Add mapping adapter to ItemWriter

[scordio]

• Closes Use existing ItemWriter with CompositeItemWriter expecting different item types #4735

[scordio]

@fmbenhassine this is now ready for review.

See #4735 (comment) on why I didn't add the same to ItemReader and ItemProcessor.

[fmbenhassine]

Hi @scordio , this is interesting! Thank you for the feature request and the PR!

Since this is a specific use case, I don't think adding the method to the item writer interface is the best option. I see this as a very similar to the ClassifierCompositeItemWriter but with a mapping function instead of a classifier.

What about adding a MappingCompositeItemWriter that uses a mapping Function in the org.springframework.batch.infrastructure.item.support package (next to other support writers).

Let me know what you think about that.

[scordio]

I see this as a very similar to the ClassifierCompositeItemWriter but with a mapping function instead of a classifier.

What about adding a MappingCompositeItemWriter that uses a mapping Function

I think in my use case it would just be a MappingItemWriter, without composition, as the mapping function is specific to each adapted writer.

This could still be combined with a CompositeItemWriter, but not every delegate would have to use it, e.g.:

@Bean
ItemWriter<Person> personItemWriter() {
  return new CompositeItemWriter<>(List.of(
    new MappingItemWriter(Person::name, nameItemWriter), // ItemWriter<String>
    new MappingItemWriter(Person::age, ageItemWriter),   // ItemWriter<Integer>
    new PersonAddressItemWriter()                        // ItemWriter<Person>
  )); 
}

In this example, it's still possible to use an ItemWriter<Person> like PersonAddressItemWriter alongside the other mapped item writers.

Or did you imagine something different?

I suppose when this feature goes to a dedicated class, there won't be any mapping static method, right? I considered it valuable for the readability of the configuration code, but that probably makes sense only if it's acting as a factory method in a "central" place like the ItemWriter interface (which indeed deviates from the rest of Spring Batch).

Summarizing, if you agree, I'd create a MappingItemWriter under org.springframework.batch.infrastructure.item.support, without any factory method.