单例模式【SINGLETON PATTERN】

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

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