Kotlin data class support for FlatFileItemReaderBuilder

[0x1306e6d]

Please do a quick search on Github issues first, the feature you are about to request might have already been requested.

Expected Behavior

Make FlatFileItemReaderBuilder detect whether the target type is Kotlin data class and sets proper FieldSetMapper which is not BeanWrapperFieldSetMapper.

RecordFieldSetMapper works with a Kotlin data class but we might be better to introduce a new dedicated FieldSetMapper.

Current Behavior

The FlatFileItemReaderBuilder only detects whether the target type is record or not. So it sets the BeanWrapperFieldSetMapper which instantiates the target type by the default constructor (no-args) and causes NotWritablePropertyException:

spring-batch/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/file/builder/FlatFileItemReaderBuilder.java

Lines 463 to 467 in d1bd771

	if (this.targetType != null && this.targetType.isRecord()) {
	RecordFieldSetMapper<T> mapper = new RecordFieldSetMapper<>(this.targetType);
	lineMapper.setFieldSetMapper(mapper);
	}
	else {

Context

As a workaround, we might make the data class fields nullable and mutable like:

data class Player(var lastName: String? = null)

but as you know, it doesn't leverage the Kotlin's features.