Kotlin.Java的单例在Kotlin的5种实现
1.饿汉式
2.懒汉式
3.线程安全的懒汉式
4.双重校验锁式
静态内部类式
1.饿汉式
Java

public class Singleton {
    private static Singleton instance=new Singleton();
    
    private Singleton(){
    }
    
    public static Singleton getInstance(){
        return instance;
    }
}

Kotlin

object Singleton {}

2.懒汉式
Java

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

Kotlin

//方法一
class Singleton private constructor() {
    companion object {
        private var instance: Singleton? = null
            get() {
                if (field == null) {
                    field = Singleton()
                }
                return field
            }
        fun get(): Singleton{
            return instance!!
        }
    }
}

//方法二
class Singleton private constructor() {
    companion object {
        private var instance: Singleton? = null
            
        fun get(xx: String): Singleton{
            if (instance== null) {
                instance= Singleton(xx)
            }
        }
    }
    
    private constructor(xxx: String){
    }
}

这里不用getInstance作为为方法名，是因为在伴生对象声明时，内部已有getInstance方法，所以只能取其他名字

3.线程安全的懒汉式
Java

public class Singleton {
    private static Singleton instance;
    
    private Singleton(){}
    
    public static synchronized Singleton getInstance(){//使用同步锁
        if(instance==null){
            instance=new Singleton();
        }
        return instance;
    }
}

Kotlin

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

4.双重校验锁式
Java

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

kotlin

//方式一
class Singleton private constructor() {
    companion object {
        val instance: Singleton by lazy(mode = LazyThreadSafetyMode.SYNCHRONIZED) {
            Singleton()
        }
    }
}

//方式二
class Singleton private constructor(private val xxx: String) {//这里可以根据实际需求发生改变
  
    companion object {
        @Volatile 
        private var instance: Singleton? = null
        
        fun getInstance(xx: String) =
                instance ?: synchronized(this) {
                    instance ?: Singleton(xx).also { instance = it }
                }
    }
}

静态内部类式
Java

public class Singleton {
    private static class SingletonProvider{
        private static Singleton instance=new Singleton();
    }
    
    private Singleton(){}
    
    public static Singleton getInstance(){
        return SingletonProvider.instance;
    }
}

kotlin

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

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