Singleton for multithreading in Java

​Everyone knows what is singleton pattern, but how to implement a singleton class in multithreaded environment? Next, I’m going to present best approaches for this use case.

Bill Pugh Singleton Implementation

public class Singleton {
	private Singleton() { }

	private static class SingletonHolder {
		private static final Singleton INSTANCE = new Singleton();
	}

	public static Singleton getInstance() {
		return SingletonHolder.INSTANCE;
	} 
}

Thread Safe Double Checked Singleton Implementation

public class Singleton {
	private static Singleton instance;

	private Singleton() { }

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