1 Exchanger 介绍
前面分别介绍了CyclicBarrier、CountDownLatch、Semaphore,现在介绍并发工具类中的最后一个Exchange
。
Exchanger
是一个用于线程间协作的工具类,Exchanger
用于进行线程间的数据交换,它提供一个同步点,在这个同步点,两个线程可以交换彼此的数据。这两个线程通过exchange
方法交换数据,如果第一个线程先执行exchange
方法,它会一直等待第二个线程也执行exchange
方法,当两个线程都到达同步点时,这两个线程就可以交换数据。
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
2 Exchanger 实例
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() + "正在把数据 " + data + " 交换出去");
Thread.sleep((long) (Math.random() * 1000));
String exchangeData = (String) exchanger.exchange(data);
System.out.println(Thread.currentThread().getName() + "交换得到数据 " + exchangeData);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
pool-1-thread-1正在把数据 data1 交换出去
pool-1-thread-2正在把数据 data2 交换出去
pool-1-thread-2交换得到数据 data1
pool-1-thread-1交换得到数据 data2
当线程A调用Exchange
对象的exchange()
方法后,他会陷入阻塞状态,直到线程B也调用了exchange()
方法,然后以线程安全的方式交换数据,之后线程A和B继续运行。
exchange等待超时
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() + "正在把数据 " + data + " 交换出去");
//远小于3秒抛出异常
String exchangeData = (String) exchanger.exchange(data,1, TimeUnit.SECONDS);
System.out.println(Thread.currentThread().getName() + "交换得到数据 " + exchangeData);
} catch ( TimeoutException e) {
e.printStackTrace();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
pool-1-thread-1正在把数据 data1 交换出去
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正在把数据 data2 交换出去
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)
实战场景:
设计一个定时任务,每日凌晨执行。在定时任务中启动两个线程,一个线程负责对业务明细表(xxx_info)进行查询统计,把统计的结果放置在内存缓冲区,另一个线程负责读取缓冲区中的统计结果并插入到业务统计表(xxx_statistics)中。
亲,这样的场景是不是听起来很有感觉?没错!两个线程在内存中批量交换数据,这个事情我们可以使用Exchanger去做!
3 实现原理
Exchanger
(交换者)是一个用于线程间协作的工具类。Exchanger
用于进行线程间的数据交换。它提供一个同步点,在这个同步点两个线程可以交换彼此的数据。这两个线程通过exchange
方法交换数据, 如果第一个线程先执行exchange
方法,它会一直等待第二个线程也执行exchange
,当两个线程都到达同步点时,这两个线程就可以交换数据,将本线程生产出来的数据传递给对方。因此使用Exchanger
的重点是成对的线程使用exchange()方法,当有一对线程达到了同步点,就会进行交换数据。因此该工具类的线程对象是成对的。
Exchanger类提供了两个方法,String exchange(V x):用
于交换,启动交换并等待另一个线程调用exchange
;String
exchange(V x,long timeout,TimeUnit unit):用于交换,启动交换并等待另一个线程调用exchange
,并且设置最大等待时间,当等待时间超过timeout
便停止等待。
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