单例模式

public class Singleton {  
 
    private volatile static Singleton singleton;  
      
    private Singleton(){ }  
   
   
    public static Singleton getInstance(){  
        // 双重检查加锁  
        if(singleton==null){  
            synchronized(Singleton.class){  
                // 延迟实例化,需要时才创建  
                if(singleton==null)  
                    singleton = new Singleton();
                System.out.println("==========111===");
            }  
        }  
        return singleton;  
    }  
}