Allow adding packages to entity scanning without disabling the default scanning

[odrotbohm]

I would like to provide a library that adds JPA entities to the persistence context bootstrapped. Ideally I would like to achieve that without the user having to explicitly configure @EntityScan to include the package of that library solely by registering some auto configuration.

I've tried both @EntityScan on a configuration class and a BeanDefinitionRegistryPostProcessor to trigger EntityScanPackages.register(…) but EntityScanner will only apply the autoconfiguration package as fallback if no entity package is configured at all.

I can see that the semantics of @EntityScan are supposed to stay unchanged. I'd be fine with a programmatic approach, so maybe an explicit method EntityScanPackage.registerAdditional(…) could populate an additional list that EntityScanner adds on top of the defaulted base list?

[wilkinsona]

See also #19024, #27549, and #28232.

@odrotbohm, does @AutoConfigurationPackage come close to what you want?

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[odrotbohm]

It does indeed. At least until someone explicitly uses @EntityScan somewhere in their application configuration in which case the configuration would break. So some way of adding to the entity packages would still be appreciated. I think it'll also make the feature a bit more discoverable. The docs currently don't mention @ACP and the Javadoc of both annotations don't mention that fallback mechanism either, and the sole presence of @EntityScan makes it very likely that developers just try to use that if what they want to do is adding entities.

[wilkinsona]

Thanks, @odrotbohm. I'm a bit concerned that providing a mechanism that's always additive will surprise users as it's not very Boot-like to have something that can't be switched off or overridden.

The documentation (both reference and javadoc) for @AutoConfigurationPackage is definitely lacking at the moment. That's what #27549 will address. I wonder if that would be sufficient here? I would expect it to update the reference documentation and the javadoc for both @EntityScan and @AutoConfigurationPackage to make the purpose of the two more clear and to highlight that auto-configuration packages are the default packages used for entity scanning, with @EntityScan being used to take complete control and override those defaults.

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