Simple CompositeItemReader analagous to CompositeItemWriter and CompositeItemProcessor

[spring-projects-issues]

Elliot Korte opened BATCH-2857 and commented

There may be a reason for this, but why doesn't there exist a composite item reader? It would be useful to chain different readers in the same way as CompositeItemWriter and CompositeItemProcessor. There's a MultipleResourceItemReader but I'm wondering why does the only reader which uses this delegate pattern have the restriction of being "ResourceAware"?

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No further details from BATCH-2857

[fmbenhassine]

There may be a reason for this, but why doesn't there exist a composite item reader? It would be useful to chain different readers in the same way as CompositeItemWriter and CompositeItemProcessor.

I couldn't find any context about why there is no composite item reader, but I guess the main reasons are how to respect the "forward-only" contract of an item reader and how to deal with restartability. Similar to the composite writer, a composite reader could delegate reading items to a list of delegates in their registration order (once a delegate returns null, it moves to the next delegate, and so on), but making this kind of composition restartable is quite challenging.

An example of such a composite item reader can be found here. I would be grateful for anyone who gives it a try and share feedback here. A typical use case for this is reading the same data from different datasources for aggregation (the example shows a composite reader that delegates to 3 readers, reading the same data from two files and a database table).

There's a MultipleResourceItemReader but I'm wondering why does the only reader which uses this delegate pattern have the restriction of being "ResourceAware"?

I see no obvious reason for that, but since items could come from different resources, one should probably be able to know the original resource from which an item was read.