5 分鐘掌握 JDK 源碼之 CyclicBarrier

CyclicBarrier

實現所有線程都到達某一點之後開始執行

初始化

private static class Generation {
    // 爲 true 的情況
    // 1. 超時
    // 2. 被中斷
    // 3. 運行拋出異常
    // 4. 被重置
    boolean broken = false;
}


private final ReentrantLock lock = new ReentrantLock();
private final Condition trip = lock.newCondition();
private final int parties;

// 當 count 減少到 0 的時候。
private final Runnable barrierCommand;
private Generation generation = new Generation();

// 總數
private int count;

await

private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
        TimeoutException {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        final Generation g = generation;
        // 四種情況
        if (g.broken)
            throw new BrokenBarrierException();

        // 中斷
        if (Thread.interrupted()) {
            breakBarrier();
            throw new InterruptedException();
        }

        int index = --count;
        if (index == 0) {  // tripped
            boolean ranAction = false;
            try {
                final Runnable command = barrierCommand;
                // 執行hook
                if (command != null)
                    command.run();
                ranAction = true;
                nextGeneration();
                return 0;
            } finally {
                if (!ranAction)
                    breakBarrier();
            }
        }

        // loop until tripped, broken, interrupted, or timed out
        for (;;) {
            try {
                // 沒有設置超時，一直阻塞
                if (!timed)
                    trip.await();
                    // 設置了超時，等待 nanos 納秒
                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();
                }
            }

            // reset() 或 中斷
            if (g.broken)
                throw new BrokenBarrierException();

            // reset() 或 count 爲 0
            if (g != generation)
                return index;

            // 超時
            if (timed && nanos <= 0L) {
                breakBarrier();
                throw new TimeoutException();
            }
        }
    } finally {
        lock.unlock();
    }
}

其他函數都很簡單，沒分析的必要

總結

CyclicBarrier 整個實現還是比較好理解的。有鎖的基本知識即可。

思考題

爲什麼需要 Generation ？