摘要:多线程工具箱之前言这一篇谈一下信号量。信息信息信息信息信息信息信息信息信息信息信息小结适用于多线程请求数量资源的场景,但无法解决单多个线程对同一资源访问的竞争性访问。在后面我们在我们的多线程工具箱里面陆续会提到。
Java多线程工具箱之Semaphore 前言
这一篇谈一下Semaphore:信号量。
将Semaphore类比为为信号灯,被继承Runable的线程类比为列车:理解信号量可以参考控制列车的信号灯:铁道上有多个列车,由信号灯通知其可用情况。若列车拿到的信号灯为绿灯,列车可以通过,否则列车不能通过。
Semaphore用于限制访问线程多于可用资源的场景。
1.1 基本API 构造器 Semaphore(int premits)构造传入初始化的允许数量。
方法 acquire()请求允许,未有有效的允许或者中断之前,线程将会阻塞。
方法 release()释放允许。
1.2 范例在范例中,我们使用一个机场请求跑道的模拟程序,机场有3个跑道,这个时段降落的航班一共有10个。由于跑道远没有飞机的数量多,因此需要飞机需要等待可用的跑道资源。这个程序就是模拟了这个过程。
UML App.javaimport java.util.Random; import lombok.extern.java.Log; @Log public class App { static Random ran = new Random(System.currentTimeMillis()); public static void main(String[] args) { Airport ap = new Airport("Baiyun Airport", 3); for (int index = 0; index < 10; index++) { AirPlane plane = new AirPlane(index, String.valueOf(index), String.valueOf(index), ap, ran.nextInt(2) == 0 ? Action.landing : Action.takeOff); Thread thread = new Thread(plane); thread.start(); } } }
import java.util.Arrays; import java.util.concurrent.Semaphore; import lombok.Data; import lombok.extern.java.Log; @Log @Data public class Airport { private String name; private Semaphore semaphore; private Runway[] runways; public Airport(String name, int numberOfRunway) { this.name = name; this.semaphore = new Semaphore(numberOfRunway); runways = new Runway[numberOfRunway]; for (int index = 0; index < numberOfRunway; index++) { Runway currentRunway = new Runway(name, index); runways[index] = currentRunway; } log.info("Airpot is ready :" + Arrays.toString(runways)); } public Runway requestRunway() throws InterruptedException { semaphore.acquire(); while (true) { for (int index = 0; index < runways.length; index++) { Runway runway = runways[index]; if (!runway.isInUse()) { runway.setInUse(true); return runway; } } log.warning("all runway is using, and wait till there is runay release."); } } public void releaseRunway(Runway release) { for (int index = 0; index < runways.length; index++) { Runway runway = runways[index]; if (runway.equals(release)) { runway.setInUse(false); semaphore.release(); } } } }Airplane.java
import java.util.Random; import lombok.Data; import lombok.extern.java.Log; @Data @Log public class AirPlane implements Runnable { private int id; private String name; private String company; private Airport airport; private Action action; private static Random rand = new Random(System.currentTimeMillis()); public AirPlane(int id, String name, String company, Airport airport, Action action) { this.id = id; this.name = name; this.company = company; this.airport = airport; this.action = action; } @Override public void run() { Runway runway = null; try { runway = airport.requestRunway(); log.info(this.toString() + "is using " + runway.toString() + " to " + this.action); int sleepSecond = rand.nextInt(10) * 1000; Thread.sleep(sleepSecond); } catch (InterruptedException e) { log.warning("request runway error"); e.printStackTrace(); } finally { if (airport != null && runway != null) { airport.releaseRunway(runway); } } } }Runway.java
import lombok.Data; @Data public class Runway { private String name; private boolean inUse = false; public Runway(String airportName, int runwayIndex) { this.name = airportName + "_" + runwayIndex; } }运行结果
可以从运行结果中看到,程序运行的第一秒,首先几架的飞机(线程)都第一时间的获得了跑道(资源),而后面几架飞机(线程),则在前面的飞机退出(sleep)跑道(资源)后获得使用权,进行起飞或降落。
Aug 13, 2018 12:56:32 AM online.tangbk.thread.study.semaphore.Airport1.3 Semaphore小结信息: Airpot is ready :[Runway(name=Baiyun Airport_0, inUse=false), Runway(name=Baiyun Airport_1, inUse=false), Runway(name=Baiyun Airport_2, inUse=false)] Aug 13, 2018 12:56:33 AM online.tangbk.thread.study.semaphore.AirPlane run 信息: AirPlane(id=1, name=1, company=1, airport=Airport(name=Baiyun Airport, semaphore=java.util.concurrent.Semaphore@61a09c6b[Permits = 1], runways=[Runway(name=Baiyun Airport_0, inUse=true), Runway(name=Baiyun Airport_1, inUse=true), Runway(name=Baiyun Airport_2, inUse=false)]), action=landing)is using Runway(name=Baiyun Airport_1, inUse=true) to landing Aug 13, 2018 12:56:33 AM