Java常见Lock(五)： lock之CyclicBarrier

常见Lock(五)： lock之CyclicBarrier"/>

Java常见Lock(五)： lock之CyclicBarrier

CyclicBarrier

public class CyclicBarrier

1. 又名回环栅栏或循环栅栏。
2. 可重用。

成员变量

    /*** 内部类 */private static class Generation {boolean broken = false;}/** 可重复入锁 */private final ReentrantLock lock = new ReentrantLock();/** 利用Condition进行阻塞和唤醒 */private final Condition trip = lock.newCondition();/** 参与者数量 */private final int parties;/* 参与者都执行完之后，从参与者中找一个线程执行该barrierCommand */private final Runnable barrierCommand;/** The current generation */private Generation generation = new Generation();/*** 还在等待的参与者数量*/private int count;

构造方法

    /*** 创建一个新的 CyclicBarrier，它将在给定数量的参与者（线程）处于等待状态时启动，并在启动 barrier 时执行给定的屏障操作，该操作由最后一个进入 barrier 的线程执行。*/public CyclicBarrier(int parties, Runnable barrierAction) {if (parties <= 0) throw new IllegalArgumentException();this.parties = parties;this.count = parties;this.barrierCommand = barrierAction;}/***   创建一个新的 CyclicBarrier，它将在给定数量的参与者（线程）处于等待状态时启动，但它不会在启动 barrier 时执行预定义的操作。*/public CyclicBarrier(int parties) {this(parties, null);}

核心方法

boolean await()： 在所有参与者都已经在此 barrier 上调用 await 方法之前，将一直等待。

    public int await() throws InterruptedException, BrokenBarrierException {try {return dowait(false, 0L);} catch (TimeoutException toe) {throw new Error(toe); // cannot happen;}}/*** 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) {  // trippedboolean 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 outfor (;;) {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();}}

int await(long timeout, TimeUnit unit)：在所有参与者都已经在此屏障上调用 await 方法之前将一直等待,或者超出了指定的等待时间。

    public int await(long timeout, TimeUnit unit)throws InterruptedException,BrokenBarrierException,TimeoutException {return dowait(true, unit.toNanos(timeout));}

void reset()： 将屏障重置为其初始状态。

    public void reset() {final ReentrantLock lock = this.lock;lock.lock();try {breakBarrier();   // break the current generationnextGeneration(); // start a new generation} finally {lock.unlock();}}/*** 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();}/*** Updates state on barrier trip and wakes up everyone.* Called only while holding lock.*/private void nextGeneration() {// signal completion of last generationtrip.signalAll();// set up next generationcount = parties;generation = new Generation();}

从源码中看出，CyclicBarrier利用ReentrantLock保证线程安全，Condition保证等待和唤醒线程。
主要思想是初始化CyclicBarrier时，指定N个参与者，及一个栅栏动作barrierAction。
每个参与者完成一次任务，即执行await方法，还在等待参与者数量–count，当发现count !=0 时，说明还有参与者未参与，每个参与者自旋等待，等所有参与者都参与了，即count == 0时，所有参与者等待线程被唤醒，通俗的解释是每个参与者await方法后面代码可以继续执行，若设置了barrierAction，则第一个被唤醒的参与者等待线程执行该方法。
count可被重置，是与CountDownLatch的最大区别。
CyclicBarrier强调的是n个线程，参与者互相等待，只要有一个没完成count != 0，所有参与者都得等着。
CountDownLatch强调的是一个线程（或多个）需要等待另外的n个线程干完某件事情之后才能继续执行。