JUC的CyclicBarrier怎么实现

public class CyclicBarrier {

    /** 支撑 CyclicBarrier 的重入锁 */
    private final ReentrantLock lock = new ReentrantLock();
    /** 条件队列，已经到达屏障的线程会在条件队列中等待其它线程 */
    private final Condition trip = lock.newCondition();
    /** 参与的线程数 */
    private final int parties;
    /** 当所有线程都到达屏障时的回调函数 */
    private final Runnable barrierCommand;
    /** 当前年代对象 */
    private Generation generation = new Generation();
    /** 当前剩余未完成的线程数 */
    private int count;

    // ... 省略方法定义

}

private static class Generation {
    boolean broken = false;
}

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;
}

public void reset() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        this.breakBarrier();   // break the current generation
        this.nextGeneration(); // start a new generation
    } finally {
        lock.unlock();
    }
}

private void breakBarrier() {
    // 标识当前屏障被打破
    generation.broken = true;
    // 重置 count 字段值
    count = parties;
    // 唤醒所有等待的线程
    trip.signalAll();
}

private void nextGeneration() {
    // 唤醒所有等待的线程
    trip.signalAll();
    // 重置 count 值
    count = parties;
    generation = new Generation();
}

private int dowait(boolean timed, long nanos) throws InterruptedException, BrokenBarrierException, TimeoutException {
    final ReentrantLock lock = this.lock;
    // 加锁
    lock.lock();
    try {
        // 获取当前年代信息
        final Generation g = generation;

        // 当前屏障被打破，抛出异常
        if (g.broken) {
            throw new BrokenBarrierException();
        }

        // 当前线程被中断，打破屏障，并唤醒所有等待的线程
        if (Thread.interrupted()) {
            this.breakBarrier();
            throw new InterruptedException();
        }

        int index = --count;
        // 如果 count 值为 0，说明所有的线程都已经到达屏障
        if (index == 0) {  // tripped
            boolean ranAction = false;
            try {
                // 如果设置了回调，则执行
                final Runnable command = barrierCommand;
                if (command != null) {
                    command.run();
                }
                ranAction = true;
                // 唤醒所有等待的线程，并重置屏障
                this.nextGeneration();
                return 0;
            } finally {
                // 如果执行回调异常
                if (!ranAction) {
                    this.breakBarrier();
                }
            }
        }

        // count 值不为 0，说明存在还未到达屏障的线程，则进入条件队列等待

        // loop until tripped, broken, interrupted, or timed out
        for (; ; ) {
            try {
                if (!timed) {
                    // 进入条件队列等待
                    trip.await();
                } else if (nanos > 0L) {
                    // 进入条件队列超时等待
                    nanos = trip.awaitNanos(nanos);
                }
            } catch (InterruptedException ie) {
                // 当前线程被中断，响应中断
                if (g == generation && !g.broken) {
                    this.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();
                }
            }

            // 屏障被打破
            if (g.broken) {
                throw new BrokenBarrierException();
            }

            // 当前 CyclicBarrier 已经被重置
            if (g != generation) {
                return index;
            }

            // 等待超时
            if (timed && nanos <= 0L) {
                this.breakBarrier();
                throw new TimeoutException();
            }
        }
    } finally {
        // 释放锁
        lock.unlock();
    }
}