SpringBootの起動時にCustomなPropertySourceを読み込ませる例

Application.kt

@SpringBootApplication
class Application
fun main(args: Array<String>) {
    // SpringApplication.run(*args)
    SpringApplicationBuilder(Application::class.java)
        // ここで起動時に追加したい Initializer を渡す
        .initializers(CustomContextInitializer())
        .run(*args)
}

CustomContextInitializer.kt

class CustomContextInitializer : ApplicationContextInitializer<ConfigurableApplicationContext> {
    private fun getSsmClient(): SsmClient {
        val credentialsProvider = AwsCredentialsProviderChain.builder()
            .credentialsProviders(
                EnvironmentVariableCredentialsProvider.create(),
                SystemPropertyCredentialsProvider.create(),
                ProfileCredentialsProvider.create(),
                ContainerCredentialsProvider.builder().build())
            .build()
        return SsmClient.builder()
            .credentialsProvider(credentialsProvider)
            .region(Region.AP_NORTHEAST_1)
            .build()
    }
    override fun initialize(applicationContext: ConfigurableApplicationContext) {
        val environment = applicationContext.environment
        val propertySources = environment.propertySources
        
        // これが今回最大のお仕事をする部分です。
        // applicationContext にある PropertySource 群の「先頭」に SSM ParameterStore から取得してくる PropertySource を追加します。 
        propertySources.addFirst(
            CustomPropertySource(
                "ssmParamStorePropertySource",
                CustomSource(getSsmClient())
            )
        )
    }
}

CustomPropertySource.kt

class CustomPropertySource(name: String, source: CustomSource)
    : PropertySource<CustomSource>(name, source) {

    companion object {
        const val SPLIT_CHARACTER = "/"
    }

    override fun getProperty(name: String): Any? {
        return if (name.startsWith(SPLIT_CHARACTER)) {
            source.getProperty(name)
        } else null
    }
}

CustomSource.kt

class CustomSource(private val ssmClient: SsmClient) {
    fun getProperty(propertyName: String): Any? {
        try {
            return ssmClient.getParameter(
                GetParameterRequest.builder()
                    .name(propertyName)
                    .withDecryption(true)
                    .build())
                .parameter()
                .value()
        } catch (e: ParameterNotFoundException) {
            // 指定されたParameterが無い場合に飛んできます。適宜処理してください。
        } catch (e: Exception) {
            // 適宜処理してください。
        }

        return null
    }
}