online.tangbk.thread.study.semaphore.AirPlane run 信息: AirPlane(id=2, name=2, company=2, airport=Airport(name=Baiyun Airport, semaphore=java.util.concurrent.Semaphore@61a09c6b[Permits = 0], runways=[Runway(name=Baiyun Airport_0, inUse=true), Runway(name=Baiyun Airport_1, inUse=true), Runway(name=Baiyun Airport_2, inUse=true)]), action=takeOff)is using Runway(name=Baiyun Airport_2, inUse=true) to takeOff Aug 13, 2018 12:56:33 AM online.tangbk.thread.study.semaphore.AirPlane run 信息: AirPlane(id=0, name=0, company=0, airport=Airport(name=Baiyun Airport, semaphore=java.util.concurrent.Semaphore@61a09c6b[Permits = 2], runways=[Runway(name=Baiyun Airport_0, inUse=true), Runway(name=Baiyun Airport_1, inUse=false), Runway(name=Baiyun Airport_2, inUse=false)]), action=landing)is using Runway(name=Baiyun Airport_0, inUse=true) to landing Aug 13, 2018 12:56:33 AM online.tangbk.thread.study.semaphore.AirPlane run 信息: AirPlane(id=8, name=8, company=8, airport=Airport(name=Baiyun Airport, semaphore=java.util.concurrent.Semaphore@61a09c6b[Permits = 0], runways=[Runway(name=Baiyun Airport_0, inUse=true), Runway(name=Baiyun Airport_1, inUse=true), Runway(name=Baiyun Airport_2, inUse=true)]), action=takeOff)is using Runway(name=Baiyun Airport_1, inUse=true) to takeOff Aug 13, 2018 12:56:35 AM online.tangbk.thread.study.semaphore.AirPlane run 信息: AirPlane(id=3, name=3, company=3, airport=Airport(name=Baiyun Airport, semaphore=java.util.concurrent.Semaphore@61a09c6b[Permits = 0], runways=[Runway(name=Baiyun Airport_0, inUse=true), Runway(name=Baiyun Airport_1, inUse=true), Runway(name=Baiyun Airport_2, inUse=true)]), action=takeOff)is using Runway(name=Baiyun Airport_0, inUse=true) to takeOff Aug 13, 2018 12:56:37 AM online.tangbk.thread.study.semaphore.AirPlane run 信息: AirPlane(id=4, name=4, company=4, airport=Airport(name=Baiyun Airport, semaphore=java.util.concurrent.Semaphore@61a09c6b[Permits = 0], runways=[Runway(name=Baiyun Airport_0, inUse=true), Runway(name=Baiyun Airport_1, inUse=true), Runway(name=Baiyun Airport_2, inUse=true)]), action=landing)is using Runway(name=Baiyun Airport_2, inUse=true) to landing Aug 13, 2018 12:56:40 AM online.tangbk.thread.study.semaphore.AirPlane run 信息: AirPlane(id=5, name=5, company=5, airport=Airport(name=Baiyun Airport, semaphore=java.util.concurrent.Semaphore@61a09c6b[Permits = 0], runways=[Runway(name=Baiyun Airport_0, inUse=true), Runway(name=Baiyun Airport_1, inUse=true), Runway(name=Baiyun Airport_2, inUse=true)]), action=landing)is using Runway(name=Baiyun Airport_2, inUse=true) to landing Aug 13, 2018 12:56:40 AM online.tangbk.thread.study.semaphore.AirPlane run 信息: AirPlane(id=6, name=6, company=6, airport=Airport(name=Baiyun Airport, semaphore=java.util.concurrent.Semaphore@61a09c6b[Permits = 0], runways=[Runway(name=Baiyun Airport_0, inUse=true), Runway(name=Baiyun Airport_1, inUse=true), Runway(name=Baiyun Airport_2, inUse=true)]), action=landing)is using Runway(name=Baiyun Airport_2, inUse=true) to landing Aug 13, 2018 12:56:41 AM online.tangbk.thread.study.semaphore.AirPlane run 信息: AirPlane(id=7, name=7, company=7, airport=Airport(name=Baiyun Airport, semaphore=java.util.concurrent.Semaphore@61a09c6b[Permits = 0], runways=[Runway(name=Baiyun Airport_0, inUse=true), Runway(name=Baiyun Airport_1, inUse=true), Runway(name=Baiyun Airport_2, inUse=true)]), action=takeOff)is using Runway(name=Baiyun Airport_1, inUse=true) to takeOff Aug 13, 2018 12:56:41 AM online.tangbk.thread.study.semaphore.AirPlane run 信息: AirPlane(id=9, name=9, company=9, airport=Airport(name=Baiyun Airport, semaphore=java.util.concurrent.Semaphore@61a09c6b[Permits = 0], runways=[Runway(name=Baiyun Airport_0, inUse=true), Runway(name=Baiyun Airport_1, inUse=true), Runway(name=Baiyun Airport_2, inUse=true)]), action=landing)is using Runway(name=Baiyun Airport_0, inUse=true) to landing
Semaphore适用于多线程请求数量资源的场景,但无法解决单多个线程对同一资源访问的竞争性访问。对于单一资源的竞争性访问,依然要使用synchronize关键字或atomic等甚至ReadWriteLock关键字或工具类进行限制。
在后面我们在我们的多线程工具箱里面陆续会提到。
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