loulanyue Kotlin 开发模式
适用于构建健壮、高效、可维护应用程序的惯用 Kotlin 模式与最佳实践。
使用时机
- 编写新的 Kotlin 代码
- 审查 Kotlin 代码
- 重构现有的 Kotlin 代码
- 设计 Kotlin 模块或库
- 配置 Gradle Kotlin DSL 构建
工作原理
本技能在七个关键领域强制执行惯用的 Kotlin 约定：使用类型系统和安全调用运算符实现空安全；通过数据类的 val 和 copy() 实现不可变性；使用密封类和接口实现穷举类型层次结构；使用协程和 Flow 实现结构化并发；使用扩展函数在不使用继承的情况下添加行为；使用 @DslMarker 和 lambda 接收器构建类型安全的 DSL；以及使用 Gradle Kotlin DSL 进行构建配置。
示例
使用 Elvis 运算符实现空安全：
fun getUserEmail(userId: String): String {
val user = userRepository.findById(userId)
return user?.email ?: "unknown@example.com"
}
使用密封类处理穷举结果：
sealed class Result<out T> {
data class Success<T>(val data: T) : Result<T>()
data class Failure(val error: AppError) : Result<Nothing>()
data object Loading : Result<Nothing>()
}
使用 async/await 实现结构化并发：
suspend fun fetchUserWithPosts(userId: String): UserProfile =
coroutineScope {
val user = async { userService.getUser(userId) }
val posts = async { postService.getUserPosts(userId) }
UserProfile(user = user.await(), posts = posts.await())
}
核心原则
1. 空安全
Kotlin 的类型系统区分可空和不可空类型。充分利用它。
// Good: Use non-nullable types by default
fun getUser(id: String): User {
return userRepository.findById(id)
?: throw UserNotFoundException("User $id not found")
}
// Good: Safe calls and Elvis operator
fun getUserEmail(userId: String): String {
val user = userRepository.findById(userId)
return user?.email ?: "unknown@example.com"
}
// Bad: Force-unwrapping nullable types
fun getUserEmail(userId: String): String {
val user = userRepository.findById(userId)
return user!!.email // Throws NPE if null
}
2. 默认不可变性
优先使用 val 而非 var，优先使用不可变集合而非可变集合。
// Good: Immutable data
data class User(
val id: String,
val name: String,
val email: String,
)
// Good: Transform with copy()
fun updateEmail(user: User, newEmail: String): User =
user.copy(email = newEmail)
// Good: Immutable collections
val users: List<User> = listOf(user1, user2)
val filtered = users.filter { it.email.isNotBlank() }
// Bad: Mutable state
var currentUser: User? = null // Avoid mutable global state
val mutableUsers = mutableListOf<User>() // Avoid unless truly needed
3. 表达式体和单表达式函数
使用表达式体编写简洁、可读的函数。
// Good: Expression body
fun isAdult(age: Int): Boolean = age >= 18
fun formatFullName(first: String, last: String): String =
"$first $last".trim()
fun User.displayName(): String =
name.ifBlank { email.substringBefore('@') }
// Good: When as expression
fun statusMessage(code: Int): String = when (code) {
200 -> "OK"
404 -> "Not Found"
500 -> "Internal Server Error"
else -> "Unknown status: $code"
}
// Bad: Unnecessary block body
fun isAdult(age: Int): Boolean {
return age >= 18
}
4. 数据类用于值对象
使用数据类表示主要包含数据的类型。
// Good: Data class with copy, equals, hashCode, toString
data class CreateUserRequest(
val name: String,
val email: String,
val role: Role = Role.USER,
)
// Good: Value class for type safety (zero overhead at runtime)
@JvmInline
value class UserId(val value: String) {
init {
require(value.isNotBlank()) { "UserId cannot be blank" }
}
}
@JvmInline
value class Email(val value: String) {
init {
require('@' in value) { "Invalid email: $value" }
}
}
fun getUser(id: UserId): User = userRepository.findById(id)
密封类和接口
建模受限的层次结构
// Good: Sealed class for exhaustive when
sealed class Result<out T> {
data class Success<T>(val data: T) : Result<T>()
data class Failure(val error: AppError) : Result<Nothing>()
data object Loading : Result<Nothing>()
}
fun <T> Result<T>.getOrNull(): T? = when (this) {
is Result.Success -> data
is Result.Failure -> null
is Result.Loading -> null
}
fun <T> Result<T>.getOrThrow(): T = when (this) {
is Result.Success -> data
is Result.Failure -> throw error.toException()
is Result.Loading -> throw IllegalStateException("Still loading")
}
用于 API 响应的密封接口
sealed interface ApiError {
