双重检查锁 & volatile保证可见性（变量值改动后及时从工作内存写回主内存）和有序性（指令不可重排）
public class Singleton { private static volatile Singleton singleton = null; private Singleton(){} public static Singleton getSingleton(){ if(singleton == null){ synchronized (Singleton.class){ if(singleton == null){ singleton = new Singleton(); } } } return singleton; } }
利用JVM类加载的原理保证只初始化一下实例，利用内部类，保证使用时才初始化加载（满足了赖加载）
public class Singleton { private static class Holder { private static Singleton singleton = new Singleton(); } private Singleton(){} public static Singleton getSingleton(){ return Holder.singleton; } }
不管采取何种方案，请时刻牢记单例的三大要点：
https://www.cnblogs.com/andy-zhou/p/5363585.html
原文：https://www.cnblogs.com/genggeng/p/10013781.html