Kotlin Micronaut is not loading application.yml properties

After following 5. Team Configuration with @ConfigurationProperties in this document https://guides.micronaut.io/latest/micronaut-configuration-maven-kotlin.html. And created a configuration bean like this

micronaut:
  application:
    name: sftp-test
netty:
  default:
    allocator:
      max-order: 3
sftp:
  known-hosts-file: "testKnownHost"
  private-key-file: "testPk"
  username: "testUser"
  password:
  remote-host: "1.0.0.0"
  remote-port: 22
  verify-timeout: 20000

// SftpConfig.kt
@ConfigurationProperties("sftp")
class SftpConfig {
    var knownHostFile: String = FileConverterConstant.SFTP_KNOWN_HOSTS_FILE_PATH
    var privateKeyFile: String = FileConverterConstant.SFTP_PRIVATE_KEY_PATH
    var privateKeyPass: CharArray = FileConverterConstant.SFTP_PRIVATE_KEY_PASS.toCharArray()

    @NotBlank
    lateinit var username: String

    @NotBlank
    lateinit var remoteHost: String
    var remotePort: Int = FileConverterConstant.SFTP_REMOTE_PORT
    var verifyTimeout: Long = FileConverterConstant.SFTP_VERIFY_TIMEOUT
}

// Inside a test function
    "Test" {
        val items: MutableMap<String, Any> = HashMap()
        items["sftp.username"] = "test"
        items["sftp.remoteHost"] = "1.0.0.0"

        val ctx = ApplicationContext.run(items)
        val sftpConfig = ctx.getBean(SftpConfig::class.java)
        println(sftpConfig.username)
    }

If I remove the properties from the items map for example items["sftp.username"] = "test", the missing properties was filled with the properties from the application.yml file I configured. Everything seems to be working in the test like this page documented.

So now I moved this to an Singleton bean class
When I use @Inject like below to create the bean and try to use the bean. It wont read anything from the application.ym

@Inject
val sftpConfig = SftpConfig()

Now it gives me this error message Message: lateinit property username has not been initialized If I allow the filed to be null. It is null after calling the bean. So the values in application.yml file is not being loaded at all.

While in the test the ApplicationContext was manually created, in here I am relying on Micronaut, am I missing certain dependency? What could be the reason the values in application.yml is not loaded? Side note, if I assign a health endpoint in the application.yml file, it does work after server starts. So the file is being read, but the values I configured for sftp is not being injected in to the configuration bean.

For testing your config, you should use @MicronautTest and inject the config-DTO like shown below.

import io.kotest.assertions.assertSoftly
import io.kotest.core.spec.style.BehaviorSpec
import io.kotest.matchers.string.shouldNotBeEmpty
import io.micronaut.test.extensions.kotest5.annotation.MicronautTest

@MicronautTest
class SftpConfigPropertiesTest(sut: SftpConfig) : BehaviorSpec({

    Given("populated sftp in application.yml") {
        Then("all fields should be populated") {
            assertSoftly(sut){
                it.username.shouldNotBeEmpty()
                /** add remaining assertions */
            }
        }
    }
})

I believe best practice is to use constructor injection instead of @Inject. Example:

@Singleton
class MyService(private val sftpConfig : SftpConfig) {

  /** class body goes here */
}