java併發編程（十一）之線程交換器Exchanger

一、定義

每個線程將條目上的某個方法呈現給 exchange 方法，與夥伴線程進行匹配，並且在返回時接收其夥伴的對象。Exchanger 可能被視爲 SynchronousQueue 的雙向形式。Exchanger 可能在應用程序（比如遺傳算法和管道設計）中很有用。

內存一致性效果：對於通過 Exchanger 成功交換對象的每對線程，每個線程中在 exchange() 之前的操作 happen-before 從另一線程中相應的 exchange() 返回的後續操作。（內存一致性效果被稱爲happen-before）,簡單的例子：當一個線程進行寫入操作，另一個線程進行讀取操作，保證一個線程寫入的結果對另一個線程的讀取是可視的。

二、用法

package cn.itcast.heima2;
import java.util.concurrent.Exchanger;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ExchangerTest {

	public static void main(String[] args) {
		ExecutorService service = Executors.newCachedThreadPool();
		final Exchanger<String> exchanger = new Exchanger<>();
		service.execute(new Runnable(){
			public void run() {
				try {				
					String data1 = "恭喜發財！";
					System.out.println("線程" + Thread.currentThread().getName() + 
					"正在把數據:" + data1 +"換出去");
					Thread.sleep((long)(Math.random()*10000));
					String data2 = exchanger.exchange(data1);
					System.out.println("線程" + Thread.currentThread().getName() + 
					"換回的數據爲:" + data2);
				}catch(Exception e){
					
				}
			}	
		});
		service.execute(new Runnable(){
			public void run() {
				try {				

					String data1 = "乖,給你一個紅包！";
					System.out.println("線程" + Thread.currentThread().getName() + 
					"正在把數據:" + data1 +"換出去");
					Thread.sleep((long)(Math.random()*10000));					
					String data2 = exchanger.exchange(data1);
					System.out.println("線程" + Thread.currentThread().getName() + 
					"換回的數據爲:" + data2);
				}catch(Exception e){
					
				}				
			}	
		});		
	}
}

三、JDK中的例子

使用 Exchanger 在線程間交換緩衝區，因此，在需要時，填充緩衝區的線程獲取一個新騰空的緩衝區，並將填滿的緩衝區傳遞給騰空緩衝區的線程。

class FillAndEmpty {
   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();
   }
  }

四、API