What is double checked locking in Singleton?

Double-checked locking prevents creating a duplicate instance of Singleton when call to getInstance() method is made in a multi-threading environment. In Double checked locking pattern as shown in below example, singleton instance is checked two times before initialization.

public class Singleton{
private static Singleton _INSTANCE;

public static Singleton getInstance()
{
	if(_INSTANCE==null) {
		synchronized(Singleton.class){
// double checked locking inside synchronized block
			if(_INSTANCE==null){_INSTANCE=new Singleton();}
			}
		}
	return _INSTANCE;
	}
}

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