java.util.concurrent.Exchanger应用范例与原理浅析

http://www.cnblogs.com/davidwang456/p/4179488.html

Exchanger是自jdk1.5起开始提供的工具套件，一般用于两个工作线程之间交换数据。在本文中我将采取由浅入深的方式来介绍分析这个工具类。首先我们来看看官方的api文档中的叙述：

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.

    在以上的描述中，有几个要点：

• 此类提供对外的操作是同步的；
• 用于成对出现的线程之间交换数据；
• 可以视作双向的同步队列；
• 可应用于基因算法、流水线设计等场景。

   接着看api文档，这个类提供对外的接口非常简洁，一个无参构造函数，两个重载的范型exchange方法：
public V exchange(V x) throws InterruptedException
public V exchange(V x, long timeout, TimeUnit unit) throws InterruptedException, TimeoutException
   从官方的javadoc可以知道，当一个线程到达exchange调用点时，如果它的伙伴线程此前已经调用了此方法，那么它的伙伴会被调度唤醒并与之进行对象交换，然后各自返回。如果它的伙伴还没到达交换点，那么当前线程将会被挂起，直至伙伴线程到达——完成交换正常返回；或者当前线程被中断——抛出中断异常；又或者是等候超时——抛出超时异常。

注意的是，

线程1

obj1 = exchanger.exchange(obj1);

线程2

obj2= exchanger.exchange(obj2);

这样对调的结果就是obj1 = obj2, obj2= obj1

在java中都是引用的交换

在下面的例子中，ReadThread交换之后的buffList就指向了null

 
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.Exchanger;
 
public class ExchangerTest {
 
	public static void main(String[] args) {
		Exchanger<List<String>> exchanger = new Exchanger<List<String>>();
		new ReadThread(exchanger).start();// 启动读数据线程
		new WriteThread(exchanger).start();// 启动写数据线程
	}
 
}
 
class ReadThread extends Thread {
	private Exchanger<List<String>> exchanger;
	List<String> bufferList = new LinkedList<String>();
	public static final Integer BUFFER_SIZE = 10;
 
	public ReadThread(Exchanger<List<String>> exchanger) {
		this.exchanger = exchanger;
	}
 
	@Override
	public void run() {
		System.out.println("ReadThread start");
		for (int i = 0; i < BUFFER_SIZE; i++) {
			bufferList.add(String.valueOf( (char)( 'A' + i))); // 模拟读数据
		}
		try {
			Thread.sleep(3000);
			bufferList = exchanger.exchange(bufferList);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		
		System.out.println("ReadThread end");
		//System.out.println("buff size " + bufferList.size());
	}
}
 
class WriteThread extends Thread {
	private Exchanger<List<String>> exchanger;
	List<String> bufferList;
 
	public WriteThread(Exchanger<List<String>> exchanger) {
		this.exchanger = exchanger;
	}
 
	@Override
	public void run() {
		try {
			System.out.println("WriteThread start");
			bufferList = exchanger.exchange(bufferList); // 从Exchanger获得缓存数据
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		for (String str : bufferList) {
			System.out.println(str);
		}
 
		System.out.println("WriteTread end");
 
	}
}