I cannot understand why ClassifierCompositeItemWriter in Spring Batch use contravariance for classify the items.
In the class the classifier property is defined with an output type of ItemWriter<? super T>:

public class ClassifierCompositeItemWriter<T> implements ItemWriter<T> {

    private Classifier<T, ItemWriter<? super T>> classifier = new ClassifierSupport<>(null);

For example I want to classify 2 type of items : PetToCreate extends PetAction{} and PetToUpdate extends PetAction{} and delegate to the corresponding writers: CreatePetWriter implements ItemWriter<PetToCreate> or UpdatePetWriter implements ItemWriter<PetToUpdate>.

So I use a SubclassClassifier:

var typeMap = Map.of(
          PetToCreate.class, createPetWriter,
          PetToUpdate.class, updatePetWriter
);
SubclassClassifier<PetAction, ItemWriter<? extends PetAction>> petActionItemWriterSubclassClassifier = new SubclassClassifier<>(typeMap, null);

But I can’t understand why cannot use this classifier for my ClassifierCompositeItemWriter<PetAction> due to the output type of the classifier property that is defined with a type ? super T.

Can someone explain to me the motivation behind this choice, shouldn’t it be possible to achieve it?

Thank in advance.