1 Exchange概述
Exchanger可以在两个线程之间交换数据，只能是2个线程，他不支持更多的线程之间互换数据。
当线程A调用Exchange对象的exchange()方法后，他会陷入阻塞状态，直到线程B也调用了exchange()方法，然后以线程安全的方式交换数据，之后线程A和B继续运行。

如下示例：
public class ExchangeThread {

	private static AtomicInteger count = new AtomicInteger(0);
	
	static class ProducerExchange {
		private Exchanger<Integer> exchanger;
		
		private Thread thread = new Thread() {
			public void run() {
				while(!this.isInterrupted()) {
					try {
						Integer tmpCount = count.getAndAdd(2);
						System.out.println("Producer before : "+tmpCount);
						tmpCount = exchanger.exchange(tmpCount);
						System.out.println("Producer after : "+tmpCount);
						Thread.sleep(2000);
					} catch (InterruptedException e) {
						System.out.println("线程中断");
						this.isInterrupted();// 重新设置中断标志
					} finally {
						System.out.println("===========================");
					}
				}
			};
		};

		public void setExchanger(Exchanger<Integer> exchanger) {
			this.exchanger = exchanger;
		}
		
		public void start() {
			thread.start();
		}
	}
	static class ConsumerExchange {
		private Exchanger<Integer> exchanger;
		
		private Thread thread = new Thread() {
			public void run() {
				while(!this.isInterrupted()) {
					try {
						Thread.sleep(4000);
						Integer tmpCount = count.decrementAndGet();
						System.out.println("Consumer before : " + tmpCount);
						tmpCount = exchanger.exchange(tmpCount);
						System.out.println("Consumer after : "+ tmpCount);
					} catch (InterruptedException e) {
						this.isInterrupted();
					}
				}
			};
		};
		
		public void setExchanger(Exchanger<Integer> exchanger) {
			this.exchanger = exchanger;
		}
		
		public void start() {
			thread.start();
		}
	}
	
	public static void main(String[] args) {
		Exchanger<Integer> exchanger = new Exchanger<Integer>();
		ProducerExchange producerExchange = new ProducerExchange();
		ConsumerExchange consumerExchange = new ConsumerExchange();
		producerExchange.setExchanger(exchanger);
		consumerExchange.setExchanger(exchanger);
		producerExchange.start();
		consumerExchange.start();
	}
}



2 Exchanger源码分析：
 private Object doExchange(Object item, boolean timed, long nanos) {
        Node me = new Node(item);                 
        int index = hashIndex();                  //根据thread id计算出自己要去的那个交易位置（slot)
        int fails = 0;                            

        for (;;) {
            Object y;                             
            Slot slot = arena[index];
            if (slot == null)                    
                createSlot(index);     //slot = null，创建一个slot，然后会回到for循环，再次开始
            else if ((y = slot.get()) != null &&  //slot里面有人等着(有Node)，则尝试和其交换
                     slot.compareAndSet(y, null)) { //关键点1：slot清空，Node拿出来，俩人在Node里面交互。把Slot让给后面的人，做交互地点
                Node you = (Node)y;               
                if (you.compareAndSet(null, item)) {//把Node里面的东西，换成自己的
                    LockSupport.unpark(you.waiter); //唤醒对方
                    return you.item; //自己把对方的东西拿走
                } //关键点2：如果你运气不好，在Node里面要交换的时候，被另一个线程抢了，回到for循环，重新开始                          
            }
            else if (y == null &&                 //slot里面为空(没有Node)，则自己把位置占住
                     slot.compareAndSet(null, me)) {
                if (index == 0)                   //如果是0这个位置，自己阻塞，等待别人来交换
                    return timed? awaitNanos(me, slot, nanos): await(me, slot);
                Object v = spinWait(me, slot);    //不是0这个位置，自旋等待
                if (v != CANCEL)  //自旋等待的时候，运气好，有人来交换了，返回
                    return v;
                me = new Node(item);     //自旋的时候，没人来交换。走执行下面的，index减半，挪个位置，重新开始for循环    
                int m = max.get();
                if (m > (index >>>= 1))           
                    max.compareAndSet(m, m - 1); 
            }
            else if (++fails > 1) { //失败 case1: slot有人，要交互，但被人家抢了  case2: slot没人，自己要占位置，又被人家抢了    
                int m = max.get();
                if (fails > 3 && m < FULL && max.compareAndSet(m, m + 1))
                    index = m + 1;   //3次匹配失败，把index扩大，再次开始for循环             
                else if (--index < 0)
                    index = m;                   
            }
        }
    }