In Spring Boot, @ConfigurationProperties is a powerful feature that allows you to externalize configuration properties and bind them to Kotlin data classes. This tutorial will walk you through the steps of using @ConfigurationProperties to simplify configuration management in your Spring Boot application.

I'd favor the @ConfigurationProperties annotation over using @Value or the Environment bean. On advantage is that @ConfigurationProperties are type safe and you can validate @ConfigurationProperties on startup of the application: https://simonscholz.github.io/tutorials/validate-spring-configuration-properties

Prerequisites

• JDK
• Kotlin
• Spring Boot
• Gradle or Maven
• An Integrated Development Environment (IDE) such as IntelliJ IDEA or Eclipse or VS Code

Create a Spring Boot Project

Start by creating a new Spring Boot project. You can use the Spring Initializer (https://start.spring.io/) or your IDE to generate a new project.

Configure Application Properties

In the application.properties or application.yml file we'd define some properties, we want to read.

simonscholz:
    third-party-url: ${THIRD_PARTY_URL}
    third-party-auth: ${THIRD_PARTY_JWT_TOKEN}

Define Configuration Properties Data Class

Create a Kotlin data class to represent your configuration properties. This class will be annotated with @ConfigurationProperties and will contain properties corresponding to your configuration properties.

import org.springframework.boot.context.properties.ConfigurationProperties
import org.springframework.stereotype.Component

@ConfigurationProperties(prefix = "simonscholz")
data class SimonScholzProperties(
    val thirdPartyUrl: String = "",
    val thirdPartyAuth: String = "",
)

In this example, we've created a SimonScholzProperties data class with two properties: thirdPartyUrl and thirdPartyAuth. The @ConfigurationProperties annotation specifies a prefix that will be used to bind properties from the configuration file.

Make sure to use the prefix, in this case simonscholz, specified in your @ConfigurationProperties annotation.

Enable Configuration Properties

The @EnableConfigurationProperties annotation must be used to enable the initialization of the data classes annotated with @ConfigurationProperties. The @EnableConfigurationProperties annotation can be used in any @Configuration class, e.g., @SpringBootApplication.

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.context.properties.EnableConfigurationProperties
import org.springframework.boot.runApplication

@SpringBootApplication
@EnableConfigurationProperties(SimonScholzProperties::class)
class Application

fun main(args: Array<String>) {
    runApplication<Application>(*args)
}

or

@Configuration
@EnableConfigurationProperties(SimonScholzProperties::class)
class ApplicationConfigurationProperties

Inject Configuration Properties

With these steps completed, you can now inject the SimonScholzProperties data class into your Spring components.

import org.springframework.stereotype.Service

@Service
class SomeService(private val simonScholzProperties: SimonScholzProperties) {
    fun someMethod() {
        println(simonScholzProperties.thirdPartyUrl)
        println(simonScholzProperties.thirdPartyAuth)
    }
}

Sources

• https://reflectoring.io/spring-boot-configuration-properties/