ouble 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

​

Read more: https://javarevisited.blogspot.com/2011/03/10-interview-questions-on-singleton.html#ixzz7oRERmqvA

// Java Program to write double checked locking 

// of Singleton class

class Singleton {

    private volatile static Singleton instance;

    private Singleton() {}

    // 1st version: creates multiple instances if two thread

    // access this method simultaneously

    public static Singleton getInstance1()

    {

        if (instance == null) {

            instance = new Singleton();

        }

        return instance;

    }

    // 2nd version : this is thread-safe and only

    // creates one instance of Singleton on concurrent

    // environment but it is unnecessarily expensive due to

    // cost of synchronization at every call.

    public static synchronized Singleton getInstance2()

    {

        if (instance == null) {

            instance = new Singleton();

        }

        return instance;

    }

    // 3rd version : An implementation of double checked

    // locking of Singleton. Intention is to reduce cost

    // of synchronization and improve performance, by only

    // locking critical section of code, the code which

    // creates instance of Singleton class.

    public static Singleton getInstance3()

    {

        if (instance == null) {

            synchronized (Singleton.class)

            {

                if (instance == null) {

                    instance = new Singleton();

                }

            }

        }

        return instance;

    }

}