单例模式如何防止反射攻击

单例模式例子：

public class Single {
    private Single ()  {//私有化构造方法
    }
    private static volatile Single instance = null;
    public static synchronized Single getInstance() {
        if(instance==null){
            instance = new Single();//在此方法内创建对象
        }
        return instance;//返回所创建的对象
    }
}

以上代码若采用反射来创建对象就会翻车了！！
例子：

class Main{
    public static void main(String[] args) throws Exception {
        Class clazz = Single.class;
        Constructor constructor = clazz.getDeclaredConstructor();
        constructor.setAccessible(true);
        Single single = Single.getInstance();
        Single single1 = (Single) constructor.newInstance();
        System.out.println(single==single1);
    }
}

输出：

这方法如何解决呢？
暴力反射调取了类中的私有化构造器方法，而instance属性是静态的，所以可以判断instance是否为null，若不是null，则抛出异常警告

代码如下：

public class Single {
    private Single ()  {
        if(instance!=null){
            throw new RuntimeException("禁止使用反射调用！");
        }
    }
    private static volatile Single instance = null;
    public static synchronized Single getInstance() {
        if(instance==null){
            instance = new Single();
        }
        return instance;
    }
}

运行结果如下：

成功解决！

在这读者可能会有个问题，以上只使用了一次反射来调取构造方法创建对象，若两次创建对象都采用反射的方式呢？该如何解决？
例如

class Main{
    public static void main(String[] args) throws Exception {
        Class clazz = Single.class;
        Constructor constructor = clazz.getDeclaredConstructor();
        constructor.setAccessible(true);
        Single single1 = (Single) constructor.newInstance();
        Single single = (Single) constructor.newInstance();
        System.out.println(single==single1);
    }
}

很遗憾，这种情况下若要坚持采用懒汉式的单例模式是不能解决的。
只能够使用饿汉式的单例模式解决：

public class Single {
    private Single ()  {
        if(instance!=null){
            throw new RuntimeException("禁止使用反射调用！");
        }
    }
    private static volatile Single instance = new Single();
    public static Single getInstance() {
//        if(instance==null){
//            instance = new Single();
//        }
        return instance;
    }
}
class Main{
    public static void main(String[] args) throws Exception {
//        Single single1 = Single.getInstance();
//        Single single2 = Single.getInstance();
//        System.out.println(single1==single2);
        Class clazz = Single.class;
        Constructor constructor = clazz.getDeclaredConstructor();
        constructor.setAccessible(true);
//        Single single = Single.getInstance();
        Single single1 = (Single) constructor.newInstance();
        Single single = (Single) constructor.newInstance();
        System.out.println(single==single1);
    }
}