Skip to content

Kotlin Support in SpringFlow

Overview

SpringFlow includes comprehensive Kotlin support, allowing you to use Kotlin data classes as JPA entities with all SpringFlow features.

Configuration

Kotlin support is fully configured in: - Parent POM: Kotlin version 1.9.22, kotlin-stdlib, kotlin-reflect - springflow-core POM: Kotlin Maven plugin for test compilation - Test entities: Example Kotlin data classes in src/test/kotlin/

Features Supported

Kotlin Data Classes

  • Full support for Kotlin data class as JPA entities
  • Support for @AutoApi annotation on data classes
  • All SpringFlow annotations work with Kotlin

Nullable Types

  • Kotlin nullable types (String?, Int?) are fully supported
  • Proper detection of nullable/non-nullable fields
  • Validation annotations work with nullable types

Annotation Targets

  • Support for @field:NotBlank, @field:Email, etc.
  • Support for @get:NotBlank annotation targets
  • All JSR-380 validation annotations compatible

Default Values

  • Kotlin default parameter values are preserved
  • Works with JPA no-arg constructor requirement

SpringFlow Features

  • Metadata resolution from Kotlin classes
  • DTO mapping (entity ↔ Map)
  • Repository generation
  • Service generation
  • Controller generation
  • OpenAPI documentation

Java Version Compatibility

IMPORTANT: Kotlin 1.9.22 is optimized for Java 17.

<properties>
    <java.version>17</java.version>
    <kotlin.version>1.9.22</kotlin.version>
</properties>

Java 21+ Compatibility Note

If using Java 21, you may encounter Kotlin compiler issues with JRT filesystem. In this case: 1. Use Java 17 for maximum compatibility 2. OR upgrade to Kotlin 2.0+ when available 3. OR use Java entities instead of Kotlin for now

Example Kotlin Entity

@Entity
@AutoApi(path = "/products")
data class Product(
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    val id: Long? = null,

    @field:NotBlank
    @field:Size(min = 3, max = 100)
    val name: String,

    @field:Min(0)
    val price: Double = 0.0,

    val description: String? = null,

    @Hidden
    val secretKey: String? = null,

    @ReadOnly
    val createdAt: String? = null
) {
    // No-arg constructor required by JPA
    constructor() : this(id = null, name = "", price = 0.0)
}

Testing

Kotlin test entities and tests are provided in: - src/test/kotlin/io/springflow/core/kotlin/KotlinEntity.kt - src/test/kotlin/io/springflow/core/kotlin/KotlinSupportTest.kt

To run Kotlin tests: 

# Ensure you're using Java 17
java -version

# Run tests
./mvnw test -Dtest=KotlinSupportTest

Limitations

Current Limitations

  • Kotlin support tested with Java 17
  • Some Java 21 users may need to use Java 17 for Kotlin compilation
  • Extension functions and DSL features are not yet available
  • Coroutines support is not yet available

Planned Enhancements

  • Kotlin extension functions for repositories/services
  • Suspend function support
  • Coroutines for reactive operations
  • Kotlin DSL for configuration

Migration from Java

Converting Java entities to Kotlin is straightforward:

// Java
@Entity
@AutoApi
public class User {
    @Id
    @GeneratedValue
    private Long id;

    @NotBlank
    private String name;

    // ... getters/setters
}

// Kotlin
@Entity
@AutoApi
data class User(
    @Id
    @GeneratedValue
    val id: Long? = null,

    @field:NotBlank
    val name: String
) {
    constructor() : this(id = null, name = "")
}

Troubleshooting

Kotlin compilation fails

  • Check Java version (java -version should show 17.x)
  • Verify Kotlin version in parent POM
  • Ensure kotlin-maven-plugin is configured correctly

Validation annotations not working

  • Use @field: target: @field:NotBlank
  • Don't use @get: or @param: for JPA fields

JPA errors with data classes

  • Ensure no-arg constructor is provided
  • Use var instead of val if JPA requires mutability
  • Mark JPA properties as open if using inheritance

Conclusion

Kotlin support in SpringFlow is production-ready for Java 17 environments. All core features work seamlessly with Kotlin data classes, providing a more concise and expressive way to define JPA entities with auto-generated REST APIs.