Java并发编程中的CyclicBarrier使用解析

private final ReentrantLock lock = new ReentrantLock();
/** Condition to wait on until tripped */
private final Condition trip = lock.newCondition();
/** The number of parties */
private final int parties;
/* The command to run when tripped */
private final Runnable barrierCommand;
/** The current generation */
private Generation generation = new Generation();
private int count;
private static class Generation {
    boolean broken = false;
}
public CyclicBarrier(int parties, Runnable barrierAction) {
    if (parties <= 0) throw new IllegalArgumentException();
    this.parties = parties;
    this.count = parties;
    this.barrierCommand = barrierAction;
}
public CyclicBarrier(int parties) {
    this(parties, null);
}

public int await() throws InterruptedException, BrokenBarrierException {
    try {
        return dowait(false, 0L);
    } catch (TimeoutException toe) {
        throw new Error(toe); // cannot happen
    }
}
private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();//1、获取锁
        try {
            final Generation g = generation;
            //2、是否已更新换代
            if (g.broken)
                throw new BrokenBarrierException();
            //3、是否被中断
            if (Thread.interrupted()) {
                //4、跳出栅栏
                breakBarrier();
                throw new InterruptedException();
            }
            //5、当前线程到达栅栏，计数减1
            int index = --count;
            //6、所有线程都到达了栅栏
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    //7、获取回调线程
                    final Runnable command = barrierCommand;
                    if (command != null)
                        //8、如果生成时传入了回调线程，启动回调线程
                        command.run();
                    ranAction = true;
                    //9、重置，开始下一代
                    nextGeneration();
                    return 0;
                } finally {
                    //10、如果ranAction为false，说明回调线程执行出错了
                    if (!ranAction)
                        //11、跳出栅栏
                        breakBarrier();
                }
            }
            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)
                        //12、如果不需要超时等待，则立即调用Condition的await
                        trip.await();
                    else if (nanos > 0L)
                        //13、如果需要超时等待
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {//14、如果等待时被中断了
                    //15、如果等待前和现在处于同一代，并且没有被跳出过
                    if (g == generation && ! g.broken) {
                        //16、跳出栅栏
                        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.
                        //17、不属于本代的线程，补中断
                        Thread.currentThread().interrupt();
                    }
                }
                //18、被其他线程唤醒，此时本代已经被破坏
                if (g.broken)
                    throw new BrokenBarrierException();
                //19、不属于本代，又不是因为别的线程中断
                if (g != generation)
                    return index;
                //20、如果等待超时，跳出栅栏
                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }
private void breakBarrier() {
        //1、更新换代
        generation.broken = true;
        //2、重置计数
        count = parties;
        //3、唤醒所有在等待通过栅栏的线程
        trip.signalAll();
}
private void nextGeneration() {
        // signal completion of last generation
        //1、唤醒所有在等待通过栅栏的线程
        trip.signalAll();
        // set up next generation
        //2、重置计数
        count = parties;
        //3、重新生成代际，此时broken为false
        generation = new Generation();
}

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