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

Exchanger是自jdk1.5起開始提供的工具套件，一般用于兩個(gè)工作線程之間交換數(shù)據(jù)。在本文中我將采取由淺入深的方式來介紹分析這個(gè)工具類。首先我們來看看官方的api文檔中的敘述：

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，有幾個(gè)要點(diǎn)：

• 此類提供對(duì)外的操作是同步的；
• 用于成對(duì)出現(xiàn)的線程之間交換數(shù)據(jù)；
• 可以視作雙向的同步隊(duì)列；
• 可應(yīng)用于基因算法、流水線設(shè)計(jì)等場(chǎng)景。

?? 接著看api文檔，這個(gè)類提供對(duì)外的接口非常簡(jiǎn)潔，一個(gè)無(wú)參構(gòu)造函數(shù)，兩個(gè)重載的范型exchange方法：
public V exchange(V x) throws InterruptedException
public V exchange(V x, long timeout, TimeUnit unit) throws InterruptedException, TimeoutException
?? 從官方的javadoc可以知道，當(dāng)一個(gè)線程到達(dá)exchange調(diào)用點(diǎn)時(shí)，如果它的伙伴線程此前已經(jīng)調(diào)用了此方法，那么它的伙伴會(huì)被調(diào)度喚醒并與之進(jìn)行對(duì)象交換，然后各自返回。如果它的伙伴還沒到達(dá)交換點(diǎn)，那么當(dāng)前線程將會(huì)被掛起，直至伙伴線程到達(dá)——完成交換正常返回；或者當(dāng)前線程被中斷——拋出中斷異常；又或者是等候超時(shí)——拋出超時(shí)異常。

注意的是，

線程1

obj1 = exchanger.exchange(obj1);

線程2

obj2= exchanger.exchange(obj2);

這樣對(duì)調(diào)的結(jié)果就是obj1 = obj2, obj2= obj1

在java中都是引用的交換

在下面的例子中，ReadThread交換之后的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();// 啟動(dòng)讀數(shù)據(jù)線程new WriteThread(exchanger).start();// 啟動(dòng)寫數(shù)據(jù)線程}}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;}@Overridepublic void run() {System.out.println("ReadThread start");for (int i = 0; i < BUFFER_SIZE; i++) {bufferList.add(String.valueOf( (char)( 'A' + i))); // 模擬讀數(shù)據(jù)}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;}@Overridepublic void run() {try {System.out.println("WriteThread start");bufferList = exchanger.exchange(bufferList); // 從Exchanger獲得緩存數(shù)據(jù)} catch (InterruptedException e) {e.printStackTrace();}for (String str : bufferList) {System.out.println(str);}System.out.println("WriteTread end");} }

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