Java多线程工具箱之Semaphore

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：理解信号量能够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状况。若列车拿到的信号灯为绿灯，列车能够经过，不然列车不能经过。

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)

构造传入初始化的容许数量。

方法 acquire()

请求容许，未有有效的容许或者中断以前，线程将会阻塞。

方法 release()

释放容许。

1.2 范例

在范例中，咱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。因为跑道远没有飞机的数量多，所以须要飞机须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

能够从运行结果中看到，程序运行的第一秒，首先几架的飞机（线程）都第一时间的得到了跑道（资源），然后面几架飞机（线程），则在前面的飞机退出（sleep）跑道（资源）后得到使用权，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，但没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。
在后面咱们在咱们的多线程工具箱里面陆续会提到。