Singleton: Throwing exception in constructor

Question

In order to prevent Singleton from being broken by using reflection one way is to throw an exception in private constructor as shown in code below:

public final class Foo {

    private static final Foo INSTANCE = new Foo();

    private Foo() {
        if (INSTANCE != null) {
            throw new IllegalStateException("Already instantiated");
        }
    }

    public static Foo getInstance() {
        return INSTANCE;
    }
}

Above is a standard code but what i am thinking is that is it thread safe? If multiple threads try to create the instance using reflection at same time [i.e. before the class is loaded in main memory which implies instance will be null ] then will they succeed?

score 1 · Accepted Answer · edited May 23 '17 at 11:57

You can't access the static members of a class (whether directly or through reflection) before it is loaded. And static final members are initialised during the loading process (step 9 of the JLS description).

So in your case there is no way a thread could:

see INSTANCE before it is properly constructed.
see INSTANCE as null (unless the first call to new Foo() throws an exception)
initialise a second instance (unless that code relies on a different class loader)

score 0 · Answer 2 · edited May 23 '17 at 11:51

0

The best way to create a reflection-safe singleton is to use an enum. It is described here

edited May 23 '17 at 11:51

Community

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answered May 27 '13 at 16:22

mmirwaldt

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i know that but my question is about this method :) – Lokesh May 27 '13 at 16:23
then I did not understand your question ;-) – mmirwaldt May 27 '13 at 16:28

Singleton: Throwing exception in constructor

2 Answers2