kotlin五种单例的写法

object 饿汉


class 懒汉 private constructor() {
    companion object {
        private var instance: 懒汉? = null
            get() {
                if (field == null) {
                    field = 懒汉()
                }
                return field
            }
        fun getIns(): 懒汉 {
            return instance!!
        }
    }
}

class 线程安全懒汉 private constructor() {
    companion object {
        private var instance: 线程安全懒汉? = null
            get() {
                if (field == null) {
                    field = 线程安全懒汉()
                }
                return field
            }

        @Synchronized
        fun getIns(): 线程安全懒汉 {
            return instance!!
        }
    }
}


class 双重校验懒汉 private constructor() {
    companion object {
        val instance: 双重校验懒汉 by lazy(mode = LazyThreadSafetyMode.SYNCHRONIZED) { 双重校验懒汉() }
    }
}

class 静态内部类 private constructor() {
    companion object {
        val instance: 静态内部类 = Holder.holder
    }

    private object Holder {
        val holder = 静态内部类()
    }
}