Avoid multiple instances of Singleton?

Question

I believe when a singleton object gets serialized and is then deserialized upon a plethora of times in series, there are multiple instances of singletons created.

How can you avoid such a situation?

What's the point of the "scala" tag here? – ach Jun 20 '14 at 21:13 — ach, Jun 20 '14 at 21:13

TyMarc · Accepted Answer · 2014-06-20T20:58:19.097

4

Normally, when you are creating a Singleton, you make sure that there is only one instance of it. you could do it that way:

public static class Singleton{
    private final static Singleton instance;

    private Singleton(){}

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


}

Putting the constructor to private makes sure that no one except this class can create an instance of Singleton.

You only need to call the Singleton with getInstance and the Singleton do all the work on its side.

edited Jun 20 '14 at 20:58

answered Jun 20 '14 at 20:45

TyMarc

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It's best to make the class final and the constructor private, so that no one can create a subclass which allows additional objects of type Singleton to be created. – Bobulous Jun 20 '14 at 20:48
@Arkanon Thanks! I made the changes. – TyMarc Jun 20 '14 at 20:49
Also, the Singleton `instance` should be static. – Bobulous Jun 20 '14 at 20:49
2

I don't think this answer addresses the serialize/de-serialize issue – Amir Afghani Jun 20 '14 at 20:59
@AmirAfghani I also think it doesn't. – peter.petrov Jun 20 '14 at 21:01
This answer does NOT provide a secure possibility to really have only one instance of this class. See Amir Afghani answer. – copakawusau Jun 20 '14 at 21:26

score 4 · Answer 2 · answered Jun 20 '14 at 20:49

4

Joshua Bloch talks about using an enum as a singleton to avoid this specific issue in Effective Java. Check out this article: Why Enums are Better Singletons -or- read it directly from Bloch here.

answered Jun 20 '14 at 20:49

Amir Afghani

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+1 from me. Very good reference/link. – peter.petrov Jun 20 '14 at 21:04
Enums requires more memory, see this https://www.youtube.com/watch?v=Hzs6OBcvNQE – Gastón Saillén Dec 05 '18 at 17:03
That video is specific to Android, and it’s the difference between 2K and 4K bytes......... – Amir Afghani Dec 05 '18 at 21:18

Avoid multiple instances of Singleton?

2 Answers2