单例模式

class Singleton{
    // 1:私有化构造方法
    private Singleton(){};

    // 2:私有的静态内部类，类加载器负责加锁
    private static class SingletonHolder{
        private static Singleton singleton = new Singleton();
    }

    // 3:自行对外提供实例
    public static Singleton getInstance(){
        return SingletonHolder.singleton;
    }
}