Java并发-CyclicBarrier

CyclicBarrier

它允许一组线程互相等待，知道到达某个公共屏障点。barrier在释放等待线程后可以重用。
CyclicBarrier的内部是使用重入锁ReentrantLock和Condition。

    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;
    }

parties表示拦截线程的数量。
barrierAction为CyclicBarrier接受的Runnable命令，用于在线程到达屏障时，优先执行barrierAction，用于处理更加复杂的业务场景。

await

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

await()方法内部调用dowait()方法

private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
        //分代
            final Generation g = generation;
        //当前generation“已损坏“，抛出BrokenBarrierException
            if (g.broken)
                throw new BrokenBarrierException();
        //如果线程中断，终止CyclicBarrier
            if (Thread.interrupted()) {
                breakBarrier();
                throw new InterruptedException();
            }
        //如果count==0，表示所有线程都已经到barrier，触发Runnable任务
            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();
        }
    }

如果该线程不是到达的最后一个线程，则他会一直处于等待状态，除非发生以下情况：
1.最后一个线程到达，即index == 0
2.超出了指定时间（超时等待）
3.其他的某个线程中断当前线程
4.其他的某个线程中断另一个等待的线程
5.其他的某个线程在等待barrier超时
6.其他的某个线程在此barrier调用reset()方法。reset()方法用于将屏障重置为初始状态

适用场景

用于多线程计算数据，最后合并计算结果的场景