Avoid cloning of singleton object

[Juni Panda]

Hi,

Can anybody clear my confusion on the object cloning concept.
I wrote a singleton class ... then somebody told me with cloning I can get one more object. Then I started searching over the internet.

I read many articles saying to avoid cloning we need to override the Object clone() method.. but what is the need of it..

Followed this discussion - cloning singleton object

As per the documentation,
"Invoking Object's clone method on an instance that does not implement the Cloneable interface results in the exception CloneNotSupportedException being thrown."

Calling the clone method on an object, will result in throwing an exception at run time.

Hence, when I am writing a singleton class and not implementing Cloneable interface, there is no chance of anyone cloning this singleton object. Even though clone method comes from Object super class..
I guess, any object whose class implements this Cloneable interface is eligible for cloning.. Please clear me if I am wrong..

Thanks

[Sumit Patil]

Hence, when I am writing a singleton class and not implementing Cloneable interface, there is no chance of anyone cloning this singleton object. Even though it comes from Object super class..

There are some ways by which you can clone it, one of them being

...
SingletonObject obj =	  SingletonObject.getSingletonObject();
SingletonObject clone = (SingletonObject) obj.clone();

Here are few links so that you can find more about singletons

Link 1
Link 2

Hope it clears your doubt.

[rohit chavan]

Juni Panda wrote:Hi,

Can anybody clear my confusion on the singleton object cloning concept.
I read many articles saying to avoid cloning we need to override the Object clone() method..
But as per the documentation,
"Invoking Object's clone method on an instance that does not implement the Cloneable interface results in the exception CloneNotSupportedException being thrown."

Hence, when I am writing a singleton class and not implementing Cloneable interface, there is no chance of anyone cloning this singleton object. Even though it comes from Object super class..
I guess, any object whose class implements this Cloneable interface is eligible for cloning.. Please clear me if I am wrong..

Thanks

If you want to create a Singleton Class, which should not be cloned by any other Class, simply DO OVERRIDE clonable interface's clone() method with a blank implementation.[Edited the post after referring Sumit's post above]
Hope this clears your doubt, else please reframe the question.

[Juni Panda]

But when I am writing a singleton class and not implementing Cloneable interface, there is no chance of anyone cloning this singleton object. right?
Then why to override the clone method?

[Sumit Patil]

Please go through the links provided above.

[Santosh Kumar Nayak]

public final class SingeltonCloneTest implements Cloneable {

    /**
     * @param args
     * @return
     */
    private static SingeltonCloneTest instance = null;

    private SingeltonCloneTest() {
     System.out.println("Rahul Tripathi");
    }

    public static SingeltonCloneTest getInstance() {

        if (instance == null) {
            instance = new SingeltonCloneTest();
            return instance;
        }
        return instance;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {

        // TODO Auto-generated method stub
        /*
         * Here forcibly throws the exception for preventing to be cloned
         */
        throw new CloneNotSupportedException();
        // return super.clone();
    }

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        SingeltonCloneTest test1 = SingeltonCloneTest.getInstance();

        try {
            SingeltonCloneTest test2 = (SingeltonCloneTest) test1.clone();
        } catch (CloneNotSupportedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

[Campbell Ritchie]

I have corrected the code tags for you.
You would appear to have a contradiction there, with implementing the Cloneable interface and throwing an Exception from clone().
I also think your attempt at checked locking will not work.

Anyway, everybody now knows the best way to implement singletons is to create an enum.

[Santosh Kumar Nayak]

Yes Enum rocks