Kotlin lazy 实现原理

/**
 * Creates a new instance of the [Lazy] that uses the specified initialization function [initializer]
 * and the default thread-safety mode [LazyThreadSafetyMode.SYNCHRONIZED].
 *
 * If the initialization of a value throws an exception, it will attempt to reinitialize the value at next access.
 *
 * Note that the returned instance uses itself to synchronize on. Do not synchronize from external code on
 * the returned instance as it may cause accidental deadlock. Also this behavior can be changed in the future.
 */
public actual fun <T> lazy(initializer: () -> T): Lazy<T> = SynchronizedLazyImpl(initializer)

private class SynchronizedLazyImpl<out T>(initializer: () -> T, lock: Any? = null) : Lazy<T>, Serializable {
    private var initializer: (() -> T)? = initializer
    // 用内存可见性来检查是否在其他线程初始化过，这样来保证多线程安全
    @Volatile private var _value: Any? = UNINITIALIZED_VALUE
    // final field is required to enable safe publication of constructed instance
    private val lock = lock ?: this
    // 重写 get 来保证懒加载，只在使用的时候才执行函数
    override val value: T
        get() {
            val _v1 = _value
            // 如果不为默认值则说明被其他线程或该线程已经初始化过，这也就是 lazy 只执行一次后续直接取值的原因
            if (_v1 !== UNINITIALIZED_VALUE) {
                @Suppress("UNCHECKED_CAST")
                // 转为 T 类型返回
                return _v1 as T
            }

            return synchronized(lock) {
                val _v2 = _value
                if (_v2 !== UNINITIALIZED_VALUE) {
                    @Suppress("UNCHECKED_CAST") (_v2 as T)
                } else {
                    // lazy 返回值为该函数的执行结果
                    val typedValue = initializer!!()
                    _value = typedValue
                    initializer = null
                    typedValue
                }
            }
        }
    // 判断是否初始化过
    override fun isInitialized(): Boolean = _value !== UNINITIALIZED_VALUE

    override fun toString(): String = if (isInitialized()) value.toString() else "Lazy value not initialized yet."

    private fun writeReplace(): Any = InitializedLazyImpl(value)
}