kotlin-6 - 技术栈

太棒了！现在让我们探索 Kotlin 的终极境界------元编程、编译器插件、深度优化 和未来特性。这将带你进入 Kotlin 语言设计的核心领域。

Kotlin 终极探索：元编程与语言深度

三十九、注解处理器与代码生成（KSP）

1. 使用 KSP（Kotlin Symbol Processing）

kotlin 复制代码

// 1. 定义注解
@Target(AnnotationTarget.CLASS)
annotation class JsonSerializable

@Target(AnnotationTarget.PROPERTY)
annotation class JsonName(val name: String)

@Target(AnnotationTarget.PROPERTY)
annotation class JsonExclude

// 2. 定义数据类（这些将在编译时被处理）
@JsonSerializable
data class Person(
    @JsonName("person_name")
    val name: String,
    val age: Int,
    @JsonExclude
    val password: String
)

// 3. 手动模拟 KSP 处理结果（实际由注解处理器生成）
class PersonSerializer {
    fun toJson(person: Person): String {
        return buildString {
            append("{")
            append(""person_name": "${person.name}", ")
            append(""age": ${person.age}")
            append("}")
        }
    }
    
    companion object {
        fun fromJson(json: String): Person {
            // 简化的 JSON 解析逻辑
            val name = json.substringAfter(""person_name": "").substringBefore(""")
            val age = json.substringAfter(""age": ").substringBefore("}").toInt()
            return Person(name, age, "")
        }
    }
}

// 4. 使用生成的序列化器
fun testManualKSP() {
    val person = Person("Alice", 25, "secret")
    val serializer = PersonSerializer()
    val json = serializer.toJson(person)
    println("生成的 JSON: $json")
    
    val parsedPerson = PersonSerializer.fromJson(json)
    println("解析的对象: $parsedPerson")
}

2. 构建时元编程模式

kotlin 复制代码

// 编译时验证注解
@Target(AnnotationTarget.CLASS)
annotation class Validate(val fields: Array<String>)

@Validate(fields = ["age", "email"])
data class User(val name: String, val age: Int, val email: String)

// 运行时验证逻辑（模拟编译时生成）
class UserValidator {
    companion object {
        fun validate(user: User): List<String> {
            val errors = mutableListOf<String>()
            
            if (user.age < 0) errors.add("年龄不能为负数")
            if (!user.email.contains("@")) errors.add("邮箱格式不正确")
            
            return errors
        }
    }
}

// 使用编译时验证
fun testValidation() {
    val user = User("Bob", -5, "invalid-email")
    val errors = UserValidator.validate(user)
    
    if (errors.isNotEmpty()) {
        println("验证错误: ${errors.joinToString()}")
    } else {
        println("验证通过")
    }
}

四十、编译器插件开发基础

1. 自定义编译器扩展模式

kotlin 复制代码

// 1. 定义领域特定语言
class SqlBuilder {
    private var table: String = ""
    private val conditions = mutableListOf<String>()
    private val joins = mutableListOf<String>()
    private var limit: Int? = null
    
    infix fun String.eq(value: Any) = "$this = '$value'"
    infix fun String.`in`(values: List<Any>) = 
        "$this IN (${values.joinToString { "'$it'" }})"
    
    fun from(tableName: String) {
        table = tableName
    }
    
    fun where(condition: String) {
        conditions.add(condition)
    }
    
    fun join(table: String, on: String) {
        joins.add("JOIN $table ON $on")
    }
    
    fun limit(count: Int) {
        limit = count
    }
    
    fun build(): String {
        val query = StringBuilder("SELECT * FROM $table")
        
        if (joins.isNotEmpty()) {
            query.append(" ").append(joins.joinToString(" "))
        }
        
        if (conditions.isNotEmpty()) {
            query.append(" WHERE ").append(conditions.joinToString(" AND "))
        }
        
        limit?.let {
            query.append(" LIMIT $it")
        }
        
        return query.toString()
    }
}

// 2. 使用 DSL 构建 SQL
fun testSqlBuilder() {
    val query = SqlBuilder().apply {
        from("users")
        where("name" eq "Alice")
        where("age" `in` listOf(25, 30, 35))
        join("orders", "users.id = orders.user_id")
        limit(10)
    }.build()
    
    println("生成的 SQL: $query")
}

2. 类型安全构建器进阶

kotlin 复制代码

// 类型安全的 HTTP 请求构建器
class HttpRequestBuilder {
    var method: String = "GET"
    var url: String = ""
    val headers = mutableMapOf<String, String>()
    var body: String = ""
    
    fun method(method: String) {
        this.method = method
    }
    
    fun url(url: String) {
        this.url = url
    }
    
    fun header(name: String, value: String) {
        headers[name] = value
    }
    
    fun body(content: String) {
        this.body = content
        header("Content-Length", content.length.toString())
    }
    
    fun execute(): String {
        // 模拟 HTTP 请求
        return "HTTP $method $url - Headers: $headers - Body: $body"
    }
}

