Exchanger学习

1. Java并发新构件之Exchanger
2. JDK API

Exchaner 介绍

JDK API 解释

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.

Sample Usage: Here are the highlights of a class that uses an Exchanger to swap buffers between threads so that the thread filling the buffer gets a freshly emptied one when it needs it, handing off the filled one to the thread emptying the buffer.

我的翻译：

Exchanger可以看成是一个同步点，在这个同步点上两个线程可以结对并且交换各自的数据。每个线程都可传入某个对象给exchange方法，然后与对应的伙伴线程匹配，并且接收伙伴线程交换的某个对象。Exchanger可以看成是一个双向的SynchronousQueue队列。Exchanger可以被用在诸如遗传算法和通道设计等应用里。

张孝祥老师的比喻

Exchaner好比两个毒贩要进行交易，一手交money，一手交drug，不管是谁先到接头地点后，就处于等待状态；当另外一个方也达到接头地点(所谓到达接头地点，也就是到达了准备接头的状态)时，两者的数据就立即交换了，然后就可以各忙各的了。

我的理解

Exchanger的作用：两个结对线程之间交换数据的工具类。

简单使用

import java.util.concurrent.Exchanger;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * Created by 58 on 2017-7-11.
 */
public class ExchangerTest1 {

    private static final Exchanger<String> exchanger = new Exchanger<>();

    private static ExecutorService threadPool = Executors.newFixedThreadPool(3) ;

    public static void main(String[] args) {

        threadPool.execute(() -> {
            String A = "A" ;
            try {
                System.out.println("A--->" + exchanger.exchange(A)) ;
            } catch (Exception e) {
                e.printStackTrace();
            }

        });
        threadPool.execute(() -> {

            String B = "B" ;
            try {
                System.out.println("B--->" + exchanger.exchange(B));
            } catch (Exception e) {
                e.printStackTrace();
            }

        });
        threadPool.execute(() -> {

            String C = "C" ;
            try {
                System.out.println("C--->" + exchanger.exchange(C));
            } catch (Exception e) {
                e.printStackTrace();
            }

        });
    }
}

应用场景

一个任务在创建对象，而这些对象的生产代价很高，另一个任务在消费这些对象。通过这种方式，可以有很多的对象被创建的同时被消费。

import java.util.List;
import java.util.concurrent.*;

public class ExchangerDemo {
    static int size = 10;
    static int delay = 5; //秒
    public static void main(String[] args) throws Exception {
        ExecutorService exec = Executors.newCachedThreadPool();
        // 这个特定的List变体允许列表在被遍历的时候调用remove()方法，而不会抛出ConcurrentModifiedException异常
        List<Fat> producerList = new CopyOnWriteArrayList<>();
        List<Fat> consumerList = new CopyOnWriteArrayList<>();
        Exchanger<List<Fat>> exchanger = new Exchanger<>();
        exec.execute(new ExchangerProducer(exchanger, producerList));
        exec.execute(new ExchangerConsumer(exchanger, consumerList));
        TimeUnit.SECONDS.sleep(delay);
        exec.shutdownNow();
    }
}

class ExchangerProducer implements Runnable {
    private List<Fat> holder;
    private Exchanger<List<Fat>> exchanger;
    public ExchangerProducer(Exchanger<List<Fat>> exchanger, List<Fat> holder) {
        this.exchanger = exchanger;
        this.holder = holder;
    }
    @Override
    public void run() {
        try {
            while(!Thread.interrupted()) {
                //生产对象
                for (int i = 0;i < ExchangerDemo.size; i++) {
                    holder.add(new Fat());
                }
                //生产完毕后 等待进行交换
                holder = exchanger.exchange(holder);
            }
        } catch (InterruptedException e) {
        }
        System.out.println("Producer stopped.");
    }
}

class ExchangerConsumer implements Runnable {
    private List<Fat> holder;
    private Exchanger<List<Fat>> exchanger;
    private volatile Fat value;
    private static int num = 0;
    public ExchangerConsumer(Exchanger<List<Fat>> exchanger, List<Fat> holder) {
        this.exchanger = exchanger;
        this.holder = holder;
    }
    @Override
    public void run() {
        try {
            while(!Thread.interrupted()) {
                //等待交换
                holder = exchanger.exchange(holder);
                //取到生产者生产的数据 开始模拟消费的场景
                for (Fat x : holder) {
                    num ++;
                    value = x;
                    //消费 在循环内删除元素，这对于CopyOnWriteArrayList是没有问题的
                    holder.remove(x);
                }
                if (num % 10000 == 0) {
                    System.out.println("Exchanged count=" + num);
                }
            }
        } catch (InterruptedException e) {

        }
        System.out.println("Consumer stopped. Final value: " + value);
    }
}

class Fat {
    private volatile double d;
    private static int counter = 1;
    private final int id = counter++;
    public Fat() {
        //执行一段耗时的操作
        for (int i = 1; i<10000; i++) {
            d += (Math.PI + Math.E) / (double)i;
        }
    }
    public String toString() {return "Fat id=" + id;}
}