Add MappingItemWriter

[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.

[fmbenhassine]

I suppose when this feature goes to a dedicated class, there won't be any mapping static method, right?

That's right. The mapping function would be passed to the MappingItemWriter, just like a Classifier is passed to a Classifier-based item writer.

This won't be a breaking change, so we still have time to include in v6. Looking forward to your update on this. Thanks again for everything you do for Spring Batch! 🙏

[fmbenhassine]

Thank you for the updates! Rebased and merged as 2621b20.

I realise that this new item writer allows for a kind of "deconstruction pattern" where a big item can be deconstructed into smaller parts (thanks to the mapping function, and the tests are great examples) where each part is written to a different location. This is awesome!

[scordio]

@fmbenhassine thanks for merging it! I realized only now that this might deserve an entry in the user guide. If you agree, I would raise another small PR for it.

[fmbenhassine]

I was planning to update that as part of the big documentation overhaul for v6, but if you want to open a PR you are welcome!

[scordio]

Sure, I can take care of it 🙂

[scordio]

I reviewed my use cases to reconfirm these changes would integrate correctly and I realized one more detail a bit late, sorry about it 🙁

There is one case where the existing writer is an ItemStream, implementing open(), update(), and close(). Ideally, MappingItemWriter should also implement ItemStream and check if downstream is an ItemStream too, delegating the corresponding calls if that's the case.

@fmbenhassine do you have any concerns with this? If not, I'd be happy to raise a PR for it (combined with the docs updates).

[fmbenhassine]

State management is not mandatory, we have some decorators that are not restartable. But if you can make this mapping writer an item stream, then yes that would be great!

[fmbenhassine]

@scordio Just FYI about documentation updates, I added a section about this new item writer in the reference docs: 114f0f3

[scordio]

Oh, you beat me to it 😄 I just raised #5046, I'll rebase it soon and align it with your changes.