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

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

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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。

复制代码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 }

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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

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

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

• static block initialization

复制代码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(){ } }

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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。

复制代码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; } }

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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;
}
}