SpringBoot List with object in configuration properties are not mapped into object from ENV variables

I have list of objects in my properties and I want populate them by env variables. Spring boot version = 3.5.3.

Example of my properties structure:

my:
  tests:
    - a: foo
      b: bar
    - a: hello
      b: world

My kotlin code:

data class Test(val a: String, val b: String)

@ConfigurationProperties(prefix = "my")
data class MyProperties(val tests: List<Test>)

and I want fill them with environment variables:

MY_TESTS_0_A=foo
MY_TESTS_0_B=bar
MY_TESTS_1_A=hello
MY_TESTS_1_B=world

But the tests is always empty.

I tried multiple combination with list of Strings or object with single field. Both works.

@ConfigurationProperties(prefix = "my")
data class MyProperties(val tests: List<String>)

MY_TESTS_0=foo
MY_TESTS_1=bar

data class Test(val a: String, val b: String)

@ConfigurationProperties(prefix = "my")
data class MyProperties(val tests: List<String>)

MY_TESTS_0=foo
MY_TESTS_1=bar

The mapping somehow works, but not for objects with multiple fields.

---

Workaround is to use env SPRING_APPLICATION_JSON and pass values as JSON.

Comment From: philwebb

I'm not sure exactly what's going on, but I can't seem to replicate this with a pure Java application.

If you build this project then run the following:

MY_TESTS_0_A=foo MY_TESTS_0_B=bar MY_TESTS_1_A=hello MY_TESTS_1_B=world java -jar target/demobind-0.0.1-SNAPSHOT.jar

You get:

2025-07-31T22:22:02.069+01:00  INFO 82614 --- [demobind] [           main] c.example.demobind.DemobindApplication   : Started DemobindApplication in 0.445 seconds (process running for 0.669)
MyProperties[tests=[Test[a=foo, b=bar], Test[a=hello, b=world]]]

@radeklos Could you please share a project that replicates the problem?