常见的五种单例模式

• 饿汉式
• 懒汉式
• 线程安全的懒汉式
• 双重校验锁式
• 静态内部类式

一、饿汉实现方式

Kotlin
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//Java实现
public class SingletonDemo {  
    private static SingletonDemo instance=new SingletonDemo();  
    private SingletonDemo(){  
  
    }  
    public static SingletonDemo getInstance(){  
        return instance;  
    }  
}

//Kotlin实现
object SingletonDemo

二、懒汉实现方式

Kotlin知识点：函数和属性与字段

Kotlin
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//Java实现
public class SingletonDemo {  
    private static SingletonDemo instance;  
    private SingletonDemo(){}  
    public static SingletonDemo getInstance(){  
        if(instance == null){  
            instance = new SingletonDemo();  
        }  
        return instance;  
    }  
}

//Kotlin实现
class SingletonDemo private constructor() {  
    companion object {  
        private var instance: SingletonDemo? = null  
            get() {  
                if (field == null) {  
                    field = SingletonDemo()  
                }  
                return field  
            }  
        fun get(): SingletonDemo{  
        //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，所以只能用其他名字
         return instance!!  
        }  
    }  
}

三、线程安全的懒汉实现方式

Kotlin
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复制代码
//Java实现
public class SingletonDemo {  
    private static SingletonDemo instance;  
    private SingletonDemo(){}  
    //使用同步锁 
    public static synchronized SingletonDemo getInstance(){ 
        if(instance == null){  
            instance = new SingletonDemo();  
        }  
        return instance;  
    }  
}

//Kotlin实现
class SingletonDemo private constructor() {  
    companion object {  
        private var instance: SingletonDemo? = null  
            get() {  
                if (field == null) {  
                    field = SingletonDemo()  
                }  
                return field  
            }  
        //方法声明为同步，需要添加@Synchronized注解    
        @Synchronized  
        fun get(): SingletonDemo{  
            return instance!!  
        }  
    }  
}

四、双重校验锁实现方式(Double Check)

Kotlin知识点：高阶函数和属性委托

Kotlin
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//Java实现
public class SingletonDemo {  
    private volatile static SingletonDemo instance;  
    private SingletonDemo(){}   
    public static SingletonDemo getInstance(){  
        if(instance == null){  
            synchronized (SingletonDemo.class){  
                if(instance == null){  
                    instance = new SingletonDemo();  
                }  
            }  
        }  
        return instance;  
    }  
}

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

五、静态内部类实现方式

Kotlin
 代码解读
复制代码
//Java实现
public class SingletonDemo {  
    private static class SingletonHolder{  
        private static SingletonDemo instance = new SingletonDemo();  
    }  
    private SingletonDemo(){  
        System.out.println("Singleton has loaded");  
    }  
    public static SingletonDemo getInstance(){  
        return SingletonHolder.instance;  
    }  
}

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

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