用枚举方式实现单例

示例demo：

public class Singleton {
    public Singleton() {
    }

    //枚举EnumHolder
    private enum EnumHolder {
        INSTANCE;
        private Singleton instance;

        EnumHolder() {
            this.instance = new Singleton();
        }

        private Singleton getInstance() {
            return instance;
        }

    }

    //提供对外的公共方法
    public static Singleton getSingleton() {
        return EnumHolder.INSTANCE.getInstance();
    }
}