[Design Pattern] LazyHolder Singleton Pattern

What is Singleton?

=> A singleton is a class that allows only a single instance of itself to be created and gives access to that created instance.

LazyHolder Singleton Pattern

public class Singleton {

    // LazyHolder Singleton pattern

    private Singleton(){};

    private static class LazyHolder{
        public static final Singleton instance = new Singleton();
    }

    public static Singleton getInstance(){
        return LazyHolder.instance;
    }
    
}

getInstance()를 호출했을때 LazyHolder 클래스가 로딩되면서 생성

장점 : 객체가 필요한 시점에서 초기화가 진행된다.

동일 인스턴스가 생성되는지 테스트

public class Main {

    public static void main(String[] args) {
        
        // singleton test
        // LazyHolder Singleton pattern test

        Singleton singleton1 = Singleton.getInstance();
        Singleton singleton2 = Singleton.getInstance();

        System.out.println(singleton1);
        System.out.println(singleton2);
        System.out.println(singleton1 == singleton2);
	
    }
}

결과값

com.example.demo.domain.Singleton@39ed3c8d
com.example.demo.domain.Singleton@39ed3c8d
true