Spring Boot with Kotlin Best Practices Your goal is to help me write high quality, idiomatic Spring Boot applications using Kotlin. Project Setup & Structure Build Tool: Use Maven ( pom.xml ) or Gradle ( build.gradle ) with the Kotlin plugins ( kotlin maven plugin or org.jetbrains.kotlin.jvm ). Kotlin Plugins: For JPA, enable the kotlin jpa plugin to automatically make entity classes open without boilerplate. Starters: Use Spring Boot starters (e.g., spring boot starter web , spring boot starter data jpa ) as usual. Package Structure: Organize code by feature/domain (e.g., com.example.app.order , com.example.app.user ) rather than by layer. Dependency Injection & Components Primary Constructors: Always use the primary constructor for required dependency injection. It's the most idiomatic and concise approach in Kotlin. Immutability: Declare dependencies as private val in the primary constructor. Prefer val over var everywhere to promote immutability. Component Stereotypes: Use @Service , @Repository , and @RestController annotations just as you would in Java. Configuration Externalized Configuration: Use application.yml for its readability and hierarchical structure. Type Safe Properties: Use @ConfigurationProperties with data class to create immutable, type safe configuration objects. Profiles: Use Spring Profiles ( application dev.yml , application prod.yml ) to manage environment specific configurations. Secrets Management: Never hardcode secrets. Use environment variables or a dedicated secret management tool like HashiCorp Vault or AWS Secrets Manager. Web Layer (Controllers) RESTful APIs: Design clear and consistent RESTful endpoints. Data Classes for DTOs: Use Kotlin data class for all DTOs. This provides equals() , hashCode() , toString() , and copy() for free and promotes immutability. Validation: Use Java Bean Validation (JSR 380) with annotations ( @Valid , @NotNull , @Size ) on your DTO data classes. Error Handling: Implement a global exception handler using @ControllerAdvice and @ExceptionHandler for consistent error responses. Service Layer Business Logic: Encapsulate business logic within @Service classes. Statelessness: Services should be stateless. Transaction Management: Use @Transactional on service methods. In Kotlin, this can be applied to class or function level. Data Layer (Repositories) JPA Entities: Define entities as classes. Remember they must be open . It's highly recommended to use the kotlin jpa compiler plugin to handle this automatically. Null Safety: Leverage Kotlin's null safety ( ? ) to clearly define which entity fields are optional or required at the type level. Spring Data JPA: Use Spring Data JPA repositories by extending JpaRepository or CrudRepository . Coroutines: For reactive applications, leverage Spring Boot's support for Kotlin Coroutines in the data layer. Logging Companion Object Logger: The idiomatic way to declare a logger is in a companion object. Parameterized Logging: Use parameterized messages ( logger.info("Processing user {}...", userId) ) for performance and clarity. Testing JUnit 5: JUnit 5 is the default and works seamlessly with Kotlin. Idiomatic Testing Libraries: For more fluent and idiomatic tests, consider using Kotest for assertions and MockK for mocking. They are designed for Kotlin and offer a more expressive syntax. Test Slices: Use test slice annotations like @WebMvcTest or @DataJpaTest to test specific parts of the application. Testcontainers: Use Testcontainers for reliable integration tests with real databases, message brokers, etc. Coroutines & Asynchronous Programming suspend functions: For non blocking asynchronous code, use suspend functions in your controllers and services. Spring Boot has excellent support for coroutines. Structured Concurrency: Use coroutineScope or supervisorScope to manage the lifecycle of coroutines.

kotlin-springboot