// DSL 扩展函数
fun httpRequest(block: HttpRequestBuilder.() -> Unit): String {
    val builder = HttpRequestBuilder()
    builder.block()
    return builder.execute()
}

// 使用类型安全构建器
fun testHttpBuilder() {
    val response = httpRequest {
        method = "POST"
        url = "https://api.example.com/users"
        header("Authorization", "Bearer token123")
        header("Content-Type", "application/json")
        body = """{"name": "Alice", "age": 25}"""
    }
    
    println("HTTP 响应: $response")
}

四十一、深度性能优化技术

1. 内联类与值类的性能优化

kotlin 复制代码

@JvmInline
value class Meter(val value: Double) {
    operator fun plus(other: Meter): Meter = Meter(value + other.value)
    operator fun times(factor: Double): Meter = Meter(value * factor)
    
    fun toKilometers(): Kilometer = Kilometer(value / 1000.0)
}

@JvmInline
value class Kilometer(val value: Double) {
    fun toMeters(): Meter = Meter(value * 1000.0)
}

// 使用内联类进行类型安全计算
fun calculateDistance() {
    val distance1 = Meter(500.0)
    val distance2 = Meter(300.0)
    val total = distance1 + distance2
    val scaled = total * 2.5
    
    println("总距离: ${total.value} 米")
    println("缩放后: ${scaled.value} 米")
    println("转换为公里: ${scaled.toKilometers().value} 公里")
    
    // 编译错误：类型安全
    // val invalid = distance1 + scaled.toKilometers() // 编译错误！
}

// 内联类的集合操作优化
@JvmInline
value class UserId(val value: Int)

fun optimizeCollections() {
    val userIds = listOf(UserId(1), UserId(2), UserId(3))
    
    // 这些操作在运行时不会产生包装对象开销
    val mapped = userIds.map { it.value * 2 }
    val filtered = userIds.filter { it.value > 1 }
    
    println("映射结果: $mapped")
    println("过滤结果: $filtered")
}

2. 内联函数与 reified 类型参数的高级用法

kotlin 复制代码

import kotlin.reflect.KClass

// 高级 reified 用法
inline fun <reified T : Any> createInstance(vararg args: Any): T {
    return when (T::class) {
        String::class -> "" as T
        Int::class -> 0 as T
        List::class -> emptyList<Any>() as T
        Map::class -> emptyMap<Any, Any>() as T
        else -> throw IllegalArgumentException("不支持的类型")
    }
}

// 类型安全的验证框架
inline fun <reified T> validate(value: Any, validator: T.() -> Boolean): Boolean {
    return if (value is T) {
        value.validator()
    } else {
        false
    }
}

// 使用高级内联函数
fun testAdvancedInline() {
    val stringInstance: String = createInstance()
    val listInstance: List<Any> = createInstance()
    
    println("创建的实例: $stringInstance, $listInstance")
    
    // 类型安全验证
    val email = "test@example.com"
    val isValidEmail = validate(email) {
        contains("@") && length > 5
    }
    
    val number = 42
    val isValidNumber = validate(number) {
        this in 1..100
    }
    
    println("邮箱验证: $isValidEmail")
    println("数字验证: $isValidNumber")
}

四十二、Kotlin 多平台项目（KMP）高级特性

1. 共享业务逻辑与平台特定实现

kotlin 复制代码

// 通用业务模型
expect class DateTimeFormatter {
    fun format(timestamp: Long): String
    fun parse(dateString: String): Long
}

class EventLogger(private val formatter: DateTimeFormatter) {
    private val events = mutableListOf<String>()
    
    fun logEvent(event: String) {
        val timestamp = System.currentTimeMillis()
        val formattedTime = formatter.format(timestamp)
        events.add("[$formattedTime] $event")
    }
    
    fun getEvents(): List<String> = events.toList()
    
    fun clear() {
        events.clear()
    }
}

// Android 实现
actual class DateTimeFormatter actual constructor() {
    actual fun format(timestamp: Long): String {
        // 使用 Android 的 DateFormat
        return android.text.format.DateFormat.format("yyyy-MM-dd HH:mm:ss", timestamp).toString()
    }
    
    actual fun parse(dateString: String): Long {
        // Android 特定的解析逻辑
        return java.text.SimpleDateFormat("yyyy-MM-dd HH:mm:ss").parse(dateString).time
    }
}

