程执行之前，所有线程必须在此处等着，直到所有线程都到达才执行。下面是图解：

所有的线程会在CyclicBarrier上调用await()方法相互等待，一旦N个线程正在CyclicBarrier上等待，所有的线程都会释放并且继续执行。

创建 CyclicBarrier

当你创建一个CyclicBarrier，你需要指定在它上面有多少个体线程同时等待，下面是如何创建一个CyclicBarrier:

CyclicBarrier barrier = new CyclicBarrier(2);

有2个线程在CyclicBarrier上等待，这两个线程就会释放。

在CyclicBarrier上等待

下面是一个线程如何在CyclicBarrier上等待：

barrier.await();

你也可以指定线程等待的时间。当等待的时间过了，即使不是所有指定的线程都在CyclicBarrier上等待，线程也会被释放。下面是如何指定超时时间：

barrier.await(10, TimeUnit.SECONDS);

等待的线程会在CyclicBarrier上等待直至：

• 最后的线程达到
• 线程被另一个线程中断（另一个线程调用interrupt()方法）
• 另一个等待的线程被中断
• 另一个等待的线程等待超时
• 某些外部线程调用了set()方法

CyclicBarrier Action

CyclicBarrire支持一种栏栅行为，一但最后的线程达到时线程将会执行。Runnable的栅栏行为通过CyclicBarrier的构造函数传递，像下面这样：

Runnable      barrierAction = ... ;
CyclicBarrier barrier       = new CyclicBarrier(2, barrierAction);

CyclicBarrier示例

下面的代码展示如何使用CyclicBarrier:

Runnable barrier1Action = new Runnable() {public void run() {System.out.println("BarrierAction 1 executed ");}
};
Runnable barrier2Action = new Runnable() {public void run() {System.out.println("BarrierAction 2 executed ");}
};CyclicBarrier barrier1 = new CyclicBarrier(2, barrier1Action);
CyclicBarrier barrier2 = new CyclicBarrier(2, barrier2Action);CyclicBarrierRunnable barrierRunnable1 =new CyclicBarrierRunnable(barrier1, barrier2);CyclicBarrierRunnable barrierRunnable2 =new CyclicBarrierRunnable(barrier1, barrier2);new Thread(barrierRunnable1).start();
new Thread(barrierRunnable2).start();

下面是CyclicBarrier类：

public class CyclicBarrierRunnable implements Runnable{CyclicBarrier barrier1 = null;CyclicBarrier barrier2 = null;public CyclicBarrierRunnable(CyclicBarrier barrier1,CyclicBarrier barrier2) {this.barrier1 = barrier1;this.barrier2 = barrier2;}public void run() {try {Thread.sleep(1000);System.out.println(Thread.currentThread().getName() +" waiting at barrier 1");this.barrier1.await();Thread.sleep(1000);System.out.println(Thread.currentThread().getName() +" waiting at barrier 2");this.barrier2.await();System.out.println(Thread.currentThread().getName() +" done!");} catch (InterruptedException e) {e.printStackTrace();} catch (BrokenBarrierException e) {e.printStackTrace();}}
}

下面是上面代码在控制台的输出结果，注意每次执行线程在控制台上打印的顺序也可能不一样。有时候可能是Thread-0先打印，有时候可能是Thread-1先打印。

Thread-0 waiting at barrier 1
Thread-1 waiting at barrier 1
BarrierAction 1 executed
Thread-1 waiting at barrier 2
Thread-0 waiting at barrier 2
BarrierAction 2 executed
Thread-0 done!
Thread-1 done!