单例类 — 老实说

[shidongwa]

https://shidonghua.work/2018/06/15/design-pattern-singleton/

单例类实现注意事项 私有的构造函数 static final 的instance域 (eagly init) static volatile 的instance (layz init with double check) 多线程环境下，double check考虑 (synchronized class类，非实例) 单例类如果实现了Serializable接口（直接或者间接），需要添加一个readResolve()方法，用instance域替换掉Serialization机制自动生成的实例类对象 double check模式中volatile的原因是避免指令排序 分布对象的内存空间 初始化对象 设置instance对象指向内存空间 private Object readResolve(){ return INSTANCE;}单例1public class DraconianSingleton { private static DraconianSingleton instance; public static synchronized DraconianSingleton getInstance() { if (instance == null) { instance = new DraconianSingleton(); } return instance; } private DraconianSingleton() { }}单例2public class DclSingleton { private static volatile DclSingleton instance; public static DclSingleton getInstance() { if (instance == null) { synchronized (DclSingleton .class) { if (instance == null) { instance = new DclSingleton(); } } } return instance; } // private constructor and other methods... private DraconianSingleton() { }}单例3public class EarlyInitSingleton { private static final EarlyInitSingleton INSTANCE = new EarlyInitSingleton(); public static EarlyInitSingleton getInstance() { return INSTANCE; } // private constructor and other methods... private DraconianSingleton() { }}单例4public class InitOnDemandSingleton { private static class InstanceHolder { private static final InitOnDemandSingleton INSTANCE = new InitOnDemandSingleton(); } public static InitOnDemandSingleton getInstance() { return InstanceHolder.INSTANCE; } // private constructor and other methods... private DraconianSingleton() { }}单例5public enum EnumSingleton { INSTANCE; // other methods...}参考baeldung