[Java]Design Pattern-Singleton

Singleton Pattern은 다음과 같다.

class Singleton {
    private static Singleton uniqueInstance;

    private Singleton(){}

    public static Singleton getInstance() {
        if (uniqueInstance == null) {
            uniqueInstance = new Singleton();
        }
        return uniqueInstance;
    }

    public String getDescription(){return "I'm a Singleton!";}
}

public class Main {
    public static void main(String[] args){
        Singleton singleton = Singleton.getInstance();
        System.out.println(singleton.getDescription());
    }
}

---

멀티스레드일 경우 다음과 같은 문제점이 발생한다.

public class ChocolateBoiler {
    private boolean empty;
    private boolean boiled;

    private static ChocolateBoiler OnlychocolateBoiler = null;

    //객체의 생성을 막음
    private ChocolateBoiler(){
        empty = true;
        boiled = false;
        System.out.println(this);
    }

    public static ChocolateBoiler getInstance(){
        if(OnlychocolateBoiler==null){
            OnlychocolateBoiler=new ChocolateBoiler();
        }
        return OnlychocolateBoiler;
    }

    public void fill() {
        if (isEmpty()) {
            empty = false;
            boiled = false;
            // fill the boiler with a milk/chocolate mixture
        }
    }

    public void drain() {
        if (!isEmpty() && isBoiled()) {
            // drain the boiled milk and chocolate
            empty = true;
        }
    }

    public void boil() {
        if (!isEmpty() && !isBoiled()) {
            // bring the contents to a boil
            boiled = true;
        }
    }

    public boolean isEmpty() {
        return empty;
    }

    public boolean isBoiled() {
        return boiled;
    }
}

//MultiThread.java

public class MultiThread extends Thread{
    private String name;
    public MultiThread(String name) {
        this.name = name;
    }

    public void run() {
        int count = 0;

        for(int i=0; i<5; i++) {
            count++;
            ChocolateBoiler chocolateBoiler = ChocolateBoiler.getInstance();
            System.out.println(name+ "의"+ count+"번째 쓰레드의 singleton 객체  : " + chocolateBoiler.toString());

            try {
                Thread.sleep(400);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
}

public class Main {
    public static void main(String[] args) {

        MultiThread AAA = new MultiThread("A");
        MultiThread BBB = new MultiThread("B");

        AAA.start();
        BBB.start();

    }
}

<출력결과>

B의1번째 쓰레드의 singleton 객체  : ChocolateBoiler@150a7ec8
A의1번째 쓰레드의 singleton 객체  : ChocolateBoiler@1e73dd
B의2번째 쓰레드의 singleton 객체  : ChocolateBoiler@150a7ec8
A의2번째 쓰레드의 singleton 객체  : ChocolateBoiler@150a7ec8
B의3번째 쓰레드의 singleton 객체  : ChocolateBoiler@150a7ec8
A의3번째 쓰레드의 singleton 객체  : ChocolateBoiler@150a7ec8
B의4번째 쓰레드의 singleton 객체  : ChocolateBoiler@150a7ec8
A의4번째 쓰레드의 singleton 객체  : ChocolateBoiler@150a7ec8
B의5번째 쓰레드의 singleton 객체  : ChocolateBoiler@150a7ec8
A의5번째 쓰레드의 singleton 객체  : ChocolateBoiler@150a7ec8