kotlin

name	kotlin
description	Use for Kotlin code in telegram-home-bot: JVM 17, Spring Boot Kotlin, Gradle Groovy DSL, src/main/java Kotlin layout, null-safety, constructor DI, ktlint.

Kotlin Skill

Language Context

This project uses Kotlin 1.9.25 targeting JVM 17. All source files are .kt located under src/main/java/ (legacy layout — do not move them).

Key Kotlin Conventions for This Project

Null Safety

• Use ?.let { } for null-safe chaining — this is the dominant pattern
• lateinit var for @Value-injected Spring properties
• Optional<Host?> return types are used in some repository/service methods — unwrap with .orElse(null)
• Never use !! — prefer safe calls or early returns

Data Classes vs Regular Classes

• JPA entities are regular classes (not data class) — required by Hibernate
• DTOs like PasswordDto, TimeLogDto, GenericResponse are regular classes with var/val
• Use object for constants (AppConstants), enums for type-safe constants

Expression Style

• Prefer expression bodies: fun getHostByMac(mac: String?): Host? = hostRepository.findHostByMac(mac)
• Use when as expression (exhaustive), not statement
• Use if-else as expression when possible

Annotations

• @field:NotBlank — JSR-380 annotations require @field: target in Kotlin
• @JvmStatic — not needed in this project (no Java interop from outside)
• @Throws(Exception::class) — used sparingly, only in SecurityConfiguration.filterChain()

Collections & Functional Style

• Use .filter { }, .map { }, .sortedWith(), .joinToString(), .none { }
• ArrayList() for mutable lists when building keyboard rows
• Pair(a, b) for destructuring declarations

Logging

• val log = logger() from random.telegramhomebot.utils.logger — project utility
• Use structured logging: log.debug("[{}] command was executed", update.message)
• Guard debug logs with if (log.isDebugEnabled) for expensive formatting

Constructors & DI

• Constructor injection only — no @Autowired on fields or setters
• Primary constructor with private val for injected dependencies
• Secondary constructors used in Host entity for convenience

Do NOT

• Do NOT add semicolons
• Do NOT use !! (force unwrap)
• Do NOT use data class for JPA entities
• Do NOT use @Autowired on fields — always constructor injection
• Do NOT create interfaces with single implementation unless there's a clear strategy pattern
• Do NOT move Kotlin sources out of src/main/java/ — it's the established layout