val message: String
data class NotFound(override val message: String) : ApiError
data class Unauthorized(override val message: String) : ApiError
data class Validation(
override val message: String,
val field: String,
) : ApiError
data class Internal(
override val message: String,
val cause: Throwable? = null,
) : ApiError
}
fun ApiError.toStatusCode(): Int = when (this) {
is ApiError.NotFound -> 404
is ApiError.Unauthorized -> 401
is ApiError.Validation -> 422
is ApiError.Internal -> 500
}
作用域函数
何时使用各个函数
// let: Transform nullable or scoped result
val length: Int? = name?.let { it.trim().length }
// apply: Configure an object (returns the object)
val user = User().apply {
name = "Alice"
email = "alice@example.com"
}
// also: Side effects (returns the object)
val user = createUser(request).also { logger.info("Created user: ${it.id}") }
// run: Execute a block with receiver (returns result)
val result = connection.run {
prepareStatement(sql)
executeQuery()
}
// with: Non-extension form of run
val csv = with(StringBuilder()) {
appendLine("name,email")
users.forEach { appendLine("${it.name},${it.email}") }
toString()
}
反模式
// Bad: Nesting scope functions
user?.let { u ->
u.address?.let { addr ->
addr.city?.let { city ->
println(city) // Hard to read
}
}
}
// Good: Chain safe calls instead
val city = user?.address?.city
city?.let { println(it) }
扩展函数
在不使用继承的情况下添加功能
// Good: Domain-specific extensions
fun String.toSlug(): String =
lowercase()
.replace(Regex("[^a-z0-9\\s-]"), "")
.replace(Regex("\\s+"), "-")
.trim('-')
fun Instant.toLocalDate(zone: ZoneId = ZoneId.systemDefault()): LocalDate =
atZone(zone).toLocalDate()
// Good: Collection extensions
fun <T> List<T>.second(): T = this[1]
fun <T> List<T>.secondOrNull(): T? = getOrNull(1)
// Good: Scoped extensions (not polluting global namespace)
class UserService {
private fun User.isActive(): Boolean =
status == Status.ACTIVE && lastLogin.isAfter(Instant.now().minus(30, ChronoUnit.DAYS))
fun getActiveUsers(): List<User> = userRepository.findAll().filter { it.isActive() }
}
协程
结构化并发
// Good: Structured concurrency with coroutineScope
suspend fun fetchUserWithPosts(userId: String): UserProfile =
coroutineScope {
val userDeferred = async { userService.getUser(userId) }
val postsDeferred = async { postService.getUserPosts(userId) }
UserProfile(
user = userDeferred.await(),
posts = postsDeferred.await(),
)
}
// Good: supervisorScope when children can fail independently
suspend fun fetchDashboard(userId: String): Dashboard =
supervisorScope {
val user = async { userService.getUser(userId) }
val notifications = async { notificationService.getRecent(userId) }
val recommendations = async { recommendationService.getFor(userId) }
Dashboard(
user = user.await(),
notifications = try {
notifications.await()
} catch (e: CancellationException) {
throw e
} catch (e: Exception) {
emptyList()
},
recommendations = try {
recommendations.await()
} catch (e: CancellationException) {
throw e
} catch (e: Exception) {
emptyList()
},
)
}
Flow 用于响应式流
// Good: Cold flow with proper error handling
fun observeUsers(): Flow<List<User>> = flow {
while (currentCoroutineContext().isActive) {
val users = userRepository.findAll()
emit(users)
delay(5.seconds)
}
}.catch { e ->
logger.error("Error observing users", e)
emit(emptyList())
}
// Good: Flow operators
fun searchUsers(query: Flow<String>): Flow<List<User>> =
query
.debounce(300.milliseconds)
.distinctUntilChanged()
.filter { it.length >= 2 }
.mapLatest { q -> userRepository.search(q) }
.catch { emit(emptyList()) }
取消与清理
// Good: Respect cancellation
suspend fun processItems(items: List<Item>) {
items.forEach { item ->
ensureActive() // Check cancellation before expensive work
