面试题2：实现一个Singleton模式

public class Singleton {
//内部持有Singleton唯一引用
private static Singleton instance;
//构造器私有化,防止外部new对象
private Singleton(){}
//利用锁加双重判断生成单例对象实例,并返回
public static Singleton getInstance(){
if(instance==null){
synchronized (Singleton.class) {
if(instance==null)
instance = new Singleton();
}
}
return instance;
}
}