kotlin-springboot

# 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.