@ConfigurationProperties requires class to be mutable in Kotlin

[jasperblues]

Is it possible to get @ConfiguraitonProperties to inject a matching constructor? Otherwise in Kotlin it is necessary to declare the configured properties as public and mutable, which is not ideal.

Also, in Kotlin, the @value annotation is a little ugly, since "${something}" is a language feature and must be escaped.

I generally love using Spring Boot and Kotlin together - what could be more fun? But things get a little ugly when it comes to external configuration.

[snicoll]

What's Kotlin specific exactly? @ConfigurationProperties requires a regular "POJO" with public properties. Isn't that the exact same thing? What do you mean by a matching constructor?

[jasperblues]

It is my understanding that in Java we can make the @ConfigurationProperties object immutable, by:

• Having private fields
• Including public getters

. . and that the object will be configured using reflection. In Kotlin, this didn't work. The properties had to be mutable. (I'll attach a sample in a moment).

What do you mean by a matching constructor?

• I mean that rather that set fields by reflection, look for a constructor whose parameter names match required properties and inject that. Ah, but I recall now that in Java/JVM languages constructors don't provide parameter-name reflection, right? So only type-signature matching would be possible, with isn't very practical in the case of a configuration holder (likely lots of strings, etc).

[snicoll]

It is my understanding that in Java we can make the @ConfigurationProperties object immutable

No. As I said, in Java you need a regular JavaBean property (public getter/setter). We don't support anything else at the moment and your request is not kotlin specific either (unless I am mistaken).

Duplicates #1254

[jasperblues]

Ah, I see! I assumed config objects would naturally lean towards being immutable.

👍 on 1254 for myself as well then :)

Thank you.

[sdeleuze]

@jasperblues Do you have any Kotlin @Value usage example to share?

[jasperblues]

Sure:

@Component
class TokenAuthenticationProvider : AuthenticationProvider
{
    val profileRepo : ProfileRepository
    val apiKey : String

    //---------------------------------------------------------------------------------------
    //MARK: - Constructors
    //---------------------------------------------------------------------------------------

    @Autowired constructor(profileRepo: ProfileRepository, @Value("\${service.api.apiKey}") apiKey: String)
    {
        this.profileRepo = profileRepo
        this.apiKey = apiKey
    }


    //etc . . . 

Unfortunately as "${something}" is a language feature in Kotlin, it needs to be escaped. I asked on Stackoverflow for a quick/easy way to customize the property placeholder prefix and suffix in Spring Boot - waiting for advice.

[sdeleuze]

@jasperblues Could you try with #{something} instead of ${something}?

[jasperblues]

@sdeleuze Out of the box, with standard Spring Boot, that does not work. However, I suspect if you changed the PropertyPlaceholderConfigurer with to one that had appropriate prefix and suffix it would.

[sdeleuze]

@jasperblues In fact, using # is probably not good idea since this prefix is already used for evaluating SpEL expressions, but it is indeed possible to customize the prefix used by declaring a PropertySourcesPlaceholderConfigurer (not PropertyPlaceholderConfigurer) in your configuration.

More details on my answer on Stackoverflow.

[jasperblues]

@sdeleuze Thank you, sir :)