processItem(item)
}
}
// Good: Cleanup with try/finally
suspend fun acquireAndProcess() {
val resource = acquireResource()
try {
resource.process()
} finally {
withContext(NonCancellable) {
resource.release() // Always release, even on cancellation
}
}
}
委托
属性委托
// Lazy initialization
val expensiveData: List<User> by lazy {
userRepository.findAll()
}
// Observable property
var name: String by Delegates.observable("initial") { _, old, new ->
logger.info("Name changed from '$old' to '$new'")
}
// Map-backed properties
class Config(private val map: Map<String, Any?>) {
val host: String by map
val port: Int by map
val debug: Boolean by map
}
val config = Config(mapOf("host" to "localhost", "port" to 8080, "debug" to true))
接口委托
// Good: Delegate interface implementation
class LoggingUserRepository(
private val delegate: UserRepository,
private val logger: Logger,
) : UserRepository by delegate {
// Only override what you need to add logging to
override suspend fun findById(id: String): User? {
logger.info("Finding user by id: $id")
return delegate.findById(id).also {
logger.info("Found user: ${it?.name ?: "null"}")
}
}
}
DSL 构建器
类型安全构建器
// Good: DSL with @DslMarker
@DslMarker
annotation class HtmlDsl
@HtmlDsl
class HTML {
private val children = mutableListOf<Element>()
fun head(init: Head.() -> Unit) {
children += Head().apply(init)
}
fun body(init: Body.() -> Unit) {
children += Body().apply(init)
}
override fun toString(): String = children.joinToString("\n")
}
fun html(init: HTML.() -> Unit): HTML = HTML().apply(init)
// Usage
val page = html {
head { title("My Page") }
body {
h1("Welcome")
p("Hello, World!")
}
}
配置 DSL
data class ServerConfig(
val host: String = "0.0.0.0",
val port: Int = 8080,
val ssl: SslConfig? = null,
val database: DatabaseConfig? = null,
)
data class SslConfig(val certPath: String, val keyPath: String)
data class DatabaseConfig(val url: String, val maxPoolSize: Int = 10)
class ServerConfigBuilder {
var host: String = "0.0.0.0"
var port: Int = 8080
private var ssl: SslConfig? = null
private var database: DatabaseConfig? = null
fun ssl(certPath: String, keyPath: String) {
ssl = SslConfig(certPath, keyPath)
}
fun database(url: String, maxPoolSize: Int = 10) {
database = DatabaseConfig(url, maxPoolSize)
}
fun build(): ServerConfig = ServerConfig(host, port, ssl, database)
}
fun serverConfig(init: ServerConfigBuilder.() -> Unit): ServerConfig =
ServerConfigBuilder().apply(init).build()
// Usage
val config = serverConfig {
host = "0.0.0.0"
port = 443
ssl("/certs/cert.pem", "/certs/key.pem")
database("jdbc:postgresql://localhost:5432/mydb", maxPoolSize = 20)
}
用于惰性求值的序列
// Good: Use sequences for large collections with multiple operations
val result = users.asSequence()
.filter { it.isActive }
.map { it.email }
.filter { it.endsWith("@company.com") }
.take(10)
.toList()
// Good: Generate infinite sequences
val fibonacci: Sequence<Long> = sequence {
var a = 0L
var b = 1L
while (true) {
yield(a)
val next = a + b
a = b
b = next
}
}
val first20 = fibonacci.take(20).toList()
Gradle Kotlin DSL
build.gradle.kts 配置
// Check for latest versions: https://kotlinlang.org/docs/releases.html
plugins {
kotlin("jvm") version "2.3.10"
kotlin("plugin.serialization") version "2.3.10"
id("io.ktor.plugin") version "3.4.0"
id("org.jetbrains.kotlinx.kover") version "0.9.7"
id("io.gitlab.arturbosch.detekt") version "1.23.8"
}
group = "com.example"
version = "1.0.0"
kotlin {
jvmToolchain(21)
}
dependencies {
// Ktor
implementation("io.ktor:ktor-server-core:3.4.0")
implementation("io.ktor:ktor-server-netty:3.4.0")
implementation("io.ktor:ktor-server-content-negotiation:3.4.0")
implementation("io.ktor:ktor-serialization-kotlinx-json:3.4.0")
// Exposed
