Use existing `ItemWriter` with `CompositeItemWriter` expecting different item types

[scordio]

I've recently encountered the need for an adapter to use an existing ItemWriter<SimpleType> in a step that works with ComplexType items, where a ComplexType instance encapsulates a SimpleType one (imagine a ComplexType::getSimpleType getter available).

While PropertyExtractingDelegatingItemWriter seems to be a solution at first sight, it's actually not compatible with my use case. I also looked at #4672, but they don't seem to help my use case either.

Taking inspiration from java.util.stream.Collectors.mapping(Function, Collector), I came up with a custom ItemWriter adapter for this purpose, statically imported as mapping(Function, ItemWriter), which allows writing a step definition like the following:

factory.get("step")
  .<ComplexType, ComplexType>chunk(10)
  .reader(/* an ItemReader<ComplexType> */)
  .writer(mapping(ComplexType::getSimpleType, new SimpleTypeItemWriter()))
  .build()

I heavily simplified the example to give a feeling about the core idea. Arguably, that example can also be solved with a processor:

factory.get("step")
  .<ComplexType, SimpleType>chunk(10)
  .reader(/* an ItemReader<ComplexType> */)
  .processor(/* an ItemProcess<ComplexType, SimpleType> */)
  .writer(new SimpleTypeItemWriter())
  .build()

However, my real-life use case also involves a CompositeItemWriter where each delegate expects items of a different type, all encapsulated by ComplexType. Something like the following:

factory.get("step")
  .<ComplexType, ComplexType>chunk(10)
  .reader(/* an ItemReader<ComplexType> */)
  .writer(new CompositeItemWriterBuilder<>()
    .delegates(
      mapping(ComplexType::getSimpleType, new SimpleTypeItemWriter())),
      mapping(ComplexType::getAnotherType, new AnotherTypeItemWriter())),
      mapping(ComplexType::getOneMoreType, new OneMoreTypeItemWriter())),
      /* etc */
    )
    .build())
  .build()

Would Spring Batch be interested in introducing such an adapter, maybe as a static factory method under ItemWriter for better discoverability?

If yes, I would be happy to provide a PR for it.

[scordio]

This can also be relevant for ItemReader and ItemProcessor.

[scordio]

While PropertyExtractingDelegatingItemWriter seems to be a solution

After some experimentation, PropertyExtractingDelegatingItemWriter is not a suitable solution for my use case, as it attempts to invoke the delegate method (in this case, write(Chunk)) with each mapped value, rather than building up a new chunk with the mapped values.

This is just to confirm that I was unable to find an out-of-the-box solution for my use case.

[scordio]

This can also be relevant for ItemReader and ItemProcessor.

After checking better, CompositeItemReader and CompositeItemProcessor have a different delegation pattern compared to CompositeItemWriter, and I don't see the usefulness of adding such an adapter to ItemReader or ItemProcessor.