Proposal: Automatically register ItemHandler as StepListener instead of only StepExecutionListener in ChunkOrientedStepBuilder

[KILL9-NO-MERCY]

Hello, Spring Batch team! I would like to submit a proposal regarding how item handlers are automatically registered as listeners on ChunkOrientedStepBuilder

Context
Starting from commit 52875e7, an ItemReader, ItemProcessor, or ItemWriter is automatically registered to stepListeners(as a StepExecutionListener) only when it directly implements StepExecutionListener:

if (this.reader instanceof StepExecutionListener listener) {
    this.stepListeners.add(listener);
}
if (this.writer instanceof StepExecutionListener listener) {
    this.stepListeners.add(listener);
}
if (this.processor instanceof StepExecutionListener listener) {
    this.stepListeners.add(listener);
}

In Batch 5, however, these components were automatically registered as listeners if:

1. they implemented StepListener, or
2. they had methods annotated with any listener annotation defined in StepListenerMetaData
   (handled internally by StepListenerFactoryBean.isListener(itemHandler) in SimpleStepBuilder#registerAsStreamsAndListeners())

This allowed ItemReader/ItemProcessor/ItemWriter to be detected as a listener even when implementing more specific listener interfaces (e.g., ItemReadListener, ItemProcessListener, etc.) or when using listener annotations such as @BeforeRead, @AfterRead, @OnReadError, etc.

Proposal
Although I haven’t personally used this pattern extensively, allowing item handlers that implement ItemReadListener/ItemProcessListener/ItemWriteListener to be automatically registered via the StepListener interface could increase their practical utility, as the listener could access internal state of the item handler directly.

if (this.reader instanceof StepListener listener) {
    this.stepListeners.add(listener);
}
if (this.writer instanceof StepListener listener) {
    this.stepListeners.add(listener);
}
if (this.processor instanceof StepListener listener) {
    this.stepListeners.add(listener);
}

Thank you for your time and consideration!

[fmbenhassine]

Thank you for the proposal! I think that was targeted at polymorphic objects, which act for example as an item reader and an item read listener at the same time. I have no objection to add this in v6 to reduce the gap with v5 and make migrations as smooth as possible.

[KILL9-NO-MERCY]

Thank you for the quick review and decision to include this in v6! I am happy that this change will help smooth out migrations for v5 users.