单例模式

1、私有化构造器，不能直接创建，调用方法创建对象；

　　

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

2、饿汉式：instance在类装载时就实例化

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

3、静态内部类：

　　

    public class Singleton {  
        private static class SingletonHolder {  
        　　private static final Singleton INSTANCE = new Singleton();  
        }  
        private Singleton (){}  
        public static final Singleton getInstance() {  
        　　return SingletonHolder.INSTANCE;  
        }  
    }