Java核心知识点学习----多线程 倒计时记数器CountDownLatch和数据交换的Exchanger
本文将要介绍的内容都是Java5中的新特性,一个是倒计时记数器---CountDownLatch,另一个是用于线程间数据交换的Exchanger.
一.CountDownLatch
1.什么是CountDownLatch?
倒计时计数器,调用CountDownLatch对象的CountDown()方法就将计数器减一,当计数到达0时,则所有等待者或者全部等待者开始执行.
2.如何用?
new CountDownLatch(1);
直接new,其构造函数必须传一个int类型的参数,参数的意思是:
count the number of times countDown must be invoked before threads can pass through await
大致可理解成,有一个门,有N个门闩,要想打开门必须把所有门闩都打开,对应到线程上是说在线程通过等待前必须要执行的倒计时操作.
3.举例
package com.amos.concurrent; import java.util.Random; import java.util.concurrent.CountDownLatch; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; /** * @ClassName: Count_Down_Latch_Test * @Description: 倒计时学习 * @author: amosli * @email:[email protected] * @date Apr 27, 2014 11:51:43 PM */ public class Count_Down_Latch_Test { public static void main(String[] args) { ExecutorService executorService = Executors.newCachedThreadPool(); final CountDownLatch countdownOrder = new CountDownLatch(1);// an order final CountDownLatch countdownAnwser = new CountDownLatch(3);// anwser for (int i = 0; i < 3; i++) { Runnable runnable = new Runnable() { public void run() { try { countdownOrder.await(); System.out.println("线程" + Thread.currentThread().getName() + " 正准备接受命令"); System.out.println("线程"+Thread.currentThread().getName()+" 已经接受命令!"); Thread.sleep(new Random().nextInt(1000)); System.out.println("线程"+Thread.currentThread().getName()+" 回应处理结果!"); countdownAnwser.countDown(); } catch (Exception e) { e.printStackTrace(); } } }; executorService.execute(runnable);//启动线程池 } try { Thread.sleep(new Random().nextInt(1000)); System.out.println("线程"+Thread.currentThread().getName()+" 即将下达命令!!"); countdownOrder.countDown(); System.out.println("线程"+Thread.currentThread().getName()+" 已经下达命令,正在等待返回结果!"); countdownAnwser.await(); System.out.println("线程"+Thread.currentThread().getName()+" 已经收到所有处理结果!"); } catch (InterruptedException e) { e.printStackTrace(); } } }
1).效果如下图所示:
2)程序说明
首先是创建了一个可缓存的线程池--->接着,创建两个CountDownLatch类,一个赋值为1,一个赋值为3;----->然后,执行一个for循环,在循环中,首先是实现了一个Runnable接口,然后,将Runnable接口加入到线程池中; 其中Runnable接口,首先是等待计数器为0,然后如果为0那么将计数器2的值减一,每循环一次减一,当第三次循环时,线程执行完毕;----->在Runnable接口中等待计数器为0,整个程序无法向下走,这时main方法,即主线程执行CountDown方法,计数器减一-------->最后等待所有的线程都执行完毕,返回最终的结果.
４.扩展--官方例子
package com.amos.concurrent; import java.util.concurrent.CountDownLatch; public class CountDownLatchTest { public static void main(String[] args) { try { new CountDownLatchTest().new Driver().main(); } catch (InterruptedException e) { e.printStackTrace(); } } class Driver { // ... void main() throws InterruptedException { CountDownLatch startSignal = new CountDownLatch(1); CountDownLatch doneSignal = new CountDownLatch(3); for (int i = 0; i < 3; ++i) // create and start threads new Thread(new worker(startSignal, doneSignal)).start(); dosomethingelse(); // don't let run yet startSignal.countDown(); // let all threads proceed dosomethingelse(); doneSignal.await(); // wait for all to finish } private void dosomethingelse() { System.out.println("dosomethingelse..."); } } class worker implements Runnable { private final CountDownLatch startsignal; private final CountDownLatch donesignal; worker(CountDownLatch startsignal, CountDownLatch donesignal) { this.startsignal = startsignal; this.donesignal = donesignal; } public void run() { try { startsignal.await(); dowork(); donesignal.countDown(); } catch (Exception ex) { } // return; } void dowork() { System.out.println("dowork...."); } } }
跟上面例子差不多,首先都是设置一个等待,然后再调用计数器减一,执行最后的操作.
CountDownLatch很适用于跑步比赛,当发令枪一声令下,所有选手开始跑起来.
二.Exchanger
1.什么是Exchange?作用是什么?
用于实现两人之间的数据交换,每个人在完成一定的事务后想与对方交换数据;只有两人见面才会有交换.就像是情人间的约会,不见不散.
2.如何使用?
new Exchanger<Ｖ>();
这里用到了泛型,即可以指定任意格式的数据,基本类型,对象等等都可以.
这里要注意的是线程要成对出现才能进行数据交换.用来交换的方法为exchange(x);
Parameters:
x the object to exchange
参数为要进行交换给对方的数据.
3.举例:
package com.amos.concurrent; import java.util.Random; import java.util.concurrent.Exchanger; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; /** * @ClassName: ExchangerTest * @Description: 线程间的数据交换Exchanger * @author: amosli * @email:[email protected] * @date Apr 28, 2014 12:26:48 AM */ public class ExchangerTest { public static void main(String[] args) { final Exchanger<String> exchanger = new Exchanger<String>(); ExecutorService newCachedThreadPool = Executors.newCachedThreadPool(); //线程一 newCachedThreadPool.execute(new Runnable() { public void run() { try { String data1="111"; System.out.println("线程:"+Thread.currentThread().getName()+" 要换出去的数据为:"+data1); Thread.sleep(new Random().nextInt(1000)); String exchange = exchanger.exchange(data1); System.out.println("线程:"+Thread.currentThread().getName()+" 换回来的数据为:"+exchange); } catch (InterruptedException e) { e.printStackTrace(); } } }); //线程二 newCachedThreadPool.execute(new Runnable() { public void run() { try { String data1="hi_amos"; System.out.println("线程:"+Thread.currentThread().getName()+" 要换出去的数据为:"+data1); Thread.sleep(new Random().nextInt(1000)); String exchange = exchanger.exchange(data1); System.out.println("线程:"+Thread.currentThread().getName()+" 换回来的数据为:"+exchange); } catch (InterruptedException e) { e.printStackTrace(); } } }); } }
这里只需要注意使用exchange()方法即可.
效果:
4.扩展---官方例子
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(); } } }
官方的例子,也比较简单,启动两个线程,然后调用exchange()方法进行两个线程间的数据交换.