Day 10: Creational patterns - Singleton

目的

建立一个「唯一」物件，专责於服务只能单一连线的情境，例如跟资料库的沟通，同时确保全域内都可以呼叫该物件。

说明

Singleton 是相当好懂的模式，用在只能服务单一连线的情境下，避免多重连线产生时间差，导致执行完毕的结果不是我们要的，例如：

与资料库沟通的写入，时间差可能会让写入的顺序错乱。
权限的许可，时间差可能让权限判定失效。

UML 图

Singleton Pattern UML Diagram

使用 Java 实作

Singleton: EmergencyTelephone

public class EmergencyTelephone {
    private static EmergencyTelephone instance;

    private EmergencyTelephone() {
    }

    private void useTelephone(String phoneNumber) {
        System.out.println("（号码：" + phoneNumber + "）拿起话筒");
        System.out.println("（号码：" + phoneNumber + "）输入号码");
        System.out.println("（号码：" + phoneNumber + "）等待接通");
        System.out.println("（号码：" + phoneNumber + "）确认电话已经打通");
    }

    private void tellDetails(String phoneNumber) {
        System.out.println("（号码：" + phoneNumber + "）详述情况");
        System.out.println("（号码：" + phoneNumber + "）告知地点");
        System.out.println("（号码：" + phoneNumber + "）记录指示");
    }

    private void finishTelephoneTalk(String phoneNumber) {
        System.out.println("（号码：" + phoneNumber + "）挂上电话");
        System.out.println("（号码：" + phoneNumber + "）思考指示");
        System.out.println("（号码：" + phoneNumber + "）行动");
    }

    public synchronized void execute(String phoneNumber) {
        useTelephone(phoneNumber);
        tellDetails(phoneNumber);
        finishTelephoneTalk(phoneNumber);
    }

    public static EmergencyTelephone getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new EmergencyTelephone();
                }
            }
        }

        return instance;

    }
}

进行测试

public class EmergencyTelephoneSample extends Thread {
    String phoneNumber;

    public EmergencyTelephoneSample(String phoneNumber) {
        this.phoneNumber = phoneNumber;
    }

    public void run() {
        EmergencyTelephone emergencyTelephone = EmergencyTelephone.getInstance();
        if (emergencyTelephone != null) {
            System.out.println("这通电话号码是：" + phoneNumber + "，这台电话的生产序号是：" + emergencyTelephone.hashCode());
            emergencyTelephone.execute(phoneNumber);
        }
    }

    public static void main(String[] args) {
        EmergencyTelephone emergencyTelephone1 = EmergencyTelephone.getInstance();
        EmergencyTelephone emergencyTelephone2 = EmergencyTelephone.getInstance();

        if (emergencyTelephone1.hashCode() == emergencyTelephone2.hashCode()) {
            System.out.println("两个是同一个物件");
        }

        Thread t1 = new EmergencyTelephoneSample("119");
        Thread t2 = new EmergencyTelephoneSample("110");
        t1.start();
        t2.start();
    }
}

使用 JavaScript 实作

Singleton: EmergencyTelephone

const EmergencyTelephone = (() => {
  let instance = null;

  function initialize() {
    let hashCode = Math.floor(Math.random() * 10000000);

    function useTelephone(phoneNumber) {
      console.log("（号码：" + phoneNumber + "）拿起话筒");
      console.log("（号码：" + phoneNumber + "）输入号码");
      console.log("（号码：" + phoneNumber + "）等待接通");
      console.log("（号码：" + phoneNumber + "）确认电话已经打通");
    }

    function tellDetails(phoneNumber) {
      console.log("（号码：" + phoneNumber + "）详述情况");
      console.log("（号码：" + phoneNumber + "）告知地点");
      console.log("（号码：" + phoneNumber + "）记录指示");
    }

    function finishTelephoneTalk(phoneNumber) {
      console.log("（号码：" + phoneNumber + "）挂上电话");
      console.log("（号码：" + phoneNumber + "）思考指示");
      console.log("（号码：" + phoneNumber + "）行动");
    }

    return {
      execute: function (phoneNumber) {
        useTelephone(phoneNumber);
        tellDetails(phoneNumber);
        finishTelephoneTalk(phoneNumber);
      },
      getHashCode: function () {
        return hashCode;
      }
    };
  }

  return {
    getInstance: function () {
      if (instance === null) {
        instance = initialize();
      }

      return instance;
    }
  }
})();

进行测试

const emergencyTelephoneSample = () => {
  const emergencyTelephone1 = EmergencyTelephone.getInstance();
  const emergencyTelephone2 = EmergencyTelephone.getInstance();

  if (emergencyTelephone1.getHashCode() === emergencyTelephone2.getHashCode()) {
    console.log("两个是同一个物件");
  } else {
    console.log("出问题，两个不是同一个物件");
  }

  console.log("进行第一通电话拨打");
  const phoneNumber1 = "119";
  console.log("这通电话号码是：" + phoneNumber1 + "，这台电话的生产序号是：" + emergencyTelephone1.getHashCode());
  emergencyTelephone1.execute(phoneNumber1);

  console.log("进行第二通电话拨打");
  const phoneNumber2 = "110";
  console.log("这通电话号码是：" + phoneNumber2 + "，这台电话的生产序号是：" + emergencyTelephone2.getHashCode());
  emergencyTelephone2.execute(phoneNumber2);
}

emergencyTelephoneSample();