如何防止单例对象被克隆？

public class Singleton implements Cloneable {
    private static Singleton instance;

    private Singleton() {
        // 私有构造函数
    }

    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        throw new CloneNotSupportedException();
    }
}

public class Singleton implements Cloneable {
    private static Singleton instance;
    private boolean isCloned;

    private Singleton() {
        // 私有构造函数
        isCloned = false;
    }

    public static synchronized Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException {
        if (isCloned) {
            throw new CloneNotSupportedException();
        }
        isCloned = true;
        return super.clone();
    }
}

public enum Singleton {
    INSTANCE;

    // 单例对象的其他方法和属性
}