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, I didn't find it type-safe due to its underlying dependency on the MethodInvoker. I also looked at #4672, but they don't seem to help my use case.

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.