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.java
import 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();
}
}
}
Airport.java
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.Airport <init>
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
1.3 Semaphore小结
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。
java
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