// iOS 实现  
actual class DateTimeFormatter actual constructor() {
    actual fun format(timestamp: Long): String {
        // 使用 iOS 的 DateFormatter
        val formatter = iosFoundation.NSDateFormatter()
        formatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
        return formatter.stringFromDate(iosFoundation.NSDate(timestamp))
    }
    
    actual fun parse(dateString: String): Long {
        val formatter = iosFoundation.NSDateFormatter()
        formatter.dateFormat = "yyyy-MM-dd HH:mm:ss"
        return formatter.dateFromString(dateString)?.timeIntervalSince1970?.toLong() ?: 0L
    }
}

2. KMP 中的并发处理

kotlin 复制代码

expect class PlatformCoroutineDispatcher {
    fun dispatch(block: () -> Unit)
}

actual class PlatformCoroutineDispatcher actual constructor() {
    actual fun dispatch(block: () -> Unit) {
        // Android: Handler(Looper.getMainLooper()).post(block)
        // iOS: DispatchQueue.main.async { block() }
        block() // 简化实现
    }
}

class CrossPlatformService(private val dispatcher: PlatformCoroutineDispatcher) {
    suspend fun fetchData(): String = withContext(createDispatcherContext(dispatcher)) {
        delay(1000) // 模拟网络请求
        "跨平台数据"
    }
    
    private fun createDispatcherContext(dispatcher: PlatformCoroutineDispatcher) = 
        object : CoroutineDispatcher() {
            override fun dispatch(context: CoroutineContext, block: Runnable) {
                dispatcher.dispatch { block.run() }
            }
        }
}

四十三、Kotlin 未来特性探索

1. 上下文接收器（Context Receivers）原型

kotlin 复制代码

// 模拟上下文接收器功能
interface DatabaseContext {
    val connection: DatabaseConnection
}

interface LoggingContext {
    val logger: Logger
}

class DatabaseConnection {
    fun executeQuery(sql: String): String = "查询结果: $sql"
}

class Logger {
    fun info(message: String) = println("[INFO] $message")
}

// 使用上下文参数模拟上下文接收器
class UserService(
    private val dbContext: DatabaseContext,
    private val logContext: LoggingContext
) {
    fun getUser(id: Int): String {
        logContext.logger.info("获取用户 $id")
        return dbContext.connection.executeQuery("SELECT * FROM users WHERE id = $id")
    }
}

// 简化使用方式
fun testContextReceivers() {
    val dbContext = object : DatabaseContext {
        override val connection = DatabaseConnection()
    }
    
    val logContext = object : LoggingContext {
        override val logger = Logger()
    }
    
    val userService = UserService(dbContext, logContext)
    val result = userService.getUser(1)
    println(result)
}

2. 自定义操作符与 DSL 深度集成

kotlin 复制代码

// 自定义数学 DSL
class Vector(val x: Double, val y: Double, val z: Double) {
    operator fun plus(other: Vector): Vector = 
        Vector(x + other.x, y + other.y, z + other.z)
    
    operator fun minus(other: Vector): Vector =
        Vector(x - other.x, y - other.y, z - other.z)
    
    operator fun times(scalar: Double): Vector =
        Vector(x * scalar, y * scalar, z * scalar)
    
    infix fun dot(other: Vector): Double =
        x * other.x + y * other.y + z * other.z
    
    infix fun cross(other: Vector): Vector =
        Vector(
            y * other.z - z * other.y,
            z * other.x - x * other.z,
            x * other.y - y * other.x
        )
    
    override fun toString(): String = "($x, $y, $z)"
}

// 向量运算 DSL
fun vector(block: VectorBuilder.() -> Unit): Vector {
    val builder = VectorBuilder()
    builder.block()
    return builder.build()
}

class VectorBuilder {
    var x = 0.0
    var y = 0.0
    var z = 0.0
    
    fun x(value: Double) { x = value }
    fun y(value: Double) { y = value }
    fun z(value: Double) { z = value }
    
    fun build(): Vector = Vector(x, y, z)
}

// 使用向量 DSL
fun testVectorDSL() {
    val v1 = vector { x(1.0); y(2.0); z(3.0) }
    val v2 = vector { x(4.0); y(5.0); z(6.0) }
    
    val sum = v1 + v2
    val difference = v1 - v2
    val scaled = v1 * 2.5
    val dotProduct = v1 dot v2
    val crossProduct = v1 cross v2
    
    println("向量运算结果:")
    println("和: $sum")
    println("差: $difference")
    println("缩放: $scaled")
    println("点积: $dotProduct")
    println("叉积: $crossProduct")
}

