What is double checked locking in Singleton?

Answer 1

Double checked locking is a technique to prevent creating another instance of Singleton when call to getInstance() method is made in multi-threading environment. In Double checked locking pattern as shown in below example, singleton instance is checked two times before initialization. See here to learn more about double-checked-locking in Java.

public static Singleton getInstance(){
     if(_INSTANCE == null){
         synchronized(Singleton.class){
         //double checked locking - because second check of Singleton instance with lock
                if(_INSTANCE == null){
                    _INSTANCE = new Singleton();
                }
            }
         }
     return _INSTANCE;
}