Singletons and Serializable

[Barry Andrews]

Hi Folks,

Maybe not a Java puzzler, but here goes.

I have a Singleton class X. So that supposedly guarantees only 1 instance per JVM right? But then I have a state manager class that manages the persistence of certain classes based on options and such. One of the classes it serializes is class X. When the program starts again, class X is loaded from disk. Now I am no longer guaranteed only 1 instance. Now I can potentially have 2 instances floating around, the one that was loaded and if someone calls getX(). That is because my static variable in X is still null.

Now I can easily create a method public static void setX(X x); and call it from the state manager class when X is loaded. But I am sensing a design flaw. I could also put my persistence code in X, but then I would have duplicate code and I really like the idea of having all of my persistence handled in 1 place. Is there a pattern that handles this sort of issue?


many thanks,

Barry

[Ernest Friedman-Hill]

Yep. It's the DontMakeSingletonClassesSerializable pattern! Don't make them Cloneable, either.

[John Smith]

If your singleton is serializable, it must have a readResolve() method that looks like this:

private Object readResolve() throws ObjectStreamException {
    return INSTANCE;
}