---
description: Rule set for Kotlin coding best practices in Spring Boot applications.
globs: **/*.kt
---
# Kotlin Coding Best Practices for Spring Boot Development

## Project Structure and Organization

1.	Group your source code into clearly defined packages like controller, service, repository, and model to separate concerns and improve maintainability.
2.	Organize your file system so that each directory mirrors the Kotlin package name (e.g. put com.myapp.users under src/main/kotlin/com/myapp/users).
3.	Name each Kotlin file after the primary class or concept it contains to make the codebase easier to navigate and understand.
4.	Avoid vague file names like Utils.kt; instead, use concise and meaningful names that reflect the purpose of the file’s contents.
5.	Place your Spring Boot application entry point in the root package and structure sub-packages by layer or feature to help Spring scan and organize components efficiently.

## Coding Style and Conventions

1.	Use PascalCase for class and object names, camelCase for functions and variables, and UPPER_SNAKE_CASE for constants to follow Kotlin naming conventions and improve readability.
2.	Declare variables using `val` by default, and only use `var` when mutation is necessary to promote safer, more predictable code.
    ```kotlin
    val maxConnections = 10    // immutable reference
    var currentUsers = 0       // mutable, try to avoid if possible
    ``` 
3.	Limit the scope of variables to where they are actually used—inside functions or smaller blocks—to avoid accidental misuse and make code easier to follow.
4.	Format your code consistently using 4-space indentation, proper spacing around operators and commas, and short, focused functions to improve clarity and maintainability.
5.	Write clear and expressive code instead of clever one-liners; break complex logic into intermediate variables or well-named functions to improve readability.
6.	Name classes, functions, and variables descriptively to convey intent, and avoid vague suffixes like '-Manager' or '-Helper' that don’t add meaning.
7.	Keep property getters and setters simple and free of heavy logic; if complex behavior is needed, move it into a separate method to keep property access predictable.

## Idiomatic Kotlin Usage

1.	Use data class to define DTOs and entities so you get useful methods like `equals()` and `copy()` without writing boilerplate code.
2.	Replace overloaded constructors with default and named parameters to simplify function calls and make them more expressive.
    ```kotlin
    // Kotlin – use default parameters
    fun createConnection(host: String, secure: Boolean = true) { … }

    createConnection("example.com")                      // uses default secure=true
    createConnection(host = "test.com", secure = false)  // named arg for clarity
    ``` 
3.	Use `when` expressions instead of long `if-else` chains to write cleaner, more readable conditional logic that clearly handles each case.
4.	Create extension functions instead of utility classes to add reusable behavior to existing types in a more natural and readable way.
    ```kotlin
    fun String.capitalizeFirst(): String = replaceFirstChar { it.uppercaseChar() }

    println("kotlin".capitalizeFirst())  // prints "Kotlin"
    ```
5.	Use scope functions like `apply`, `let`, `also`, `run`, and `with` to reduce repetition and clearly express object configuration or null-safe operations.
6.	Declare variables as nullable only when necessary, and handle them using safe-call operators (`?.`) and the Elvis operator (`?:`) to avoid runtime crashes.
7.	Avoid using the not-null assertion (`!!`) and instead provide fallback values or explicit null checks to write safer and more predictable code.
8.	Handle platform types from Java APIs immediately by explicitly casting them to `String` or `String?` to avoid spreading nullability uncertainty in your Kotlin code.
9.	Use Kotlin’s functional collection operations like `filter`, `map`, and `forEach` instead of manual loops to write concise and expressive data transformation logic.
    ```kotlin
    // Imperative approach
    val activeUsers = mutableListOf<User>()
    for (user in users) {
        if (user.isActive) activeUsers.add(user)
    }

    // Idiomatic functional approach
    val activeUsers = users.filter { it.isActive }
    ``` 
10.	Convert simple functions into single-expression functions when the logic is clear, to eliminate unnecessary syntax and improve code brevity.
    ```kotlin
    fun toDto(entity: User) = UserDto(name = entity.name, email = entity.email)
    ``` 
11.	Build strings using string templates (`$var` or `${expression}`) instead of concatenation, and use triple-quoted strings for clean multi-line text.

## Implementation Patterns and Design

1.	Inject dependencies via constructor parameters using `val` to keep them immutable and to align with Spring and Kotlin idioms.
    ```kotlin
    @Service
    class OrderService(
        private val orderRepo: OrderRepository,
        private val notifier: Notifier
    ) {
        // ...
    }
    ``` 
2.	Keep classes `final` by default, and let Spring’s 'all-open' plugin handle proxy generation so you don’t need to manually add the open modifier.
3.	Use Kotlin’s `object` declaration for true singletons or stateless utility holders instead of static methods or Java-style singletons.
4.	Favor composition by combining small, focused classes or using higher-order functions instead of relying on deep inheritance hierarchies.
5.	Define sealed classes when a type has a limited, closed set of variants to enforce exhaustive handling and improve type safety in `when` expressions.
    ```kotlin
    sealed class Result<out T>
    data class Success<T>(val data: T): Result<T>()
    data class Error(val exception: Throwable): Result<Nothing>()
    ``` 
6.	Use enum class to model fixed sets of constants that may contain logic, avoiding magic strings or raw values in business logic.
7.	Return nullable types, sealed classes, or result wrappers instead of throwing exceptions for expected scenarios like “not found” or “invalid input”.
8.	Always `use` the use function to safely manage and close resources like streams and file handles, ensuring they are closed even if an exception occurs.
    ```kotlin
    FileInputStream("data.txt").use { stream ->
        // read from stream 
    } // stream is automatically closed here
    ``` 
9.	Minimize visibility of your components by using `private` or `internal` where possible, and only expose what’s truly necessary as public.
10.	Use Kotlin coroutines with suspend functions and coroutine builders like `launch` or `async` to write clean, asynchronous backend code without callback hell.
11.	Leverage Kotlin’s standard library features like `lazy`, `observable`, `infix`, and operator overloading to write concise, expressive, and idiomatic code.
12.	Use immutable data class entities with `val` fields and Kotlin’s JPA plugin to satisfy JPA requirements while keeping your models safe and thread-friendly.
13.	Write unit tests for your business logic using dependency injection and pure functions to make testing straightforward and independent from Spring’s context.