Spring Boot with Kotlin Best Practices

# 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.
  ```kotlin
  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.