Creating java singleton with static inner class

Question

I want to use the following pattern to create a singleton in java

public class Singleton {
        // Private constructor prevents instantiation from other classes
        private Singleton() { }

        /**
        * SingletonHolder is loaded on the first execution of Singleton.getInstance() 
        * or the first access to SingletonHolder.INSTANCE, not before.
        */
        private static class SingletonHolder { 
                public static final Singleton INSTANCE = new Singleton();
        }

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

But what happens when the private constructor I want to call is

 private Singleton(Object stuff) {... }

How do I pass stuff to INSTANCE = new Singleton()? As in INSTANCE = new Singleton(stuff);

Rewriting the above snippet:

public class Singleton {
        // Private constructor prevents instantiation from other classes
        private Singleton(Object stuff) { ... }

        /**
        * SingletonHolder is loaded on the first execution of Singleton.getInstance() 
        * or the first access to SingletonHolder.INSTANCE, not before.
        */
        private static class SingletonHolder { 
                public static final Singleton INSTANCE = new Singleton();
        }

        public static Singleton getInstance(Object stuff) {
                return SingletonHolder.INSTANCE;//where is my stuff passed in?
        }
}

EDIT:

for those of you claiming this pattern is not thread safe, read here: http://en.wikipedia.org/wiki/Singleton_pattern#The_solution_of_Bill_Pugh.

The object I am passing in is the android application context.

Your description is probably a sign that you should not use a singleton. In particular, what happens if you call: `Singleton.getInstance(object1); Singleton.getInstance(object2);`??? Do you now have two instances of your singleton? Do you mutate the first instance? etc. — assylias, Jul 24 '13 at 16:15
@LuiggiMendoza if you want to make a point make it. I am not going to write a dissertation on why I need a singleton. — Cote Mounyo, Jul 24 '13 at 16:15
My question is about trying to understand the real problem that you're surely *trying to solve* with a singleton. — Luiggi Mendoza, Jul 24 '13 at 16:16
A singleton doesn't make sense if it depends on some parameter coming from outside the class. — GriffeyDog, Jul 24 '13 at 16:17
you should have a look at [this](http://en.wikipedia.org/wiki/Singleton_pattern)..you are making it way too complicated — Anirudha, Jul 24 '13 at 16:18
@GriffeyDog it could conceivably be startup configuration or state information that needs to be available to the singleton. — mcfinnigan, Jul 24 '13 at 16:18
@Anirudh his code sample is one of the ones listed on that page ;) — mcfinnigan, Jul 24 '13 at 16:19
@mcfinnigan Perhaps, but this design leads to calling for the "singleton" with potentially different parameters, like in assylias's comment. — GriffeyDog, Jul 24 '13 at 16:20
@GriffeyDog which leads to the point assylias and me are wondering: why to use a singleton for this? But we won't know if OP doesn't explain the real problem. — Luiggi Mendoza, Jul 24 '13 at 16:21
possible duplicate of [Singleton with Arguments in Java](http://stackoverflow.com/questions/1050991/singleton-with-arguments-in-java) — assylias, Jul 24 '13 at 16:29

score 5 · Answer 1 · edited May 23 '17 at 12:22

If you really want a singleton, there should be only one instance of it (duh!). If you add a parameter to getInstance you probably expect the returned instance to be different (otherwise there is no need for a parameter) which defeats the purpose.

If your goal is to add some configuration when the only instance is created, the simplest way would be to have your singleton query for the configuration information when it is instantiated:

public static final Singleton INSTANCE = new Singleton(getConfiguration());

where getConfiguration returns what is needed (whether by reading a file or forwarding some other variable for example).

Usual disclaimer: Singletons are evil.
Additional resource: Google guide to writing testable code (in case you were not convinced the first time).

Joris W · Answer 2 · 2013-07-24T16:31:43.587

public class Singleton {
        private static Singleton singleton;

        // Private constructor prevents instantiation from other classes
        private Singleton() { }

        public void addStuff(Object stuff){}    

        public static Singleton getInstance() {
                 if(singleton == null) singleton = new Singleton()
                 return singleton;
        }
}

and use it as:

 Singleton s = Singleton.getInstance();
 s.addStuff(stuff);

or an alternative

public class Singleton {
        private static Singleton singleton;

        // Private constructor prevents instantiation from other classes
        private Singleton() { }

        public static void redefine(Object stuff){

             singleton = new Singleton(stuff) // choose constructor based on parameters
        }


        public static Singleton getInstance() {
                 return singleton;
        }
}

I thought about this. But I was wondering about a more inclusive approach. — Cote Mounyo, Jul 24 '13 at 16:26
I edited the answer, maybe this is want you were looking for? — Joris W, Jul 24 '13 at 16:32

score 0 · Answer 3 · edited May 23 '17 at 12:14

0

You might want to read

a singleton with parameters is not a singleton

The first answer argues why a >>singleton with parameters<< is not a singleton, and doesn't come near a singleton.

edited May 23 '17 at 12:14

Community

1
1

answered Jul 24 '13 at 16:24

user2587106

315
1
5

This should be a comment rather than an answer, but thanks for the Q/A. – Luiggi Mendoza Jul 24 '13 at 16:25

score -1 · Answer 4 · answered Jul 24 '13 at 16:23

-1

Why not get rid of SingletonHolder use factory pattern. You will have to decide what to do, when try to call getInstance twice, but with different 'stuff'.

public class Singleton {

    private static Singleton singleton
    private final Object stuff;

    private Singleton(Object stuff) {
        this.stuff = stuff;
    }

    public static synchronized Singleton getInstance(Object stuff) {
         if (singleton == null) {
             singleton = new Singleton(stuff);
             return singleton;
         }

         return singleton; // or throw error because trying to re-init
   }
}

answered Jul 24 '13 at 16:23

Ayub Malik

2,488
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'or throw error because trying to re-init' This defeats the purpose of a singleton – Joris W Jul 24 '13 at 16:26
Agreed but going with the flow of the question. I would probably not use params in a singleton – Ayub Malik Jul 24 '13 at 16:29
@assylias What have I missed regarding thread safety? I can't spot it. CHeers – Ayub Malik Jul 24 '13 at 16:41
1

@AyubMalik The first version was not synchronized. Now it is fine. – assylias Jul 24 '13 at 16:42
However, the problem with this approach is if getInstance() gets called so many times in a large system, the overhead for synchronization is high. You could synchronize on the actual object. – geraldgreen Oct 20 '13 at 19:09

Creating java singleton with static inner class

4 Answers4