= Singleton Pattern =
인스턴스 하나만 만들기

{{tag>Architecture Modeling Design_Pattern Creational}}

= Double-Checking Locking =
<sxh java>
public class Singleton {
	private volatile static Singleton instance;

	private Singleton() {
	}

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

		return instance;
	}
}
</sxh>

= Initialization on demand holder idiom =
※ java.lang.ExceptionInInitializerError 발생 주의.

<sxh java>

public class Singleton {
    private Singleton(){
    }
    
    private static class SingletonHolder{
        static final Singleton instance = new Singleton();
    }
    
    public static Singleton getInstatnce(){
        return SingletonHolder.instance;
    }
}
</sxh>

= Enum initialization =
Effective Java에서 소개된 방법.
  * Thread-safe
  * 단 한번의 인스턴스 생성 보장.
  * 사용 간편
  * enum value는 전역에서 접근 가능

<sxh java>
public enum Singleton {
	INSTANCE;
	
	public static Singleton getInstance() {
		return INSTANCE;
	}
}
</sxh>