Lazy initialization of singleton

Asked Nov 19 '21 at 10:15

Active Nov 19 '21 at 12:14

Viewed 65 times

I'm wondering why the 2nd variant is considered more lazy, that he first.

public class Singleton {
        private static Singleton singleton=new Singleton();
    
        private Singleton(){}
    
        static public Singleton getInstance(){
            return singleton;
        }
    }

The singleton above is initialized when the class is loaded, that is when getInstance() is called. Because classes are loaded when they are required. It's not loaded before getInstance() call. Is that correct?

But the sinleton below is also initialized when getInstance() is called. So can't understand the benefits of the 2nd variant:

public class SingletonWithHolder {

    private SingletonWithHolder() {
    }

    public static SingletonWithHolder getInstance() {
        return Holder.instance;
    }

    private static class Holder {
        private final static SingletonWithHolder instance = new SingletonWithHolder();
    }

   
}

edited Nov 19 '21 at 12:14

asked Nov 19 '21 at 10:15

Ekaterina

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No. I suspect there must be a case when Singleton is loaded before getInstance() call, otherwise 2nd variant would be useless. But what is that case? – Ekaterina Nov 19 '21 at 10:39
The point is that `Holder` is used only by `getInstance()` so `Holder` is loaded the first time when `getInstance()` is executed. – dan1st Nov 19 '21 at 10:46
And when Singleton is loaded the first time? – Ekaterina Nov 19 '21 at 10:48
How can `Singleton` be used without it's `getInstance()`? – Ekaterina Nov 19 '21 at 10:49
1

For example `Singleton.class.getName();`. There are plenty of ways the class can be loaded without calling `getInstance()`. – Kayaman Nov 19 '21 at 10:52
Yes, some static Singleton's constant may be used somewhere, for example... – Ekaterina Nov 19 '21 at 10:54
@Kayaman, could you give some other ways? – Ekaterina Nov 19 '21 at 10:56
2

@Ekaterina `Class.forName("yourpackage.Singleton")` is one other example – f1sh Nov 19 '21 at 10:58
1

@f1sh a good example, since it's very plausible that top level classes would be iterated reflectively through some mechanism (e.g. to observe annotations or something similar), but their inner classes wouldn't. – Kayaman Nov 19 '21 at 11:17
Your first code does not allow access to the singleton. How do you get a singleton? – Nov 19 '21 at 11:48
@sadetaosa Why? Using getInstance() – Ekaterina Nov 19 '21 at 11:50
It's an instance method. It can only be called after getting a singleton. – Nov 19 '21 at 11:57
1

@sadetaosa That's right, it's a mistake. I meant static. This is now fixed. – Ekaterina Nov 19 '21 at 12:13

Lazy initialization of singleton

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