@ConfigurationProperties no longer works on a mutable Kotlin data classes

[Azbesciak]

Hello, as in discusion on Gitter there is an issue after update from 3.0.3 to 3.0.4 with @ConfigurationProperty and @Component on bean.

Repro repo: https://github.com/Azbesciak/spring-3-configuration-prop-issue/tree/spring-boot-3.0.4-config-prop-component-issue (please notice branch spring-boot-3.0.4-config-prop-component-issue)

I suppose it existed also in 3.0.0 (that repo contains the example I also reported - #33509)
@wilkinsona also mentioned #34407

[hho]

I (probably) hit the same issue when upgrading to 3.0.4. Had a Kotlin data class with var fields and default values and wanted to apply @ConfigurationProperties like this:

@Bean
@ConfigurationProperties("event-hubs.producer")
fun eventHubProducerProperties() = EventHubProducerProperties()

In 3.0.0 this didn't work, then in 3.0.2 it did, and now in 3.0.4 it doesn't work again.
I guess that fixing #34407 led to a regression for #33710?

I stepped through it with a debugger: there are two instances created, one gets the properties injected, but the other one is used for the rest of the application context.

[philwebb]

@Azbesciak You sample has the SomeConfig class annotated with @Component which makes Spring attempt to create it as a regular bean, rather than a configuration properties bean. If you remove @Component from SomeConfig and add @ConfigurationPropertiesScan to your DemoApplication class then things work.

[philwebb]

@hho I expect your problem is similar. If you define a @Bean method then you are in control of creating the bean and constructor binding cannot be used. You could try @EnableConfigurationProperties(EventHubProducerProperties.class) if you want to create the EventHubProducerProperties instance using constructor binding.

If that doesn't work, please provide a sample that I can take a look at.

[Azbesciak]

@philwebb thank you for the quick reply.
My question is: is it a temporary solution or you stated that and no step back?
You know, that once occurred, I have tens of beans like that, some of them are @Bean, some of them are @Component, some just have @Bean and @ConfigurationProperty in configuration - not declared on class directly.

If it is permanent, I need to know that, but... you know, legacy, backward compatibility... not a good way to introduce in a patch release. Not a good way at all since it was working like this for as long as I remember, and it occurs even in tutorials.

[philwebb]

Sorry, I think I messed up. I assumed that Kotlin data classes were immutable, but it looks like they can have setters. I'll need to change this logic

spring-boot/spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/DefaultBindConstructorProvider.java

Line 109 in 285097d

boolean immutableType = type.isRecord() || isKotlinDataClass(type);

[Azbesciak]

As I remember they can be immutable in spring boot, and still it is possible to use them as properties with @ConfigurationProperty, but to do that some special freamework configuration is required. @sdeleuze will know better :)

But yes, in general, Kotlin data class can be mutable.

[hho]

OK, I took an hour to minimize my repro, and now I'm four minutes late 😄

Thank you both!

[hho]

@wilkinsona Sorry, but I think that title is slightly misleading: The current issue affects all mutable data classes used with @ConfigurationProperties, not just the @Component annotated ones.