Singleton clone attack

Question

I saw a lot of people implementing the clone method for singletons, which are throwing a CloneNotSupportedException. Why?

How for example could this be hacked by cloning or any other way? Btw. I have read effective java and know about enums.

public final class Elvis implements Serializable {

    public final static transient Elvis INSTANCE = new Elvis();

    private Elvis() {
        if(INSTANCE != null) {
            throw new IllegalStateException("This is a singleton. Don't try to instantiate it.");
        }
    }

    private Object readResolve() {
        //serialization protection
        return INSTANCE;
    }
}

score 2 · Answer 1 · answered Nov 15 '12 at 16:23

2

Otherwise one can create more than one object of your singleton class using cloning.

answered Nov 15 '12 at 16:23

Subhrajyoti Majumder

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The object class has `clone()` method – Sri Harsha Chilakapati Nov 15 '12 at 16:39
But if there is no interface Cloneable implemented, how would you call that? – Christian Nov 15 '12 at 16:41
you can add method run time to this class. [how ?](http://stackoverflow.com/questions/6680674/can-a-java-class-add-a-method-to-itself-at-runtime) – Subhrajyoti Majumder Nov 15 '12 at 16:47
If you can show me an implementation of this method I will believe you. – Christian Nov 15 '12 at 17:03
Ok, but this only can happen with a patched version of the jvm. Not the standard way. – Christian Nov 15 '12 at 17:27

score 0 · Answer 2 · answered Nov 15 '12 at 16:28

0

... throwing a CloneNotSupportedException. Why?

You can check out the docs here:

protected Object clone() throws CloneNotSupportedException

Throws: CloneNotSupportedException - if the object's class does not support the Cloneable interface. Subclasses that override the clone method can also throw this exception to indicate that an instance cannot be cloned.

answered Nov 15 '12 at 16:28

Bhesh Gurung

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I didn't ask why clone throws a CloneNotSupportedException. I asked, why would anyone implement that method if there is no interface Cloneable implemented anyway? – Christian Nov 15 '12 at 16:42
I don't understand why somebody trying to implement singleton would also implement the clone method. It doesn't make sense to me. – Bhesh Gurung Nov 15 '12 at 16:51
It doesn't. But I saw a lot of mentions in forums where developers are preventing cloning by implementing clone and throwing an exception. I ask why, because I can't imagine a hack. – Christian Nov 15 '12 at 17:02

Singleton clone attack

2 Answers2