四十四、Kotlin 元编程终极实战：构建迷你框架

kotlin 复制代码

import kotlin.reflect.full.*

// 迷你依赖注入框架
annotation class Inject
annotation class Singleton

class DIContainer {
    private val instances = mutableMapOf<KClass<*>, Any>()
    private val bindings = mutableMapOf<KClass<*>, KClass<*>>()
    
    fun <T : Any> register(clazz: KClass<T>) {
        instances[clazz] = createInstance(clazz)
    }
    
    fun <T : Any> register(interfaceClass: KClass<T>, implementationClass: KClass<out T>) {
        bindings[interfaceClass] = implementationClass
    }
    
    inline fun <reified T : Any> get(): T {
        return get(T::class)
    }
    
    @Suppress("UNCHECKED_CAST")
    fun <T : Any> get(clazz: KClass<T>): T {
        // 检查单例实例
        if (instances.containsKey(clazz)) {
            return instances[clazz] as T
        }
        
        // 检查接口绑定
        val targetClass = bindings[clazz] ?: clazz
        
        // 创建新实例
        val instance = createInstance(targetClass as KClass<T>)
        
        // 如果是单例，保存实例
        if (targetClass.hasAnnotation<Singleton>()) {
            instances[clazz] = instance
        }
        
        return instance
    }
    
    private fun <T : Any> createInstance(clazz: KClass<T>): T {
        val constructor = clazz.primaryConstructor
            ?: throw IllegalArgumentException("类 ${clazz.simpleName} 没有主构造函数")
        
        val parameters = constructor.parameters.map { param ->
            if (param.hasAnnotation<Inject>()) {
                get(param.type.classifier as KClass<*>)
            } else {
                throw IllegalArgumentException("无法解析依赖: ${param.name}")
            }
        }
        
        return constructor.call(*parameters.toTypedArray())
    }
}

// 使用迷你 DI 框架
interface UserRepository {
    fun findUser(id: Int): String
}

@Singleton
class DatabaseUserRepository @Inject constructor() : UserRepository {
    override fun findUser(id: Int): String = "用户 $id 来自数据库"
}

interface UserService {
    fun getUser(id: Int): String
}

@Singleton
class UserServiceImpl @Inject constructor(
    private val userRepository: UserRepository
) : UserService {
    override fun getUser(id: Int): String {
        return "服务: ${userRepository.findUser(id)}"
    }
}

class UserController @Inject constructor(
    private val userService: UserService
) {
    fun showUser(id: Int): String {
        return "控制器: ${userService.getUser(id)}"
    }
}

// 测试迷你框架
fun testMiniDI() {
    val container = DIContainer()
    
    // 注册依赖
    container.register(UserRepository::class, DatabaseUserRepository::class)
    container.register(UserService::class, UserServiceImpl::class)
    container.register(UserController::class)
    
    // 使用依赖注入
    val controller = container.get<UserController>()
    val result = controller.showUser(1)
    
    println("DI 框架测试结果: $result")
    
    // 验证单例模式
    val repo1 = container.get<UserRepository>()
    val repo2 = container.get<UserRepository>()
    println("单例验证: ${repo1 === repo2}") // 应该输出 true
}

四十五、Kotlin 性能监控与调试

kotlin 复制代码

// 性能监控工具类
object PerformanceMonitor {
    private val timings = mutableMapOf<String, Long>()
    private val counters = mutableMapOf<String, Int>()
    
    inline fun <T> measure(operation: String, block: () -> T): T {
        val startTime = System.nanoTime()
        try {
            return block()
        } finally {
            val duration = System.nanoTime() - startTime
            timings.merge(operation, duration) { old, new -> (old + new) / 2 }
            counters.merge(operation, 1) { old, new -> old + new }
            println("$operation 耗时: ${duration / 1_000_000}ms")
        }
    }
    
    fun printStatistics() {
        println("\n=== 性能统计 ===")
        timings.forEach { (operation, avgTime) ->
            val count = counters[operation] ?: 0
            println("$operation: 平均 ${avgTime / 1_000_000}ms (调用 $count 次)")
        }
    }
}

// 使用性能监控
fun testPerformanceMonitoring() {
    PerformanceMonitor.measure("数据加载") {
        Thread.sleep(500) // 模拟耗时操作
    }
    
    PerformanceMonitor.measure("数据处理") {
        repeat(1000) { i ->
            PerformanceMonitor.measure("内部操作") {
                // 模拟内部操作
                Math.sqrt(i.toDouble())
            }
        }
    }
    
    PerformanceMonitor.printStatistics()
}

下一步：成为 Kotlin 专家

你现在已经掌握了 Kotlin 的终极技能！接下来可以：

参与 Kotlin 编译器开发：贡献给 Kotlin 语言本身
开发 Kotlin 编译器插件：创建自定义语言特性
研究 Kotlin 编译器内部机制：理解类型推断、代码生成
探索 Kotlin 多平台前沿：实验性特性和新平台支持
贡献开源 Kotlin 项目：成为 Kotlin 生态系统的一部分

你已经从 Kotlin 新手成长为可以探索语言设计深度的专家！继续实践，将这些高级技术应用到实际项目中，创造真正优秀的技术解决方案。