Clear Singleton instance in Java

Question

I have a Singleton class to save the state of an application's module. This class simply have a lot of class variables with setters and getters :

public class ModuleState{

private static ModuleState instance;

private A a;
private B b;
private C c;
..
..
..
..

private ModuleState (){}

public ModuleState getInstance(){
    if(instance==null)
        instance=new ModuleState();

    return instance;
}

}

At a precise moment of the application lifecycle, i have the need to CLEAR the module's state. What i do now is to reset ALL the variables in ModuleState by a clearAll() method like this:

public void clearAll(){
    a=null;
    b=null;
    c=null;

    ..
    ..
}

My question is the following : there is a cleaner method to do this reset? Possibly clearing the singleton instance itself, without resetting every class variable?

The problem with this approach is that i may have the need to add a new class variable to the ModuleState. In this case i must remember to add a line in the clearAll() method to reset the new variable.

Answer 1

What about ...

public static volatile ModuleState instance = null;

public static void reset() {
    instance = new ModuleState();
}

p.s.: as per discussion below: in a multithreaded environment it's very important to synchronize the access on the instance because the JVM is allowed to cache its value. You can use volatile as shown above. Thanks to all!

Cheers!

Answer 2

no, this approach is perfectly acceptable. you are of course synchronizing access to these state objects in some way, right? otherwise you risk someone seeing a half-cleared config object.

another thing you could do to future-proof yourself against any extra state added in the future is store all of your state in a HashMap, for example, instead of individual fields. this way, clear()ing the hashmap ensures that all state is wiped and adding any extra state in the future becomes safer