Resettable, lazy-getter - @Getter(lazy=true,resettable=true)

[Morten Sabroe Mortensen]

I have just had a use-case for a resettable variation of @Getter(lazy=true)!

It comes up when you want to have central collection XXXProviderIndex-something, possible obtained from a ServiceLoader per default, possible decorated upon by using Jakarta CDI or Spring DI, depending upon context. Then make a list of default suppliers, possibly decorate these with additional suppliers, possibly wipe the list clean and insert context-specific supplier. Then you ask for the supplied list -- on-demand, lazy to resolve what should be the static list to be used during the application lifetime.

Then you come too late -- the list is set before you have had a chance to modify the suppliers. Then you really want to reset the list.

What is can be boiled down to -- within a larger context -- 

https://github.com/sabroe/Topp-Standard/tree/main/module/Core/Topp-Standard-Loader-Library/src/main/java/com/yelstream/topp/standard/load/util/holder

-- is this:

public interface Container<X> {
    X getItem();
}

@AllArgsConstructor(staticName="of")
final class LazyContainer<X> implements Container<X> {
    private final Supplier<X> itemSupplier;

    @Getter(lazy=true)
    private final X item=itemSupplier.get();
}

public interface ResettableContainer<X> extends Container<X> {
    void reset();
}

@AllArgsConstructor(staticName="of")
final class ResettableLazyContainer<X> implements ResettableContainer<X> {
     private final Supplier<X> itemSupplier;

     private final AtomicReference<X> itemHolder=new AtomicReference<>();

    @Override
    public X getItem() {
        X item=itemHolder.get();
        if (item==null) {
            synchronized (itemHolder) {
                item=itemHolder.updateAndGet(v->v!=null?v:itemSupplier.get());
            }
        }
        return item;
    }

    @Override
    public void reset() {
        synchronized (itemHolder) {
            itemHolder.set(null);
        }
    }
}

THE POINT being that to get a "occationally resettable" version, we have to create code that essentially ends up replicating the complete code generated by @Getter(lazy=true)!

Hence, there could be a point with a version like  @Getter(lazy=true,resettable=true)

While LazyContainer relies fully on Lombok, ResettableLazyContainer is my own code.

Kind regards,
Morten Sabroe Mortensen