Question

Question says everything. Is there any special event occurs? I have a class which needs to implement Serializable interface. I am developing an application using Wicket. I have implemented a Factory class which will provide some links (to remove the redundancy of same code block written over and over):

public class NavigationLinkFactory implements Serializable {

    private static final long serialVersionUID = 7836960726965363785L;

    private NavigationLinkFactory() {

    }

    private static class SingletonHolder { 
        public static final NavigationLinkFactory instance = new NavigationLinkFactory();
    }

    public static NavigationLinkFactory getFactory() {          
        return SingletonHolder.instance;

    }

    private Object readResolve() throws ObjectStreamException {
        return SingletonHolder.instance;
    }

    public Link<Serializable> getUserHomeLink() {
        return new Link<Serializable>("home", new Model<Serializable>()) {

            private static final long serialVersionUID = -8282608926361995521L;

            @Override
            public void onClick() {
                EMSSession session = (EMSSession) getSession();
                session.setActiveHorizonalMenu(1);
                setResponsePage(HomePage.class);
            }
        };      
    }

    public Link<Serializable> getProfileLink() {
        return getProfileViewLink();
    }

    public Link<Serializable> getProfileViewLink() {
        return new Link<Serializable>("profileView", new Model<Serializable>()) {

            private static final long serialVersionUID = 232185489384306999L;

            @Override
            public void onClick() {
                EMSSession session = (EMSSession) getSession();
                session.setActiveHorizonalMenu(2);
                setResponsePage(ProfileViewPage.class);
            }
        };  
    }
}

If I doesn’t implement Serializable then I am getting exception at java.io.ObjectOutputStream.writeObject, which is thrown by the runtime environment of the wicket framework. If I implement it then it is gone.

So what really happens when one calls ObjectInputStream#readobject() method for some object created by Singleton pattern?

I am not adding wicket tag because I don’t think this question is related to the Wicket.

Answer 1

Unfortunately, what happens by default is you get a second (or third, or thirty-seventh) instance of your singleton — not good. Luckily there’s a way around this: by implementing the readResolve() method, which can return a replacement object — the proper singleton object, in your case — instead of the one returned by default serialization. The link describes the particulars, but it’s pretty simple — you just implement this parameterless instance method to return the singleton object.