1 Exchanger 是什么
JDK 1.5
开始 JUC 包下提供的 Exchanger
类可用于两个线程之间交换信息。Exchanger
对象可理解为一个包含2个格子的容器,通过调用 exchanger 方法向其中的格子填充信息,当两个格子中的均被填充信息时,自动交换两个格子中的信息,然后将交换的信息返回给调用线程,从而实现两个线程的信息交换。
功能看似简单,但这在某些场景下是很有用处的,例如游戏中两个玩家交换装备;交友软件男女心仪对象匹配。
下面简单模拟下两个玩家交换装备的场景。
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package com.chenpi; import java.util.concurrent.Exchanger; /** * @Description * @Author 陈皮 * @Date 2021/7/11 * @Version 1.0 */ public class ChenPiMain { public static void main(String[] args) throws InterruptedException { Exchanger<String> exchanger = new Exchanger<>(); new Thread(() -> { String str = null ; try { str = exchanger.exchange( "屠龙刀" ); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println( "交易成功," + Thread.currentThread().getName() + "获得" + str); }, "周芷若" ).start(); new Thread(() -> { String str = null ; try { str = exchanger.exchange( "倚天剑" ); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println( "交易成功," + Thread.currentThread().getName() + "获得" + str); }, "张无忌" ).start(); } } |
输出结果如下:
交易成功,张无忌获得屠龙刀
交易成功,周芷若获得倚天剑
2 Exchanger 详解
Exchager
类可用于两个线程之间交换信息,如果一个线程调用了 Exchanger 对象的 exchange 方法之后,会一直阻塞直到另一个线程来和它交换信息,交换之后的信息返回给调用线程,从而实现两个线程的信息交换。
Exchager
底层也是使用到了自旋和 cas
机制。
注意:如果超过两个线程调用同一个 Exchanger
对象 exchange
方法时,结果是不可预计的,只要有2个线程满足条件了,就认为匹配成功并交换信息。而剩下的未能得到配对的线程,则会被阻塞一直等待直到有另一个线程能与它匹配与之配对。
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package com.chenpi; import java.util.concurrent.Exchanger; /** * @Description * @Author 陈皮 * @Date 2021/7/11 * @Version 1.0 */ public class ChenPiMain { public static void main(String[] args) { Exchanger<String> exchanger = new Exchanger<>(); new Thread(() -> { String str = null ; try { str = exchanger.exchange( "屠龙刀" ); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println( "交易成功," + Thread.currentThread().getName() + "获得" + str); }, "周芷若" ).start(); new Thread(() -> { String str = null ; try { str = exchanger.exchange( "倚天剑" ); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println( "交易成功," + Thread.currentThread().getName() + "获得" + str); }, "张无忌" ).start(); new Thread(() -> { String str = null ; try { str = exchanger.exchange( "假的倚天剑" ); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println( "交易成功," + Thread.currentThread().getName() + "获得" + str); }, "成昆" ).start(); } } // 输出结果如下 交易成功,周芷若获得假的倚天剑 交易成功,成昆获得屠龙刀 |
当然,在等待交换信息的线程是可以被中断的,就比如玩家在等待交易过程中,突然玩家下线了,那就应该中断线程等待。
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package com.chenpi; import java.lang.Thread.State; import java.util.ArrayList; import java.util.List; import java.util.concurrent.Exchanger; /** * @Description * @Author 陈皮 * @Date 2021/7/11 * @Version 1.0 */ public class ChenPiMain { public static void main(String[] args) throws InterruptedException { Exchanger<String> exchanger = new Exchanger<>(); List<Thread> threads = new ArrayList<>( 3 ); Thread thread1 = new Thread(() -> { String str = null ; try { str = exchanger.exchange( "屠龙刀" ); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println( "交易成功," + Thread.currentThread().getName() + "获得" + str); }, "周芷若" ); threads.add(thread1); Thread thread2 = new Thread(() -> { String str = null ; try { str = exchanger.exchange( "倚天剑" ); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println( "交易成功," + Thread.currentThread().getName() + "获得" + str); }, "张无忌" ); threads.add(thread2); Thread