Allow @ConfigurationProperties binding for immutable POJOs

Currently, it seems we are forced to use Kotlin classes with mutable nullable properties and default constructor with @ConfigurationProperties while idiomatic Kotlin code would be using classes with immutable properties initialized via constructor. I think there is a way to supporting that by leveraging kotlin-reflect library like jackson-module-kotlin do.

More concretely, in MiXiT app I would like to be able to convert this MixitProperties class implementation to:

@ConfigurationProperties("mixit")
class MixitProperties(
    val baseUri: String,
    val admin: Credential,
    val drive: Drive
)

class Credential(
    val username: String,
    val password: String
)

class Drive(
    val fr: DriveDocuments,
    val en: DriveDocuments
)

class DriveDocuments(
    val sponsorform: String,
    val sponsor: String,
    val speaker: String,
    val press: String
)

We could imagine to support optional properties by using nullable types, like val sponsorform: String? for example.

I will be happy to help. I have also already worked with @apatrida who maintains Jackson Kotlin module, he may provide us some guidance I think.

[sdeleuze]

Update: the solution proposed bellow does not work with nested classes so in practice is pretty useless 😦

Good news, while working on this issue, I found that the no-arg plugin allows to use Kotlin mutable @ConfigurationProperties data classes with non-nullable properties.

Concretely, with the following configuration:

plugins {
  id "org.jetbrains.kotlin.plugin.noarg" version kotlinVersion
}
noArg {
    annotation("org.springframework.boot.context.properties.ConfigurationPro‌​perties")
}

It is possible to use the following mutable data classes:

@ConfigurationProperties("mixit")
data class MixitProperties(
    var baseUri: String,
    var contact: String,
    var drive: Drive,
    var aes: Aes) {

    data class Drive(
        var fr: DriveDocuments,
        var en: DriveDocuments)


    data class DriveDocuments(
        var sponsorform: String,
        var sponsor: String,
        var speaker: String,
        var press: String)

    data class Aes(
        var initvector: String,
        var key: String)
}

This issue is still needed to avoid the noArg trick and support immutable @ConfigurationProperties, but at least it provides a reasonably usable way to use @ConfigurationProperties with Kotlin + Spring Boot 2.0 regardless if we fix this issue in time for 2.0 or not.

In parallel, I am working on a Kotlin BeanBinder that supports immutable classes (including support for data classes) using Kotlin reflection. I will share the code later when I have something usable.

[sdeleuze]

Thanks a lot Boot team for fixing that!