concurrent-7-AQS-CountDownLatch，CyclicBarrier

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本文深入解析了CountDownLatch和CyclicBarrier两种并发控制组件的工作原理。CountDownLatch通过计数器实现线程间的同步等待，而CyclicBarrier则允许一组线程相互等待直到到达某个公共屏障点。文中详细展示了这两种组件的核心方法实现，包括CountDownLatch的tryAcquireShared和tryReleaseShared方法，以及CyclicBarrier的dowait方法。

CountDownLatch

CountDownLatch#tryAcquireShared

   protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;   //获取状态是不是达到条件
        }

CountDownLatch#tryReleaseShared

protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();  //获取状态
                if (c == 0)     //表示已达到条件
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))  //循环cas操作释放资源
                    return nextc == 0; //如果为true 需要唤醒等待的await节点
            }
        }

CyclicBarrier

CyclicBarrier#dowait

private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final Generation g = generation;

            if (g.broken)
                throw new BrokenBarrierException();

            if (Thread.interrupted()) {
                breakBarrier();   //唤醒条件等待队列，设置broken = true  重置资源数量
                throw new InterruptedException();
            }

            int index = --count;    //如果为0，则开启栅栏
            if (index == 0) {  // tripped
                boolean ranAction = false;
                try {
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();   //执行栅栏开启后的方法
                    ranAction = true;
                    nextGeneration();   //开启下一个栅栏循环
                    return 0;
                } finally {
                    if (!ranAction)   //如果失败重置
                        breakBarrier();
                }
            }
            //如果index ＞０　
            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    if (!timed)  //如果没有时间限制则一直等待被唤醒
                        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)  //开启了下一个栅栏则return
                    return index;

                if (timed && nanos <= 0L) {  //超过了时间限制，则抛出异常
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }