单例模式

使一个类只在内存中实例化一次

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