CyclicBarrier原理及源码解析

概念

通过AQS的互斥锁机制（ReentrantLock）+条件（Condition）实现的，用于一组线程相互之间等待，直到到达某个公共屏障点 (common barrier point)，再继续执行。常用于多线程计算数据，当所有线程都完成执行后，在CyclicBarrier回调线程中合并计算。为方便源码解释，假定将一组线程表示为组

public class CyclicBarrier {
    //Generation类在每组创建时都会生成一个该类的新的实例，用于下一组线程/reset方法后的复用，并恢复broken值
    private static class Generation {
        boolean broken = false; // 当前组有没有被强制中断
    }
    private final ReentrantLock lock = new ReentrantLock();
    //用于阻塞线程的条件变量，有未到组的线程，那么在该条件变量上等待，进入AQS条件队列
    private final Condition trip = lock.newCondition();
    //参与组的线程数
    private final int parties;
    //当所有的线程都参与到了组中后回调的方法
    private final Runnable barrierCommand;
    //代表当前组
    private Generation generation = new Generation();
    //还未到组的线程数
    private int count;
    
    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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await方法

public int await() throws InterruptedException, BrokenBarrierException {
    try {
        return dowait(false, 0L);
    } catch (TimeoutException toe) {
        throw new Error(toe);
    }
}

private int dowait(boolean timed, long nanos) throws InterruptedException, BrokenBarrierException,
TimeoutException {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        final Generation g = generation;  //保存当前组的Generation快照，避免后面步骤的更新影响到这里的实例
        if (g.broken)
            throw new BrokenBarrierException();
        //组中有中断的线程，干掉组内其他线程并重新开始，此时并没有改变组中的Generation
        if (Thread.interrupted()) {
            breakBarrier();
            throw new InterruptedException();
        }
        int index = --count;
        if (index == 0) {  //最后一个到达组的线程，回调barrierCommand（若正常完成，那么不需要手动调用reset，因为这里调用了nextGeneration）,然后唤醒所有阻塞在条件变量上的线程
            boolean ranAction = false;
            try {
                final Runnable command = barrierCommand;
                if (command != null)
                    command.run(); //执行回调
                ranAction = true;
                nextGeneration(); // 进入下一个组
                return 0;
            } finally {
                // barrierCommand回调方法发生了异常，那么设置broken标志位
                if (!ranAction)
                    breakBarrier();
            }
        }
        // 循环等待最后一个进入组的线程唤醒自己，或者被中断、或者等待超时。
        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 {
                    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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reset方法

public void reset() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        breakBarrier(); // 将所有组内线程唤醒
        nextGeneration();  // 生成下一代
    } finally {
        lock.unlock();
    }
}
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breakBarrier方法

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

private void nextGeneration() {
    trip.signalAll();
    count = parties;
    generation = new Generation(); // 生成了下一代组的实例
}
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