单例模式

public class Singleton {  
    private static class SingletonHolder {  
    private static final Singleton INSTANCE = new Singleton();  
    }  
    private Singleton (){}  
    public static final Singleton getInstance() {  
    return SingletonHolder.INSTANCE;  
    }  
}

转载：https://juejin.cn/post/6844903606659842055#heading-7
https://juejin.cn/post/6844903875443458062