thread3 = new Thread(() -> { String str = null ; try { str = exchanger.exchange( "假的屠龙刀" ); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println( "交易成功," + Thread.currentThread().getName() + "获得" + str); }, "成昆" ); threads.add(thread3); for (Thread thread : threads) { thread.start(); } // 等待5秒 Thread.sleep( 5000 ); for (Thread thread : threads) { System.out.println(thread.getName() + ":" + thread.getState()); // 如果还在阻塞等待则中断线程 if (thread.getState() == State.WAITING) { thread.interrupt(); } } } } // 输出结果如下 交易成功,张无忌获得屠龙刀 交易成功,周芷若获得倚天剑 周芷若:TERMINATED 张无忌:TERMINATED 成昆:WAITING 交易成功,成昆获得 null java.lang.InterruptedException at java.util.concurrent.Exchanger.exchange(Exchanger.java: 568 ) at com.chenpi.ChenPiMain.lambda$main$ 2 (ChenPiMain.java: 47 ) at java.lang.Thread.run(Thread.java: 748 ) |
上面演示的是线程如果等不到另一个线程和它交换信息,则会一直等待下去。其实 Exchanger
还可以设置等待指定时间。比如系统设置玩家交换装备匹配时间为60秒,如果超出时间则终止交易。
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package com.chenpi; import java.util.concurrent.Exchanger; import java.util.concurrent.TimeUnit; import java.util.concurrent.TimeoutException; /** * @Description * @Author 陈皮 * @Date 2021/7/11 * @Version 1.0 */ public class ChenPiMain { public static void main(String[] args) { Exchanger<String> exchanger = new Exchanger<>(); new Thread(() -> { try { // 超时时间设置为5秒 String str = exchanger.exchange( "屠龙刀" , 5 , TimeUnit.SECONDS); System.out.println( "交易成功," + Thread.currentThread().getName() + "获得" + str); } catch (TimeoutException e) { System.out.println( "交易超时!" ); e.printStackTrace(); } catch (InterruptedException e) { System.out.println( "交易异常终止" ); e.printStackTrace(); } }, "周芷若" ).start(); } } // 输出结果如下 交易超时! java.util.concurrent.TimeoutException at java.util.concurrent.Exchanger.exchange(Exchanger.java: 626 ) at com.chenpi.ChenPiMain.lambda$main$ 0 (ChenPiMain.java: 22 ) at java.lang.Thread.run(Thread.java: 748 ) |
3 Exchanger 应用
Exchager
在遗传算法和管道设计等应用中是非常有用的。比如两个线程之间交换缓冲区,填充缓冲区的线程在需要时从另一个线程获得一个刚清空的缓冲区,并将填充的缓冲区传递给清空缓冲区的线程。
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package com.chenpi; import java.awt.image.DataBuffer; import java.util.concurrent.Exchanger; /** * @Description * @Author 陈皮 * @Date 2021/7/11 * @Version 1.0 */ public class ChenPiMain { Exchanger<DataBuffer> exchanger = new Exchanger<DataBuffer>(); DataBuffer initialEmptyBuffer = ... a made-up type DataBuffer initialFullBuffer = ... class FillingLoop implements Runnable { public void run() { DataBuffer currentBuffer = initialEmptyBuffer; try { while (currentBuffer != null ) { addToBuffer(currentBuffer); if (currentBuffer.isFull()) { currentBuffer = exchanger.exchange(currentBuffer); } } } catch (InterruptedException ex) { ...handle ...} } } class EmptyingLoop implements Runnable { public void run() { DataBuffer currentBuffer = initialFullBuffer; try { while (currentBuffer != null ) { takeFromBuffer(currentBuffer); if (currentBuffer.isEmpty()) { currentBuffer = exchanger.exchange(currentBuffer); } } } catch (InterruptedException ex) { ...handle ...} } } void start() { new Thread( new FillingLoop()).start(); new Thread( new EmptyingLoop()).start(); } } |
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原文链接:https://www.tuicool.com/articles/yiiaumV