Exchanger类详解

　　Exchanger并发辅助类，允许在并发任务之间交换数据。具体来说Exchanger类在两个线程之间定义同步点。当两个线程到达同步点时，它们交换数据结构。需要注意的是Exchanger类只能同步两个线程。

　　内存一致性效果：对于通过Exchanger成功交换对象的每对线程，每个线程中在exchanger()之前的操作 happen-before从另一线程中相应的exchanger()返回的后续操作。

下面用一对一的生产者-消费者例子进行说明。

　　生产者代码如下：

public class Producer implements Runnable {
    private List<String> buffer;
    private Exchanger<List<String>> exchanger;
    public Producer(List<String> buffer, Exchanger<List<String>> exchanger) {
        this.buffer = buffer;
        this.exchanger = exchanger;
    }
    @Override
    public void run() {
        int cycle=1;
        for (int i=0; i<10; i++) {
            System.out.println("Producer:Cycle "+cycle);
            for (int j = 1; j <=10; j++) {
                String msg="Event "+(i*10+j);
                System.out.println("Producer: "+msg);
                buffer.add(msg);
            }
            try {
                buffer=exchanger.exchange(buffer);//和消费者交换数据结构
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Producer size: "+buffer.size());
            cycle++;
        }
    }

}

　　消费者代码：

public class Consumer implements Runnable {
    private List<String> buffer;
    private Exchanger<List<String>> exchanger;
    public Consumer(List<String> buffer, Exchanger<List<String>> exchanger) {
        this.buffer = buffer;
        this.exchanger = exchanger;
    }
    @Override
    public void run() {
        int cycle=1;
        for (int i=0; i<=9; i++) {
            System.out.println("Consumer: Cycle "+cycle);
            try {
                buffer=exchanger.exchange(buffer);//he生产者交换数据结构
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("Consumer size: "+buffer.size());
            for (int j = 1; j <=10; j++) {
                String msg=buffer.get(0);
                System.out.println("Consumer: "+msg);
                buffer.remove(0);
            }
            cycle++;
        }
    }
}

　　运行：

public class Core {
    public static void main(String[] args) {
        List<String> buffer1=new ArrayList<String>();
        List<String> buffer2=new ArrayList<String>();
        Exchanger<List<String>> exchanger=new Exchanger<List<String>>();
        Producer producer=new Producer(buffer1, exchanger);
        Consumer consumer=new Consumer(buffer2, exchanger);
        Thread thread1=new Thread(producer);
        Thread thread2=new Thread(consumer);
        thread1.start();
        thread2.start();
    }
}