implementation("org.jetbrains.exposed:exposed-core:1.0.0")
implementation("org.jetbrains.exposed:exposed-dao:1.0.0")
implementation("org.jetbrains.exposed:exposed-jdbc:1.0.0")
implementation("org.jetbrains.exposed:exposed-kotlin-datetime:1.0.0")
// Koin
implementation("io.insert-koin:koin-ktor:4.2.0")
// Coroutines
implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.10.2")
// Testing
testImplementation("io.kotest:kotest-runner-junit5:6.1.4")
testImplementation("io.kotest:kotest-assertions-core:6.1.4")
testImplementation("io.kotest:kotest-property:6.1.4")
testImplementation("io.mockk:mockk:1.14.9")
testImplementation("io.ktor:ktor-server-test-host:3.4.0")
testImplementation("org.jetbrains.kotlinx:kotlinx-coroutines-test:1.10.2")
}
tasks.withType<Test> {
useJUnitPlatform()
}
detekt {
config.setFrom(files("config/detekt/detekt.yml"))
buildUponDefaultConfig = true
}
错误处理模式
用于领域操作的 Result 类型
// Good: Use Kotlin's Result or a custom sealed class
suspend fun createUser(request: CreateUserRequest): Result<User> = runCatching {
require(request.name.isNotBlank()) { "Name cannot be blank" }
require('@' in request.email) { "Invalid email format" }
val user = User(
id = UserId(UUID.randomUUID().toString()),
name = request.name,
email = Email(request.email),
)
userRepository.save(user)
user
}
// Good: Chain results
val displayName = createUser(request)
.map { it.name }
.getOrElse { "Unknown" }
require, check, error
// Good: Preconditions with clear messages
fun withdraw(account: Account, amount: Money): Account {
require(amount.value > 0) { "Amount must be positive: $amount" }
check(account.balance >= amount) { "Insufficient balance: ${account.balance} < $amount" }
return account.copy(balance = account.balance - amount)
}
集合操作
惯用的集合处理
// Good: Chained operations
val activeAdminEmails: List<String> = users
.filter { it.role == Role.ADMIN && it.isActive }
.sortedBy { it.name }
.map { it.email }
// Good: Grouping and aggregation
val usersByRole: Map<Role, List<User>> = users.groupBy { it.role }
val oldestByRole: Map<Role, User?> = users.groupBy { it.role }
.mapValues { (_, users) -> users.minByOrNull { it.createdAt } }
// Good: Associate for map creation
val usersById: Map<UserId, User> = users.associateBy { it.id }
// Good: Partition for splitting
val (active, inactive) = users.partition { it.isActive }
快速参考：Kotlin 惯用法
| 惯用法 | 描述 |
|---|---|
val 优于 var |
优先使用不可变变量 |
data class |
用于具有 equals/hashCode/copy 的值对象 |
sealed class/interface |
用于受限的类型层次结构 |
value class |
用于零开销的类型安全包装器 |
表达式 when |
穷举模式匹配 |
安全调用 ?. |
空安全的成员访问 |
Elvis ?: |
为可空类型提供默认值 |
let/apply/also/run/with |
用于编写简洁代码的作用域函数 |
| 扩展函数 | 在不使用继承的情况下添加行为 |
copy() |
数据类上的不可变更新 |
require/check |
前置条件断言 |
协程 async/await |
结构化并发执行 |
Flow |
冷响应式流 |
sequence |
惰性求值 |
委托 by |
在不使用继承的情况下重用实现 |
应避免的反模式
// Bad: Force-unwrapping nullable types
val name = user!!.name
// Bad: Platform type leakage from Java
fun getLength(s: String) = s.length // Safe
fun getLength(s: String?) = s?.length ?: 0 // Handle nulls from Java
// Bad: Mutable data classes
data class MutableUser(var name: String, var email: String)
// Bad: Using exceptions for control flow
try {
val user = findUser(id)
} catch (e: NotFoundException) {
// Don't use exceptions for expected cases
}
// Good: Use nullable return or Result
val user: User? = findUserOrNull(id)
// Bad: Ignoring coroutine scope
GlobalScope.launch { /* Avoid GlobalScope */ }
// Good: Use structured concurrency
coroutineScope {
launch { /* Properly scoped */ }
}
// Bad: Deeply nested scope functions
user?.let { u ->
u.address?.let { a ->
a.city?.let { c -> process(c) }
}
}
// Good: Direct null-safe chain
user?.address?.city?.let { process(it) }
请记住：Kotlin 代码应简洁但可读。利用类型系统确保安全，优先使用不可变性，并使用协程处理并发。如有疑问，让编译器帮助你。