1.懒汉模式

java

public class Singleton {

    private static Singleton singleton = new Singleton();

    private Singleton() {}

    public static Singleton getInstance() {
        return singleton;
    }
}

kotlin

object Singleton{}

2.懒加载（非线程安全）

java

public class Singleton {

    private static Singleton instance;

    private Singleton() {
    }

    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }

}

kotlin

// 1 //
class Singleton {
    companion object {
        val instance by lazy(LazyThreadSafetyMode.NONE) {
            Singleton()
        }
    }
}

// 2 //
class Singleton {

    private var instance: Singleton? = null

    fun get(): Singleton {
        if (instance == null) {
            instance = Singleton()
        }
        return instance!!
    }
}

3.懒加载（线程安全）

java

public class Singleton {

    private static Singleton instance;

    private Singleton() {
    }

    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

kotlin

class Singleton private constructor() {

    private var instance: Singleton? = null

    @Synchronized
    fun get(): Singleton {
        if (instance == null) {
            instance = Singleton()
        }
        return instance!!
    }
}

4.双重锁

java

public class Singleton {

    private static volatile Singleton instance;

    private Singleton() {
    }

    public static synchronized Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                instance = new Singleton();
            }
        }
        return instance;
    }

}

kotlin

// 1 //
class Singleton private constructor() {

    companion object {
        val instance by lazy(mode = LazyThreadSafetyMode.SYNCHRONIZED) {
            Singleton()
        }
    }
 }
// 2 //
class Singleton private constructor() {

    private @Volatile var instance: Singleton? = null

    fun get(): Singleton {
        if (instance == null) {
            synchronized(this) {
                if (instance == null) {
                    instance = Singleton()
                }
            }
        }
        return instance!!
    }
}

5.最优雅的写法：静态内部类

java

public class Singleton {

    private static class Holder {
        private static Singleton instance = new Singleton();
    }

    private Singleton() {
    }

    public static Singleton getInstance() {
        return Holder.instance;
    }
}

kotlin

class Singleton private constructor() {

    companion object {
        fun getInstance() = Holder.instance
    }

    private object Holder {
        val instance = Singleton()
    }
}