1 Exchanger 介紹

前面分別介紹了CyclicBarrier、CountDownLatch、Semaphore，現(xiàn)在介紹并發(fā)工具類中的最后一個Exchange。
Exchanger 是一個用于線程間協(xié)作的工具類，Exchanger用于進(jìn)行線程間的數(shù)據(jù)交換，它提供一個同步點，在這個同步點，兩個線程可以交換彼此的數(shù)據(jù)。這兩個線程通過exchange 方法交換數(shù)據(jù)，如果第一個線程先執(zhí)行exchange 方法，它會一直等待第二個線程也執(zhí)行exchange 方法，當(dāng)兩個線程都到達(dá)同步點時，這兩個線程就可以交換數(shù)據(jù)。

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，有幾個要點：

• 此類提供對外的操作是同步的；
• 用于成對出現(xiàn)的線程之間交換數(shù)據(jù)；
• 可以視作雙向的同步隊列；
• 可應(yīng)用于基因算法、流水線設(shè)計等場景。
• 接著看api文檔，這個類提供對外的接口非常簡潔，一個無參構(gòu)造函數(shù)，兩個重載的范型exchange方法：
  

public V exchange(V x) throws InterruptedException
public V exchange(V x, long timeout, TimeUnit unit) throws InterruptedException, TimeoutException

2 Exchanger 實例

public class ExchangerTest {
    public static void main(String[] args) {
        ExecutorService executor = Executors.newCachedThreadPool();
        final Exchanger exchanger = new Exchanger();
        executor.execute(new Runnable() {
            String data = "data1";

            @Override
            public void run() {
                doExchangeWork(data, exchanger);
            }
        });

        executor.execute(new Runnable() {
            String data = "data2";

            @Override
            public void run() {
                doExchangeWork(data, exchanger);
            }
        });
        executor.shutdown();
    }

    private static void doExchangeWork(String data, Exchanger exchanger) {
        try {
            System.out.println(Thread.currentThread().getName() + "正在把數(shù)據(jù) " + data + " 交換出去");
            Thread.sleep((long) (Math.random() * 1000));

            String exchangeData = (String) exchanger.exchange(data);
            System.out.println(Thread.currentThread().getName() + "交換得到數(shù)據(jù)  " + exchangeData);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

pool-1-thread-1正在把數(shù)據(jù) data1 交換出去
pool-1-thread-2正在把數(shù)據(jù) data2 交換出去
pool-1-thread-2交換得到數(shù)據(jù)  data1
pool-1-thread-1交換得到數(shù)據(jù)  data2

當(dāng)線程A調(diào)用Exchange對象的exchange()方法后，他會陷入阻塞狀態(tài)，直到線程B也調(diào)用了exchange()方法，然后以線程安全的方式交換數(shù)據(jù)，之后線程A和B繼續(xù)運行。

exchange等待超時

public class ExchangerTest {
    public static void main(String[] args) {
        ExecutorService executor = Executors.newCachedThreadPool();
        final Exchanger exchanger = new Exchanger();
        executor.execute(new Runnable() {
            String data = "data1";

            @Override
            public void run() {
                doExchangeWork(data, exchanger);
            }
        });

        executor.execute(new Runnable() {
            String data = "data2";

            @Override
            public void run() {
                try {
                    Thread.sleep((long) (3000));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                doExchangeWork(data, exchanger);
            }
        });
        executor.shutdown();
    }

    private static void doExchangeWork(String data, Exchanger exchanger) {
        try {
            System.out.println(Thread.currentThread().getName() + "正在把數(shù)據(jù) " + data + " 交換出去");

            //遠(yuǎn)小于3秒拋出異常
            String exchangeData = (String) exchanger.exchange(data,1, TimeUnit.SECONDS);
            System.out.println(Thread.currentThread().getName() + "交換得到數(shù)據(jù)  " + exchangeData);
        } catch ( TimeoutException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

pool-1-thread-1正在把數(shù)據(jù) data1 交換出去
java.util.concurrent.TimeoutException
    at java.util.concurrent.Exchanger.exchange(Exchanger.java:626)
    at ExchangerTest.doExchangeWork(ExchangerTest.java:37)
    at ExchangerTest.access$000(ExchangerTest.java:3)
    at ExchangerTest$1.run(ExchangerTest.java:12)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)
pool-1-thread-2正在把數(shù)據(jù) data2 交換出去
java.util.concurrent.TimeoutException
    at java.util.concurrent.Exchanger.exchange(Exchanger.java:626)
    at ExchangerTest.doExchangeWork(ExchangerTest.java:37)
    at ExchangerTest.access$000(ExchangerTest.java:3)
    at ExchangerTest$2.run(ExchangerTest.java:26)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)

實戰(zhàn)場景：

設(shè)計一個定時任務(wù)，每日凌晨執(zhí)行。在定時任務(wù)中啟動兩個線程，一個線程負(fù)責(zé)對業(yè)務(wù)明細(xì)表（xxx_info）進(jìn)行查詢統(tǒng)計，把統(tǒng)計的結(jié)果放置在內(nèi)存緩沖區(qū)，另一個線程負(fù)責(zé)讀取緩沖區(qū)中的統(tǒng)計結(jié)果并插入到業(yè)務(wù)統(tǒng)計表（xxx_statistics）中。
親，這樣的場景是不是聽起來很有感覺？沒錯!兩個線程在內(nèi)存中批量交換數(shù)據(jù)，這個事情我們可以使用Exchanger去做！

3 實現(xiàn)原理

Exchanger（交換者）是一個用于線程間協(xié)作的工具類。Exchanger用于進(jìn)行線程間的數(shù)據(jù)交換。它提供一個同步點，在這個同步點兩個線程可以交換彼此的數(shù)據(jù)。這兩個線程通過exchange方法交換數(shù)據(jù)， 如果第一個線程先執(zhí)行exchange方法，它會一直等待第二個線程也執(zhí)行exchange，當(dāng)兩個線程都到達(dá)同步點時，這兩個線程就可以交換數(shù)據(jù)，將本線程生產(chǎn)出來的數(shù)據(jù)傳遞給對方。因此使用Exchanger的重點是成對的線程使用exchange()方法，當(dāng)有一對線程達(dá)到了同步點，就會進(jìn)行交換數(shù)據(jù)。因此該工具類的線程對象是成對的。
Exchanger類提供了兩個方法，String exchange(V x):用于交換，啟動交換并等待另一個線程調(diào)用exchange；String exchange(V x,long timeout,TimeUnit unit)：用于交換，啟動交換并等待另一個線程調(diào)用exchange，并且設(shè)置最大等待時間，當(dāng)?shù)却龝r間超過timeout便停止等待。

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