CyclicBarrier的介绍和源码分析
CyclicBarrier的字母意思是可循环（Cyclic）使用的屏障（Barrier）。它要做的事情是，让一组线程到达一个屏障（也可以叫做同步点）时被阻塞，直到最后一个线程到达屏障，屏障才会开门，所有被屏障拦截的线程才会继续干活。
线程进入屏障通过CyclicBarrier的await()方法。
CyclicBarrier默认的构造方法是CyclicBarrier(int parties)。其参数表示屏障拦截的线程数量，每个线程调用await方法告诉CyclicBarrier我已经到达屏障，然后当前线程被阻塞。
CyclicBarrier还提供一个更高级的构造函数CyclicBarrier(int parties，Runnable barrier Action)，用于在线程到达屏障时，优先执行barrier Action这个Runnable对象，方便处理更复杂的业务场景。
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public CyclicBarrier(int parties) {
this(parties, null);
}
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
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实现原理，在CyclicBarrier的内部定义了一个Lock对象（ReentrantLock），每当一个线程调用CyclicBarrier的await()方法时，将剩余拦截的线程数减一，然后判断剩余拦截数是否为0，如果不是，进入Lock对象的条件队列等待。如果是，执行barrierAction对象的Runnable方法，然后将所的条件队列中的所有线程放入锁等待队列中，这些线程会依次获取锁，释放锁，接着先从await()方法返回，在从CyclicBarrier的await()方法返回。
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/** The lock for guarding barrier entry */
private final ReentrantLock lock = new ReentrantLock();
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await()源码：
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public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
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dowait源码：
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/**
* Main barrier code, covering the various policies.
*/
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;
if (index == 0) { // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null)
command.run();
ranAction = true;
nextGeneration();
return 0;
} finally {
if (!ranAction)
breakBarrier();
}
}
// loop until tripped, broken, interrupted, or timed out
for (;;) {
try {
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();
if (g != generation)
return index;
if (timed && nanos <= 0L) {
breakBarrier();
throw new TimeoutException();
}
}
} finally {
lock.unlock();
}
}
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private void nextGeneration() {
// signal completion of last generation
trip.signalAll();
// set up next generation
count = parties;
generation = new Generation();
}
/**
* Sets current barrier generation as broken and wakes up everyone.
* Called only while holding lock.
*/
private void breakBarrier() {
generation.broken = true;
count = parties;
trip.signalAll();
}
nextGeneration和breakBarrier方法都可以停止阻塞。
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CyclicBarrier主要用于一组线程之间的相互等待，而CountDownLatch一般用于一组线程等待另一组线程。实际上可以通过CountDownLatch的countDown()和await()来实现CyclicBarrier的功能。
即CountDownLatch中的countDown()和await() = CyclicBarrier中的await()。注意在一个线程中先调用countDown()再调用await()
CyclicBarrier对象可以重复使用，重用之前应当调用CyclicBarrier的reset方法：
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public void reset() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
breakBarrier(); // break the current generation
nextGeneration(); // start a new generation
} finally {
lock.unlock();
}
}
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CyclicBarrier使用：
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package com.fpc.Test;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.Random;
public class CyclicBarrierTest {
private CyclicBarrier cyclicBarrier = new CyclicBarrier(4);
private Random rnd = new Random();
class taskDemo implements Runnable{
private String taskId;
public taskDemo( String taskId ) {
this.taskId = taskId;
}
@Override
public void run() {
try {
int time = rnd.nextInt(1000);
Thread.sleep(time);
System.out.println(" Thread : " + taskId + " sleep : " + time + "ms");
try {
cyclicBarrier.await();
System.out.println(" Thread : " + taskId + " sleep is over");
} catch (BrokenBarrierException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
// CyclicBarrier cyclicBarrier = new CyclicBarrier();
public static void main( String[] args ) {
CyclicBarrierTest c = new CyclicBarrierTest();
ExecutorService pool = Executors.newCachedThreadPool();
pool.submit(c.new taskDemo("1"));
pool.submit(c.new taskDemo("2"));
pool.submit(c.new taskDemo("3"));
pool.submit(c.new taskDemo("4"));
}
}
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运行结果：
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Thread : 1 sleep : 102ms
Thread : 3 sleep : 254ms
Thread : 4 sleep : 394ms
Thread : 2 sleep : 943ms
Thread : 2 sleep is over
Thread : 1 sleep is over
Thread : 4 sleep is over
Thread : 3 sleep is over
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如果构造CyclicBarrier时，给传的大小是5，但是你进入屏障的线程数只有4个，那么会发生什么现象？
这些4个线程都无法结束，因为CyclicBarrier还在等待第5个线程结束，但此时根本没有第5个线程，所以之前的4个线程根本无法结束。
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Thread : 3 sleep : 166ms
Thread : 4 sleep : 281ms
Thread : 2 sleep : 444ms
Thread : 1 sleep : 776ms
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那么如果构造CyclicBarrier时，给传递的参数是3呢，但此时有4个线程进入屏障呢？
结果是其中三个线程会执行然后结束，第四个线程永远无法结束，这是因为CyclicBarrier是可以循环利用的。
执行结果：
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Thread : 3 sleep : 500ms
Thread : 1 sleep : 567ms
Thread : 2 sleep : 912ms
Thread : 2 sleep is over
Thread : 3 sleep is over
Thread : 1 sleep is over
Thread : 4 sleep : 988ms
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