Java并发编程-同步辅助类之Exchanger

Exchanger是自jdk1.5起开始提供的工具套件，一般用于两个工作线程之间交换数据。在本文中我将采取由浅入深的方式来介绍分析这个工具类。首先我们来看看官方的api文档中的叙述：

A synchronization point at which threads can pair and swap elements within pairs. Each thread presents some object on entry to the exchange method, matches with a partner thread, and receives its partner’s object on return. An Exchanger may be viewed as a bidirectional form of a SynchronousQueue. Exchangers may be useful in applications such as genetic algorithms and pipeline designs.
在以上的描述中，有几个要点：

• 此类提供对外的操作是同步的；
• 用于成对出现的线程之间交换数据；
• 可以视作双向的同步队列；
• 可应用于基因算法、流水线设计等场景。

构造方法：
public Exchanger()
public V exchange(V x) throws InterruptedException
public V exchange(V x, long timeout, TimeUnit unit) throws InterruptedException, TimeoutException

从官方的javadoc可以知道，当一个线程到达exchange调用点时，如果它的伙伴线程此前已经调用了此方法，那么它的伙伴会被调度唤醒并与之进行对象交换，然后各自返回。如果它的伙伴还没到达交换点，那么当前线程将会被挂起，直至伙伴线程到达——完成交换正常返回；或者当前线程被中断——抛出中断异常；又或者是等候超时——抛出超时异常。

使用例子
将Exchanger运用到生产者、消费者模式当中如下：

package concurrent;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.Exchanger;

public class ExchangerTest {

    public static void main(String[] args) {
        Exchanger<List<Integer>> exchanger = new Exchanger<>();
        new Consumer(exchanger).start();
        new Producer(exchanger).start();
    }

}

class Producer extends Thread {
    List<Integer> list = new ArrayList<>();
    Exchanger<List<Integer>> exchanger = null;
    public Producer(Exchanger<List<Integer>> exchanger) {
        super();
        this.exchanger = exchanger;
    }
    @Override
    public void run() {
        Random rand = new Random();
        for(int i=0; i<10; i++) {
            list.clear();
            list.add(rand.nextInt(10000));
            list.add(rand.nextInt(10000));
            list.add(rand.nextInt(10000));
            list.add(rand.nextInt(10000));
            list.add(rand.nextInt(10000));
            try {
                list = exchanger.exchange(list);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}

class Consumer extends Thread {
    List<Integer> list = new ArrayList<>();
    Exchanger<List<Integer>> exchanger = null;
    public Consumer(Exchanger<List<Integer>> exchanger) {
        super();
        this.exchanger = exchanger;
    }
    @Override
    public void run() {
        for(int i=0; i<10; i++) {
            try {
                list = exchanger.exchange(list);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            System.out.print(list.get(0)+", ");
            System.out.print(list.get(1)+", ");
            System.out.print(list.get(2)+", ");
            System.out.print(list.get(3)+", ");
            System.out.println(list.get(4)+", ");
        }
    }
}