【Android】Java的单例在Kotlin的5种实现

饿汉式

Java

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

Kotlin

object Singleton

懒汉式

Java

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

Kotlin

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

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

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

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

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

or

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 SingletonHolder{
        private static Singleton instance=new Singleton();
    }
    
    private Singleton(){}
    
    public static SingletonDemo getInstance(){
        return SingletonHolder.instance;
    }
}

kotlin

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

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