Lazy initialization of Singleton implementation without using Synchronized Key word

Question

I was asked in an interview to suggest a design/implementation of a Singleton Pattern where I have to Lazy load the class and also not use the synchronized key word. I got choked and could not come up with anything.I then I started reading on java concurrency and concurrentHaspMap. Please check the below imlpementation and kindly confirm if you see any issue with Double check Locking or any other issues with this implementation.

package Singleton;

import java.util.concurrent.ConcurrentHashMap;

public final class SingletonMap {

    static String key = "SingletonMap";
    static ConcurrentHashMap<String, SingletonMap> singletonMap = new ConcurrentHashMap<String, SingletonMap>();
    //private constructor
    private SingletonMap(){

    }
    static SingletonMap getInstance(){

        SingletonMap map = singletonMap.get(key);       
        if (map == null){
                //SingletonMap newValue=  new SingletonMap();
                map =   singletonMap.putIfAbsent(key,new SingletonMap());
                if(map == null){
                    map = singletonMap.get(key);    
                }
        }       
        return map;
    }
}

This implementation is unnecessarily convoluted. There are simpler ways to create a singleton - See the answers posted by Peter and me below. — munyengm
– munyengm, Commented Aug 9, 2012 at 8:35

Peter Lawrey · Accepted Answer · 2012-08-09 07:54:36Z

3

Its pretty simple if you know how

enum Singleton {
    INSTANCE;
}

The INSTANCE is lazy loaded and thread safe (and doesn't use explict locking of any kind)

answered Aug 9, 2012 at 7:54

Peter Lawrey

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2 Comments

Metalhead Over a year ago

sorry but I do not know much about enums yet and I think you may be right too but can you comment on the implementation I had shared? Do see any loopholes in it?

Peter Lawrey Over a year ago

It could create more than one SingletonMap and it is far more complicated than it needs to be. Otherwise it could work.

munyengm · Accepted Answer · 2012-08-09 08:54:16Z

2

2 Comments

Metalhead Over a year ago

Just want to aks you how can this be any different if I use "public static final Singleton INSTANCE = new Singleton(); " as part of Singleton class member variable? what benefit does Inner class provides ?

munyengm Over a year ago

The static inner class is only created when either getInstance() is called or INSTANCE is accessed. A static instance variable will be created the moment it's parent class is loaded by the Classloader. See the Java Language Specification for details on how static instance varaibles are initialised.

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