java并发编程（十一）之线程交换器Exchanger

一、定义

每个线程将条目上的某个方法呈现给 exchange 方法，与伙伴线程进行匹配，并且在返回时接收其伙伴的对象。Exchanger 可能被视为 SynchronousQueue 的双向形式。Exchanger 可能在应用程序（比如遗传算法和管道设计）中很有用。

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 exchange() 之前的操作 happen-before 从另一线程中相应的 exchange() 返回的后续操作。（内存一致性效果被称为happen-before）,简单的例子：当一个线程进行写入操作，另一个线程进行读取操作，保证一个线程写入的结果对另一个线程的读取是可视的。

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 exchanger = new Exchanger<>();
		service.execute(new Runnable(){
			public void run() {
				try {				
					String data1 = "恭喜发财！";
					System.out.println("线程" + Thread.currentThread().getName() + 
					"正在把数据:" + data1 +"换出去");
					Thread.sleep((long)(Math.random()*10000));
					String data2 = exchanger.exchange(data1);
					System.out.println("线程" + Thread.currentThread().getName() + 
					"换回的数据为:" + data2);
				}catch(Exception e){
					
				}
			}	
		});
		service.execute(new Runnable(){
			public void run() {
				try {				

					String data1 = "乖,给你一个红包！";
					System.out.println("线程" + Thread.currentThread().getName() + 
					"正在把数据:" + data1 +"换出去");
					Thread.sleep((long)(Math.random()*10000));					
					String data2 = exchanger.exchange(data1);
					System.out.println("线程" + Thread.currentThread().getName() + 
					"换回的数据为:" + data2);
				}catch(Exception e){
					
				}				
			}	
		});		
	}
}

三、JDK中的例子

使用 Exchanger 在线程间交换缓冲区，因此，在需要时，填充缓冲区的线程获取一个新腾空的缓冲区，并将填满的缓冲区传递给腾空缓冲区的线程。

class FillAndEmpty {
   Exchanger exchanger = new Exchanger();
   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