读Exchanger源码

//用于线程间交换数据
public V exchange(V x) throws InterruptedException {
        if (!Thread.interrupted()) {
            Object v = doExchange((x == null) ? NULL_ITEM : x, false, 0);
            if (v == NULL_ITEM)
                return null;
            if (v != CANCEL)
                return (V)v;
            Thread.interrupted(); // Clear interrupt status on IE throw
        }
        throw new InterruptedException();
    }

 private Object doExchange(Object item, boolean timed, long nanos) {
        Node me = new Node(item);                 // Create in case occupying
        //计算hash值
	int index = hashIndex();                  // Index of current slot
        int fails = 0;                            // Number of CAS failures

        for (;;) {
            Object y;                             // Contents of current slot
            Slot slot = arena[index];
            if (slot == null) 
	        //创建slot
                createSlot(index);     
            //如果有值
            else if ((y = slot.get()) != null &&  // Try to fulfill
                     slot.compareAndSet(y, null)) {
                Node you = (Node)y;               // Transfer item
                if (you.compareAndSet(null, item)) {
		    //唤醒线程
                    LockSupport.unpark(you.waiter);
                    return you.item;
                }                                 // Else cancelled; continue
            }

            else if (y == null &&                 // Try to occupy
                     slot.compareAndSet(null, me)) {
                if (index == 0)                   // Blocking wait for slot 0
                    return timed ?
		     //阻塞当前线程
                        awaitNanos(me, slot, nanos) :
			await(me, slot);
                Object v = spinWait(me, slot);    // Spin wait for non-0
                if (v != CANCEL)
                    return v;
                me = new Node(item);              // Throw away cancelled node
                int m = max.get();
                if (m > (index >>>= 1))           // Decrease index
                    max.compareAndSet(m, m - 1);  // Maybe shrink table
            }
            else if (++fails > 1) {               // Allow 2 fails on 1st slot
                int m = max.get();
                if (fails > 3 && m < FULL && max.compareAndSet(m, m + 1))
                    index = m + 1;                // Grow on 3rd failed slot
                else if (--index < 0)
                    index = m;                    // Circularly traverse
            }
        }
    }


private void createSlot(int index) {
        // Create slot outside of lock to narrow sync region
        Slot newSlot = new Slot();
        Slot[] a = arena;
        synchronized (a) {
            if (a[index] == null)
                a[index] = newSlot;
        }
    }


private static Object spinWait(Node node, Slot slot) {
        int spins = SPINS;
        for (;;) {
            Object v = node.get();
            if (v != null)
                return v;
            else if (spins > 0)
                --spins;
            else
                tryCancel(node, slot);
        }
    }

//尝试取消
private static boolean tryCancel(Node node, Slot slot) {
        if (!node.compareAndSet(null, CANCEL))
            return false;
        if (slot.get() == node) // pre-check to minimize contention
            slot.compareAndSet(node, null);
        return true;
    }


//自旋挂起当前线程
private static Object await(Node node, Slot slot) {
        Thread w = Thread.currentThread();
        int spins = SPINS;
        for (;;) {
            Object v = node.get();
            if (v != null)
                return v;
            else if (spins > 0)                 // Spin-wait phase
                --spins;
            else if (node.waiter == null)       // Set up to block next
                node.waiter = w;
            else if (w.isInterrupted())         // Abort on interrupt
                tryCancel(node, slot);
            else                                // Block
                LockSupport.park(node);
        }
    }

//自选等待挂起当前线程指定时间
private Object awaitNanos(Node node, Slot slot, long nanos) {
        int spins = TIMED_SPINS;
        long lastTime = 0;
        Thread w = null;
        for (;;) {
            Object v = node.get();
            if (v != null)
                return v;
            long now = System.nanoTime();
            if (w == null)
                w = Thread.currentThread();
            else
                nanos -= now - lastTime;
            lastTime = now;
            if (nanos > 0) {
                if (spins > 0)
                    --spins;
                else if (node.waiter == null)
                    node.waiter = w;
                else if (w.isInterrupted())
                    tryCancel(node, slot);
                else
                    LockSupport.parkNanos(node, nanos);
            }
            else if (tryCancel(node, slot) && !w.isInterrupted())
                return scanOnTimeout(node);
        }
    }

    /**
    总结：该类的实现,是当一个线程过来先查看对应的slot中是否已经有数据在等待交换,
    没有则挂起当前线程等待交换数据。有则将数据交换然后唤醒之前等待交换的线程。
    */