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, Oct 07 '15 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, Oct 07 '15 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, Oct 07 '15 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, Oct 08 '15 at 13:59
@AndyTurner: I think it would not be semantically identical. Putting the code in a static block will make it thread-safe as it will be guaranteed to be executed only once. Please correct me if I am wrong. — displayName, Jun 14 '18 at 20:14
@displayName have you tried compiling the two versions and comparing the bytecode, as suggested in my previous comment? — Andy Turner, Jun 14 '18 at 20:16

score 4 · Answer 1 · edited May 23 '17 at 11:58

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 '17 at 11:58

Community

1
1

answered Oct 07 '15 at 13:08

Elliott Frisch

198,278
20
158
249

Thanks Elliott, but why there is a need for clone method? i never implements cloneable – Masarrat Siddiqui Oct 08 '15 at 14:00
What happens if someone `clone`s a `Singleton`? – Elliott Frisch Oct 08 '15 at 14:04

score 3 · Answer 2 · answered Oct 07 '15 at 13:04

3

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

answered Oct 07 '15 at 13:04

Andy Turner

137,514
11
162
243

I Agree Andy, added private constructor, Please review and let me know. – Masarrat Siddiqui Oct 08 '15 at 14:08

singleton with static initializer block

2 Answers2