单例的7种写法

懒汉模式-线程不安全

1 2 3 4 5 6 7 8 9 10 11 12 13

      
      

       
       public class Singleton {

      
      
      
      
   
       
        private
       
        static Singleton instance;

      
      
      
      
   
       
        private Singleton
       
        (
       
       ){

      
      
      
      


      
      
      
      

       
           }

      
      
      
      


      
      
      
      
   
       
        public
       
        static Singleton
       
        getInstance(
       
       ) {

      
      
      
      
       
       
        if (instance == null
       
       ) {

      
      
      
      
           
       
        instance =
       
        new Singleton(
       
       );

      
      
      
      

       
               }

      
      
      
      
       
       
        return instance;

      
      
      
      

       
           }

      
      
      
      

       
       }

      
      

懒汉模式-线程安全

1 2 3 4 5 6 7 8 9 10 11 12 13

      
      

       
       public class Singleton {

      
      
      
      
   
       
        private
       
        static Singleton instance;

      
      
      
      
   
       
        private Singleton
       
        (
       
       ){

      
      
      
      


      
      
      
      

       
           }

      
      
      
      


      
      
      
      
   
       
        public
       
        static synchronized Singleton
       
        getInstance(
       
       ) {

      
      
      
      
       
       
        if (instance == null
       
       ) {

      
      
      
      
           
       
        instance =
       
        new Singleton(
       
       );

      
      
      
      

       
               }

      
      
      
      
       
       
        return instance;

      
      
      
      

       
           }

      
      
      
      

       
       }

      
      

饿汉模式

1 2 3 4 5 6 7 8 9 10 11

      
      

       
       public class Singleton {

      
      
      
      
   
       
        private
       
        static Singleton
       
        instance =
       
        new Singleton(
       
       );

      
      
      
      


      
      
      
      
   
       
        private Singleton
       
        (
       
       ){

      
      
      
      


      
      
      
      

       
           }

      
      
      
      
    

      
      
      
      
   
       
        public
       
        static Singleton
       
        getInstance(
       
       ) {

      
      
      
      
    
       
       	return instance;

      
      
      
      

       
           }

      
      
      
      

       
       }

      
      

饿汉模式-变种

1 2 3 4 5 6 7 8 9 10 11 12 13 14

      
      

       
       public class Singleton {

      
      
      
      
   
       
        private Singleton
       
        instance = null;

      
      
      
      


      
      
      
      
   
       
        static {

      
      
      
      
	   
       
        instance =
       
        new Singleton(
       
       );

      
      
      
      

       
           }

      
      
      
      
    

      
      
      
      
   
       
        private Singleton
       
        (
       
       ){

      
      
      
      


      
      
      
      

       
           }

      
      
      
      
   
       
        public
       
        static Singleton
       
        getInstance(
       
       ) {

      
      
      
      
	   
       
        return this.instance;

      
      
      
      

       
           }

      
      
      
      

       
       }

      
      

静态内部类

1 2 3 4 5 6 7 8 9 10 11 12 13

       
       

        
        public 
        
        class Singleton {

       
       
       
       
    
        
        private 
        
        static 
        
        class SingletonHolder {

       
       
       
       
	    
        
        private 
        
        static 
        
        final Singleton INSTANCE = 
        
        new Singleton();

       
       
       
       

        
            }

       
       
       
       


       
       
       
       
    
        
        private Singleton (){

       
       
       
       


       
       
       
       

        
            }

       
       
       
       


       
       
       
       
    
        
        public static final Singleton getInstance() {

       
       
       
       
	    
        
        return SingletonHolder.INSTANCE;

       
       
       
       

        
            }

       
       
       
       

        
        }

       
       

双重校验锁

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

       
       

        
        public 
        
        class Singleton {

       
       
       
       
    
        
        private 
        
        volatile 
        
        static Singleton singleton;

       
       
       
       
    
        
        private Singleton (){

       
       
       
       


       
       
       
       

        
            }

       
       
       
       
    
        
        public static Singleton getSingleton() {

       
       
       
       
        
        
        if (singleton == 
        
        null) {

       
       
       
       
            
        
        synchronized (Singleton.class) {

       
       
       
       
                
        
        if (singleton == 
        
        null) {

       
       
       
       

        
                            singleton = 
        
        new Singleton();

       
       
       
       

        
                        }

       
       
       
       

        
                    }

       
       
       
       

        
                }

       
       
       
       
        
        
        return singleton;

       
       
       
       

        
            }

       
       
       
       

        
        }

       
       

枚举

1 2 3 4 5

      
      

       
       public 
       
       enum Singleton {  

      
      
      
      

       
           INSTANCE;  

      
      
      
      
    
       
       public 
       
       void whateverMethod() {  

      
      
      
      

       
           }  

      
      
      
      

       
       }