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) {  // 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();        }    }

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 generation            nextGeneration(); // 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 generation        trip.signalAll();        // set up next generation        count = 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。