什么是CyclicBarrier？

CyclicBarrier是Java的一个同步类，用于协作多线程，同时也是一个共享锁

CyclicBarrier的使用场景

多线程一起开始

异步线程之间互相等待

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public static void main(String[] args) throws IOException {
    CyclicBarrier cyclicBarrier = new CyclicBarrier(3);
    for (int i = 0; i < 6; i++) {
        new MyThread(cyclicBarrier).start();
    }
    System.in.read();
}

static class MyThread extends Thread {

    CyclicBarrier cyclicBarrier;

    public MyThread(CyclicBarrier cyclicBarrier) {
        this.cyclicBarrier = cyclicBarrier;
    }

    @Override
    public void run() {
        try {
            // 获取到锁
            Thread.sleep(1000 + new Random().nextInt(3000));
            System.out.println(Thread.currentThread().getName() + "start");
            cyclicBarrier.await();
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
        System.out.println(Thread.currentThread().getName() + "end");
    }
}

CyclicBarrier源码解析

构造器

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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;
}

成员变量

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// 每次计数都是一个新的Generation
private static class Generation {
    boolean broken = false;
}


private final ReentrantLock lock = new ReentrantLock();


private final Condition trip = lock.newCondition();

// 到达屏障需要的总线程数
private final int parties;

// 线程数到达屏障的时候执行的方法
private final Runnable barrierCommand;

// 当前的Generation
private Generation generation = new Generation();

// 到达屏障还差多少线程数
private int count;

await()

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

dowait()

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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;
        // 当前线程数到达屏障
        if (index == 0) {  // tripped
            boolean ranAction = false;
            try {
                final Runnable command = barrierCommand;
                // 执行方法
                if (command != null)
                    command.run();
                ranAction = true;
                // 唤醒条件队列所有线程，并且更新屏障
                nextGeneration();
                return 0;
            } finally {
                // 将当前屏障生成设置为已损坏并唤醒条件队列所有线程
                if (!ranAction)
                    breakBarrier();
            }
        }

        // 循环直到当前线程数到达屏障、线程被中断、超时
        for (;;) {
            try {
                // timed 表示是否采用超时机制
                if (!timed)
                    // 加入条件队列阻塞，同时会把占用的锁释放
                    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();
    }
}

nextGeneration()：开启下一轮循环，重置count，唤醒条件队列所有的线程

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private void nextGeneration() {
    // signal completion of last generation
    trip.signalAll();
    // set up next generation
    count = parties;
    generation = new Generation();
}

breakBarrier()：设置当前循环为被打断状态，重置count，唤醒条件队列所有的线程

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private void breakBarrier() {
    generation.broken = true;
    count = parties;
    trip.signalAll();
}