Singleton模式的五种实现

单例模式的特点：

• 单例类只能有一个实例。
• 单例类必须自己创建自己的唯一实例。
• 单例类必须给所有其他对象提供这一实例。 -《Java与模式》

单例类可以是没有状态的，仅用做提供工具性函数的对象。既然是提供工具性函数，也就没有必要创建多个实例。

 

下面列举单例模式的几种实现：

• 单元素的枚举类型实现Singleton – the preferred approach.enum类型是Jdk1.5引入的一种新的数据类型.其背后的基本想法：通过公有的静态final域为每个枚举常量导出实例的类。因为没有可访问的构造器，枚举类型是真正的final。既不能创建枚举类型实例，也不能对它进行扩张 。枚举类型是实例受控的，是单例的泛型化，本质上是氮元素的枚举 -《Effective Java》P129

public enum Singleton{
    INSTANCE;
    public void execute(){…}
    public static void main(String[] agrs){
        Singleton sole=Singleton.INSTANCE;
        sole.execute();
    }
}

public enum Singleton{
	INSTANCE;
	public void execute(){…}
	public static void main(String[] agrs){
		Singleton sole=Singleton.INSTANCE;
		sole.execute();
	}
}

• 懒汉式 – Lazy initialization . 构造函数是private,只有通过Singleton才能实例化这个类。

public class Singleton{
    private volatile static Singleton uniqueInstance=null;
    private Singleton(){}
    public static Singleton getInstance(){
        if(uniqueInstance==null){
            synchronized(Singleton.class){
                if(uniqueInstance==null){
                    uniqueInstance=new Singleton();
                }
            }
        }
        return uniqueInstance;
    }
    //other useful methods here
}

public class Singleton{
	private volatile static Singleton uniqueInstance=null;
	private Singleton(){}
	public static Singleton getInstance(){
		if(uniqueInstance==null){
			synchronized(Singleton.class){
				if(uniqueInstance==null){
					uniqueInstance=new Singleton();
				}
			}
		}
		return uniqueInstance;
	}
	//other useful methods here
}

• 饿汉式 – Eager initialization

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

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

• static block initialization

public class Singleton{
    private static final Singleton instance;

    static{
        try{
            instance=new Singleton();
        }catch(IOException e){
            thronw new RuntimeException("an error's occurred!");
        }
    }
    public static Singleton getInstance(){
        return instance;
    }
    private Singleton(){
    }
}

public class Singleton{
	private static final Singleton instance;
		
	static{
		try{
			instance=new Singleton();
		}catch(IOException e){
			thronw new RuntimeException("an error's occurred!");
		}
	}
	public static Singleton getInstance(){
		return instance;
	}
	private Singleton(){
	}
}

• The solution of Bill Pugh 。- From wiki <Singleton Pattern> 。通过静态成员类来实现，线程安全。

public class Singleton{
    //private constructor prevent instantiation from other classes
    private Singleton(){}

    /**
    *SingletonHolder is loaded on the first execution of Singleton.getInstance()
    *or the first access to SingletonHolder.INSTANCE,not befor.
    */
     private static class SingletonHolder{
        public static final Singleton instance=new Singleton();
     }
     private static Singleton getInstance(){
        return SingletonHolder.instance;
     }
}

public class Singleton{
	//private constructor prevent instantiation from other classes
	private Singleton(){}
    
   	/**
    *SingletonHolder is loaded on the first execution of Singleton.getInstance()
    *or the first access to SingletonHolder.INSTANCE,not befor.
    */
     private static class SingletonHolder{
		public static final Singleton instance=new Singleton();
	 }
     private static Singleton getInstance(){
		return SingletonHolder.instance;
	 }
}