@ConfigurationProperties recommendation for Kotlin doesn't produce metadata properties

[corneil]

In the process of build a stream processor I discovered the recommended use of @ConfigurationProperties for kotlin doesn't work as expected.
My test-processor repo. https://github.com/corneil/test-processor

When building the Java version a file is produced named META-INF/spring-configuration-metadata.json containing:

{
  "groups": [
    {
      "name": "com.example.testprocessor",
      "type": "com.example.testprocessor.TestConfiguration",
      "sourceType": "com.example.testprocessor.TestConfiguration"
    }
  ],
  "properties": [
    {
      "name": "com.example.testprocessor.addition",
      "type": "java.lang.String",
      "description": "Will be added to name to make fullName",
      "sourceType": "com.example.testprocessor.TestConfiguration",
      "defaultValue": "N\/A"
    }
  ],
  "hints": []
}

The container image should have a label name org.springframework.boot.spring-configuration-metadata.json with all configuration properties and a label named org.springframework.cloud.dataflow.spring-configuration-metadata.json with the configuration properties from classes named in META-INF/dataflow-configuration-metadata.properties

{
  "groups": [
    { "name": "com.example.testprocessor", "type": "com.example.testprocessor.TestConfiguration", "sourceType": "com.example.testprocessor.TestConfiguration" },
    { "name": "com.example.testprocessor2", "type": "com.example.testprocessor.TestConfiguration2", "sourceType": "com.example.testprocessor.TestConfiguration2" }
  ],
  "properties": [
    {
      "name": "com.example.testprocessor.addition",
      "type": "java.lang.String",
      "description": "Will be added to name to make fullName",
      "sourceType": "com.example.testprocessor.TestConfiguration"
    }
  ]
}

• When using the recommended data class the properties section doesn't contain the property com.example.testprocessor2.addition.
• The defaultValue is missing from the property in the case where properies are detected.

Recommendation

@ConfigurationProperties(prefix = "com.example.testprocessor2")
data class TestConfiguration2(
    /**
     * Will be added to name to make fullName
     */
    val addition: String = "N/A"
)

Working version

@ConfigurationProperties(prefix = "com.example.testprocessor")
class TestConfiguration {
    /**
     * Will be added to name to make fullName
     */
    var addition: String = "N/A"
}

Reproduce

./build-only.sh
./build-images.sh
./inspect-images.sh

[philwebb]

I think this one is probably a duplicate of #28046 or #16293. Do you agree @corneil?

[fprochazka]

I just found this issue after trying to figure out why the official guide doesn't work, when applied on current versions (Gradle 8.10, Kotlin 2.0.20, JDK 22, Spring Boot 3.3.3)

[fprochazka]

I've noticed the .java file that kapt generates in the gradle build directory and did some experiments:

when I set kapt.use.k2=true (in gradle.properties), and write the data class like this:

import org.springframework.boot.context.properties.ConfigurationProperties
import org.springframework.boot.context.properties.bind.ConstructorBinding
import org.springframework.boot.context.properties.bind.DefaultValue

@ConfigurationProperties(prefix = QueryLoggingConfigurationProperties.PREFIX)
data class QueryLoggingConfigurationProperties @ConstructorBinding constructor(
    @DefaultValue("false")
    val enabled: Boolean = false,
) {

    companion object {
        const val PREFIX: String = "query.logging"
    }

}

then the spring-configuration-metadata.json is generated as expected.

Interestingly, when I mark the data class as @JvmRecord, the @ConstructorBinding was never propagated to the final .java file, only this specific incantation works.

[corneil]

I think this one is probably a duplicate of #28046 or #16293. Do you agree @corneil?

My test used a data class with a single constructor with a single property, it excludes #16293
Maybe #28046.

The important difference from those is the APT for the configuration properties with the 'normal' class missed the defaultValue.

[corneil]

I've noticed the .java file that kapt generates in the gradle build directory and did some experiments:

when I set kapt.use.k2=true (in gradle.properties), and write the data class like this:

import org.springframework.boot.context.properties.ConfigurationProperties
import org.springframework.boot.context.properties.bind.ConstructorBinding
import org.springframework.boot.context.properties.bind.DefaultValue

@ConfigurationProperties(prefix = QueryLoggingConfigurationProperties.PREFIX)
data class QueryLoggingConfigurationProperties @ConstructorBinding constructor(
    @DefaultValue("false")
    val enabled: Boolean = false,
) {

    companion object {
        const val PREFIX: String = "query.logging"
    }

}

then the spring-configuration-metadata.json is generated as expected.

Interestingly, when I mark the data class as @JvmRecord, the @ConstructorBinding was never propagated to the final .java file, only this specific incantation works.

Adding a @DefaultValue is good news. I would prefer not doing that if the annotation is not required for Java.

[fprochazka]

After checking further, it seems that the @DefaultValue is not needed, the part that makes is work is using k2 and @ConstructorBinding constructor(

the k2 compiler for some reason generates multiple constructors in the .java file, and IMHO that's what confuses the processor. Adding the @ConstructorBinding probably forces it to focus on the correct constructor

[philwebb]

My test used a data class with a single constructor with a single property, it excludes #16293

Although your Kolin class declares a single constructor, the generated bytecode has two. Here's the output from javap

$ javap -s com.example.testprocessor.TestConfiguration2

Compiled from "TestProcessor.kt"
public final class com.example.testprocessor.TestConfiguration2 {
  public com.example.testprocessor.TestConfiguration2(java.lang.String);
    descriptor: (Ljava/lang/String;)V

  public com.example.testprocessor.TestConfiguration2(java.lang.String, int, kotlin.jvm.internal.DefaultConstructorMarker);
    descriptor: (Ljava/lang/String;ILkotlin/jvm/internal/DefaultConstructorMarker;)V

  public final java.lang.String getAddition();
    descriptor: ()Ljava/lang/String;

  public com.example.testprocessor.TestConfiguration2();
    descriptor: ()V
}

I think that makes this one a duplicate of #16293