美文网首页
CyclicBarrier

CyclicBarrier

作者: Pillar_Zhong | 来源:发表于2019-06-05 17:54 被阅读0次
    CyclicBarrier.png

    Constructor

    public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        // 参与方数目,如果都到达,那么放开限制
        this.parties = parties;
        this.count = parties;
        // 当限制放开,会执行barrierCommand
        this.barrierCommand = barrierAction;
    }
    

    await

    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;
        // 每一个参与人要进入wait流程的第一要素是,首先要拿锁
        lock.lock();
        try {
            final Generation g = generation;
    
            // 如果当前generation不可用,报错 
            if (g.broken)
                throw new BrokenBarrierException();
    
            // 中断处理
            if (Thread.interrupted()) {
                // 唤醒所有人并设置generation为broken
                breakBarrier();            
                throw new InterruptedException();
            }
            
            // 到这里说明该参与人已经就位,计数减一
            int index = --count;
            // 如果计数归零,说明全部参与人都已经就位
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        // 如果有设定barrierCommand,那么开始执行该线程
                        command.run();
                    ranAction = true;
                    // 前面是上个阶段的barrier已经放开,这里重新再复用barrier
                    nextGeneration();
                    return 0;
                } finally {
                    if (!ranAction)
                        breakBarrier();
                }
            }
    
            // 到这里,说明参与人没有全部就位,那么进入wait模式,等待人员到齐
            for (;;) {
                try {
                    // 都是等待condition,无非一个有时间限制,一个无限等待直到被唤醒或中断
                    if (!timed)
                        trip.await();
                    else if (nanos > 0L)
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    // 如果在等待的过程中,人员都没到齐,却发生了中断,且并没有broken
                    // 那么没办法提前放开barrier
                    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.
                        // 保存中断状态,或换代,或broken
                        Thread.currentThread().interrupt();
                    }
                }
    
                // 唤醒或中断后的处理。
                
                // broken直接报错
                if (g.broken)
                    throw new BrokenBarrierException();
    
                // 如果已经换代,那么说明一切正常,返回需要的参与人数量
                if (g != generation)
                    return index;
    
                // 等待超时处理
                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }
    

    Other

    nextGeneration

    // 这是复用CyclicBarrier的关键,重新生成generation
    private void nextGeneration() {
        // signal completion of last generation
        // 唤醒所有wait中的参与人
        trip.signalAll();
        // set up next generation
        count = parties;
        generation = new Generation();
    }
    

    breakBarrier

    private void breakBarrier() {
        // 设置generation的状态为broken,代表当前的barrier不可用
        generation.broken = true;
        count = parties;
        // 既然这个barrier不可用,那么参与人wait也就没有意义,全部都唤醒
        trip.signalAll();
    }
    

    相关文章

      网友评论

          本文标题:CyclicBarrier

          本文链接:https://www.haomeiwen.com/subject/yamsxctx.html