by lazy：遅延初期化を深く学ぶ

by lazyとは

val greeting: String by lazy {
    println("Computing...")
    "Hello, Kotlin!"
}
fun main() {
    println(greeting) // prints Computing... then Hello, Kotlin!
    println(greeting) // prints Hello, Kotlin! (cached)
}

Lazyの構文

val pi: Double by lazy { Math.PI }
val appName: String by lazy { "MyApp v1.0" }
fun main() {
    println(pi)      // 3.141592653589793
    println(appName) // MyApp v1.0
}

SYNCHRONIZEDモード（デフォルト）

val heavyResource: String by lazy(LazyThreadSafetyMode.SYNCHRONIZED) {
    Thread.sleep(100) // simulate work
    "Resource ready"
}
fun main() { println(heavyResource) }

PUBLICATIONモード

val config: Map<String, String> by lazy(LazyThreadSafetyMode.PUBLICATION) {
    mapOf("host" to "localhost", "port" to "8080")
}
fun main() { println(config["host"]) }

NONEモード

val singleThreadValue: Int by lazy(LazyThreadSafetyMode.NONE) {
    42
}
fun main() { println(singleThreadValue) }

クラス内でのLazy

class DatabaseConnection(val url: String) {
    val pool: String by lazy {
        println("Connecting to $url")
        "Pool@$url"
    }
}
fun main() {
    val db = DatabaseConnection("localhost:5432")
    println(db.pool)
}

Lazyとlateinitの比較

class MyService {
    val computed: List<Int> by lazy { (1..100).toList() }
    lateinit var injected: String
}
fun main() {
    val s = MyService()
    s.injected = "injected value"
    println(s.computed.size)
    println(s.injected)
}

isInitializedチェック

val myLazy: String by lazy { "initialized" }
fun main() {
    val ref = ::myLazy
    // No direct isInitialized on val lazy, but we can check via delegate
    println(myLazy)
}

依存関係のあるLazy

val baseUrl: String by lazy { "https://api.example.com" }
val usersUrl: String by lazy { "$baseUrl/users" }
val postsUrl: String by lazy { "$baseUrl/posts" }
fun main() {
    println(usersUrl)
    println(postsUrl)
}

実践：Singleton Service

object Analytics {
    val client: String by lazy {
        println("Initializing analytics client")
        "AnalyticsClient@ready"
    }
}
fun main() {
    println("App started")
    println(Analytics.client) // initialized on first use
    println(Analytics.client) // cached
}

正規表現の遅延コンパイル

val emailRegex: Regex by lazy {
    Regex("^[A-Za-z0-9+_.-]+@(.+)$")
}
fun isValidEmail(email: String) = emailRegex.matches(email)
fun main() {
    println(isValidEmail("user@example.com")) // true
    println(isValidEmail("bad-email"))         // false
}

クイックチェック

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