前言
咱们晓得，CountDownLatch的计数器是一次性的，它不能重置。也就是说，当count值变为0时，再调用await()办法会立刻返回，不会阻塞。
本文要说的CyclicBarrier就是一种能够重置计数器的线程同步工具类。CyclicBarrier字面意思是“回环屏障”，它能够让一组线程全副达到一个状态后再全副同时往下执行。之所以叫回环是因为当所有线程执行结束，并重置CyclicBarrier的状态后它能够被重用。而之所以叫屏障是因为当某个线程调用await办法后就会被阻塞，这个阻塞点就称为屏障，等其余所有线程都调用了await办法后，这组线程就会一起冲破屏障，并往下执行。
应用场景
两个子工作别离执行本人的工作，等它们都执行完后，主工作汇总子工作的后果，并做一些解决，解决实现后两个子工作又持续做其余事件。示例代码：
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
public class CyclicBarrierDemo {
private static CyclicBarrier cyclicBarrier = new CyclicBarrier(2, () -> {
try {
System.out.println("main task merge subtask result begin");
// simulate merge work
Thread.sleep(5000);
System.out.println("main task merge subtask result finished");
} catch (InterruptedException e) {
// ignore
}
});
public static void main(String[] args) {
Thread thread1 = new Thread(() -> {
try {
Thread.sleep(4000);
System.out.println("thread1 finished its work");
cyclicBarrier.await();
System.out.println("thread1 continue work");
} catch (InterruptedException | BrokenBarrierException e) {
// ignore
}
});
Thread thread2 = new Thread(() -> {
try {
Thread.sleep(5000);
System.out.println("thread2 finished its work");
cyclicBarrier.await();
System.out.println("thread2 continue work");
} catch (InterruptedException | BrokenBarrierException e) {
// ignore
}
});
thread1.start();
thread2.start();
}
}输入后果：
能够看到，线程1和线程2调用await()时，会被阻塞，等主线程工作实现后，线程1和线程2就会冲破屏障，持续往下执行。这里的主线程合并工作是可选的，也就是说能够间接new CyclicBarric(int parties)，这种状况下就没有达到屏障后的合并工作，会间接在全副线程达到屏障后同时冲破屏障往下执行。能够比喻成举办同学聚会的场景。有20集体加入团聚，第1集体达到集合地点后要等其他人，第2个，第3个，…第19集体也须要等，当最初一个人到的时候，全副的20集体就能够登程去嗨皮了。
下面介绍的是“屏障”的利用场景，再来看个“回环”的利用场景。
假如一个工作由阶段1，阶段2，阶段3这三个阶段组成，每个线程都串行的顺次执行阶段1,2,3。当多个线程执行工作时，必须保障等所有线程都执行完阶段1后，能力执行阶段2，同样地，也必须保障所有线程都执行完阶段2后，能力执行阶段3。示例代码：
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
public class CyclicBarrierDemo2 {
private static CyclicBarrier cyclicBarrier = new CyclicBarrier(2);
public static void main(String[] args) {
Thread thread1 = new Thread(() -> {
try {
System.out.println("thread1 step 1");
cyclicBarrier.await();
System.out.println("thread1 step 2");
cyclicBarrier.await();
System.out.println("thread1 step 3");
} catch (InterruptedException | BrokenBarrierException e) {
// ignore
}
});
Thread thread2 = new Thread(() -> {
try {
System.out.println("thread2 step 1");
cyclicBarrier.await();
System.out.println("thread2 step 2");
cyclicBarrier.await();
System.out.println("thread2 step 3");
} catch (InterruptedException | BrokenBarrierException e) {
// ignore
}
});
thread1.start();
thread2.start();
}
}输入后果如下：
能够看到，实现了这种同阶段期待的成果。
实现原理
先来看看重要属性：
private static class Generation {
// 屏障是否被突破
boolean broken = false;
}
/** The lock for guarding barrier entry */
private final ReentrantLock lock = new ReentrantLock();
/** Condition to wait on until tripped */
private final Condition trip = lock.newCondition();
/** The number of parties */
private final int parties;
/* The command to run when tripped */
private final Runnable barrierCommand;
/** The current generation */
private Generation generation = new Generation();
/**
* Number of parties still waiting. Counts down from parties to 0 on each generation.
* It is reset to parties on each new generation or when broken.
*/
private int count;能够看到，CyclicBarrier里用了独占锁ReentrantLock实现多线程间的计数器同步，parties示意当多少个线程达到屏障后，冲破屏障往下执行，而count示意以后还残余多少个线程还未达到屏障，当所有线程都冲破屏障后，它又会在新一轮(new generation)被重置为parties的值。也就是说，count和Generation是用来实现重置成果的。
再看看构造方法的属性赋值：
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}再来看看要害办法：
await()
public int await() throws InterruptedException, BrokenBarrierException {
try {
// false示意不设置超时
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}dowait()办法代码如下：
// timed：是否超时期待, nanos：超时工夫
private int dowait(boolean timed, long nanos)
throws InterruptedException, BrokenBarrierException,
TimeoutException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
final Generation g = generation;
if (g.broken)
throw new BrokenBarrierException();
if (Thread.interrupted()) {
breakBarrier();
throw new InterruptedException();
}
int index = --count;
// 如果index为0，示意所有线程都已达到了屏障，此时去执行初始化时设定的barrierCommand（如果有的话）
if (index == 0) { // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null)
command.run();
ranAction = true;
// 唤醒其余线程，并重置进行下一轮
nextGeneration();
// 返回
return 0;
} finally {
if (!ranAction)
breakBarrier();
}
}
// 否则须要等其余线程都达到屏障
// loop until tripped, broken, interrupted, or timed out
for (;;) {
try {
// 辨别超时期待与不超时期待
if (!timed)
trip.await();
else if (nanos > 0L)
nanos = trip.awaitNanos(nanos);
} catch (InterruptedException ie) {
if (g == generation && ! g.broken) {
breakBarrier();
throw ie;
} else {
// We're about to finish waiting even if we had not
// been interrupted, so this interrupt is deemed to // "belong" to subsequent execution.
Thread.currentThread().interrupt();
}
}
if (g.broken)
throw new BrokenBarrierException();
// g != generation 阐明被唤醒后已重置了轮次，阐明所有线程均已达到线程屏障，能够返回了。
if (g != generation)
return index;
// 期待超时，抛出超时异样
if (timed && nanos <= 0L) {
breakBarrier();
throw new TimeoutException();
}
}
} finally {
lock.unlock();
}
}其中，nextGeneration()办法如下：
private void nextGeneration() {
// signal completion of last generation
// 唤醒期待在trip条件(即屏障)上的其余所有线程
trip.signalAll();
// set up next generation
// 重置count的值为初始值parties
count = parties;
// 重置以后轮次
generation = new Generation();
}参考资料：
《Java并发编程之美》
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