Kotlin constructor binding is not applicable to target 'class' - spring-boot

Question

i am new to Kotlin. I am trying to achieve same thing, i would do in java.
in application.yaml, i have this config:
conference:
plans:
- plan-id: P1_1
product-id: product1
employee-limit: 50
- plan-id: P1_2
product-id: product2
employee-limit: 100
then i want to create data class, so that , when i run the springboot application, this class is automatically loaded and validated
in Conferences.kt file:
package com.company.conferenceplanservice.config.properties
import jakarta.validation.constraints.NotBlank
import jakarta.validation.constraints.NotEmpty
import org.springframework.boot.context.properties.ConfigurationProperties
import org.springframework.boot.context.properties.bind.ConstructorBinding
import org.springframework.context.annotation.Configuration
import org.springframework.validation.annotation.Validated
#Validated
#ConstructorBinding
#Configuration
#ConfigurationProperties(prefix = "conference")
data class Plans(
#field:NotEmpty val plans: Set<Plan>
)
#Validated
#ConstructorBinding
data class Plan(
#field:NotBlank val planId: String,
#field:NotBlank val productId: String,
val eomployeeLimit: Int
)
but always it throws this exception, on both places where ConstructorBinding is written
Kotlin: This annotation is not applicable to target 'class'
springboot 3.0.0, java 18, kotlin 1.7.21

Answer 1

I hope this helps
I no longer use #ConstructingBinding nowadays
This is How I would have implemented
application.yaml
conference:
plan-id: P1_1
product-id: product1
Conferences.kt
import org.springframework.boot.context.properties.ConfigurationProperties
#ConfigurationProperties(prefix = "conference")
data class Conferences(
val planId: String,
val productId: String,
)
Usage.kt
object MySample {
fun haveFun(conference : Conference) : String {
return conference.planId
}
}
#Component
class BootLoader(val conference: Conference) : CommandLineRunner {
override fun run(vararg args: String?) {
println(MySample.haveFun(conference))
}
}
FYI
"#ConstructingBinding No Longer Needed at the Type Level"
"Improved #ConstructorBinding Detection When using constructor bound #ConfigurationProperties the #ConstructorBinding annotation is no longer required if the class has a single parameterized constructor. If you have more than one constructor, you’ll still need to use #ConstructorBinding to tell Spring Boot which one to use."
"For most users, this updated logic will allow for simpler #ConfigurationProperties classes. If, however, you have a #ConfigurationProperties and you want to inject beans into the constructor rather than binding it, you’ll now need to add an #Autowired annotation."
reference
https://github.com/spring-projects/spring-boot
https://docs.spring.io/spring-boot/docs/current/api/org/springframework/boot/context/properties/ConstructorBinding.html

Answer 2

#Validated
#ConstructorBinding
#Configuration
#ConfigurationProperties(prefix = "conference")
data class Plans(
#field:NotEmpty val plans: Set<Plan>
)
#Validated
#ConstructorBinding
data class Plan(
#field:NotBlank val planId: String,
#field:NotBlank val productId: String,
val eomployeeLimit: Int
)
and #ConfigurationPropertiesScan
annotation is needed in main class

Answer 3

I don't use Spring, but I'm guessing #ConstructorBinding is an annotation for a constructor. You are using it on the class declaration itself. It would be like putting it in front of the class name in Java:
#ConstructorBinding
public class Plans {
//...
}
If you want to annotate the data class constructor in Kotlin, you have to declare the constructor. You have omitted the constructor keyword, which is usually fine, but not if you need to annotate it or modify its visibility:
#Validated
#Configuration
#ConfigurationProperties(prefix = "conference")
data class Plans
#ConstructorBinding constructor(
#field:NotEmpty val plans: Set<Plan>
)