singleton with static initializer block

Question

Please refer below code and help me to understand why this not a valid singleton implementation.

class A{
    private static A ob;

    private A(){}
    static {
        ob = new A();
    }
    public static A getInstance() {
        return ob;
    }
}

You can drop the static initializer block and just declare it as private static final A ob = new A();, it is semantically identical. — Andy Turner
– Andy Turner, Commented Oct 7, 2015 at 13:06
@AndyTurner But declaring a static brackets will make the code execute even if he never requires an instance of A, won't it? — user2651804
– user2651804, Commented Oct 7, 2015 at 13:09
@user2651804 it is semantically identical. Try compiling it with the static initializer and with a field initializer, the resulting bytecode is the same. The field initializer becomes a static initializer, it's just less verbose. — Andy Turner
– Andy Turner, Commented Oct 7, 2015 at 13:09
Thanks friends for your responses, I forget to add private constructor. Please let me know if updated code still required any changes for a valid singleton example — Masarrat Siddiqui
– Masarrat Siddiqui, Commented Oct 8, 2015 at 13:59

Community · Accepted Answer · 2017-05-23 11:58:25Z

4

It is not a valid singleton (multiple instances may be instantiated) because you get the default constructor. Add a private constructor (which will prevent the default constructor from being inserted). And you probably want to override clone().

private A() {
}

@Override
public Object clone() throws CloneNotSupportedException {
    throw new CloneNotSupportedException(this.getClass().getName() 
            + ": is a singleton.");
}

I would usually use an enum to implement a singleton.

edited May 23, 2017 at 11:58

CommunityBot

11 silver badge

answered Oct 7, 2015 at 13:08

Elliott Frisch

202k20 gold badges166 silver badges265 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Masarrat Siddiqui Over a year ago

Thanks Elliott, but why there is a need for clone method? i never implements cloneable

Elliott Frisch Over a year ago

What happens if someone clones a Singleton?

Andy Turner · Accepted Answer · 2015-10-07 13:04:38Z

3

Nothing stops me creating a new instance of A by calling new A(), so I can have multiple instances of it.

answered Oct 7, 2015 at 13:04

Andy Turner

141k11 gold badges169 silver badges263 bronze badges

1 Comment

Masarrat Siddiqui Over a year ago

I Agree Andy, added private constructor, Please review and let me know.

Collectives™ on Stack Overflow

singleton with static initializer block

2 Answers 2

2 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related