kotlin-springboot

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• 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.

companion object {

    private val logger = LoggerFactory.getLogger(MyClass::class.java)

}

• 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.