Exchanger学习
Exchaner 介绍
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;}
}
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