Empty yaml objects are not injected as properties

[jntakpe]

Spring Boot version: 2.2.1.RELEASE

Having the following properties

prefix:
  obj:
    foo:
      firstName: John
      lastName: Doe
    bar: {}

And given the following properties class

@ConstructorBinding
@ConfigurationProperties("prefix")
data class ObjProperties(val obj: Map<String, FooBarProperties>)

@ConstructorBinding
data class FooBarProperties(val firstName: String? = "", val lastName: String? = "")

I was expecting that injecting ObjProperties bean will result in having both foo property and bar property in obj map. Unfortunately, bar property is missing from the objmap.

[wilkinsona]

Thanks for the issue report, @jntakpe. We spent some time discussing this today and concluded that this it isn't something that we want to try and support. Configuration property binding is backed by Spring Framework's Environment and PropertySource abstractions. The later is a "source of name/value property pairs". As such, there's no way to represent the YAML-specific concept of an empty object that could then be used by the property binding to know that an empty instance of FooBarProperties should be created.