Kotlin 单例模式的常用写法

饿汉式

object Singleton

线程安全的懒汉式

class Singleton private constructor() {

    companion object {
        private var instance: Singleton? = null
            get() {
                if (field == null) field = Singleton()
                return field
            }

        @Synchronized
        fun instance(): Singleton {
            return instance!!
        }
    }
}

双重校验锁式

class KtSingleton3 private constructor() {
    companion object {
        val instance by lazy { KtSingleton3() }
    }
}

Lazy 是接受一个 lambda 并返回一个 Lazy 实例的函数，返回的实例可以作为实现延迟属性的委托。第一次调用 get() 会执行已传递给 lazy() 的 lambda 表达式并记录结果,后续调用 get() 只是返回记录的结果。Lazy 默认的线程模式就是 LazyThreadSafetyMode.SYNCHRONIZED 内部默认双重校验锁

Lazy内部实现

public fun <T> lazy(mode: LazyThreadSafetyMode, initializer: () -> T): Lazy<T> =
     when (mode) {
       LazyThreadSafetyMode.SYNCHRONIZED -> SynchronizedLazyImpl(initializer)
       LazyThreadSafetyMode.PUBLICATION -> SafePublicationLazyImpl(initializer)
       LazyThreadSafetyMode.NONE -> UnsafeLazyImpl(initializer)
    }

Lazy接口

public interface Lazy<out T> {
    //当前实例化对象，一旦实例化后，该对象不会再改变
    public val value: T
    //返回true表示，已经延迟实例化过了，false 表示，没有被实例化，
    //一旦方法返回true，该方法会一直返回true,且不会再继续实例化
    public fun isInitialized(): Boolean
}

SynchronizedLazyImpl

private class SynchronizedLazyImpl<out T>(initializer: () -> T, lock: Any? = null) : Lazy<T>, Serializable {
      private var initializer: (() -> T)? = initializer
      @Volatile private var _value: Any? = UNINITIALIZED_VALUE
      // final field is required to enable safe publication of constructed instance
      private val lock = lock ?: this
 
      override val value: T
          get() {
              val _v1 = _value
              //判断是否已经初始化过，如果初始化过直接返回，不在调用高级函数内部逻辑
              if (_v1 !== UNINITIALIZED_VALUE) {
                  @Suppress("UNCHECKED_CAST")
                  return _v1 as T
              }
 
              return synchronized(lock) {
                  val _v2 = _value
                  if (_v2 !== UNINITIALIZED_VALUE) {
                      @Suppress("UNCHECKED_CAST") (_v2 as T)
                 }
                  else {
                      //调用高级函数获取其返回值
                      val typedValue = initializer!!()
                      //将返回值赋值给_value,用于下次判断时，直接返回高级函数的返回值
                      _value = typedValue
                     initializer = null
                      typedValue
                  }
              }
          }
          //省略部分代码
  }

静态内部类式

class Singleton private constructor() {
    companion object {
        val instance = SingletonHolder.holder
    }

    private object SingletonHolder {
        val holder = Singleton()
    }
}

枚举式

enum class Singleton {
    INSTANCE;
}