单利模式有多种，其实并不需要那么多。java里面分，懒汉，饿汉，DSL，内部类，枚举，加syncronized的懒汉 等
其实最有效有用的，DSL ，枚举，内部类这三种

java

内部类

public class SingletonManager {
    private SingletonManager() {
    }

  public static SingletonManager getInstance(){
    return SingletonHolder.instance;
}

private static class SingletonHolder{
    private final static SingletonManager instance = new SingletonManager();
    }
}

Enum

public class SingletonManager {
    private SingletonManager() {
    }
   
          public static SingletonManager getInstance(){
            return SingletonEnum.INSTANCE.getSingletonManager();
          }
    
    
          private enum SingletonEnum{
            INSTANCE;
            private SingletonManager singletonManager = null;
            private SingletonEnum(){
                singletonManager = new SingletonManager();
                System.out.println("enum=======constructor");
            }
            private SingletonManager getSingletonManager(){
                System.out.println("getinstance");
                return singletonManager;
            }
          }
}

kotlin 照着java也有对应的多种，但是在kotlin中只有一种推荐，就是Object

DSL

public class SingletonManager {
    private SingletonManager() {
    }
private volatile static SingletonManager singletonManager;

public static SingletonManager getInstance() {
    if (singletonManager == null) {
        synchronized (SingletonManager.class) {
            if (singletonManager == null) {
                singletonManager = new SingletonManager();
            }
        }
    }

    return singletonManager;
    }
}

剩下懒汉，饿汉，不推荐用，太简单不列举

kotlin

object SingletonDemo

DSL

class SingletonDemo private constructor(){
companion object{
    val instance:SingletonDemo by lazy(mode = LazyThreadSafetyMode.SYNCHRONIZED) {
        SingletonDemo()
    }
}
}


class SingletonDemo private constructor(private val property: Int) {//这里可以根据实际需求发生改变

companion object {
    @Volatile private var instance: SingletonDemo? = null
    fun getInstance(property: Int) =
        instance ?: synchronized(this) {
            instance ?: SingletonDemo(property).also { instance = it }
        }
}
}

不推荐
懒汉线程安全

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

        @Synchronized
        fun get():SingletonDemo{
            return instance!!
        }
}
}

类似内部类，不推荐

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

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