java - 关于将单例的构造函数说明符从 private 更改为 protected

Question

如果我们将单例的构造函数从私有(private)更改为 protected ，会发生什么？在这种情况下我们如何防止它破裂？

单例:

public class SingletonObject
{
    private static SingletonObject ref;

    private SingletonObject () //private constructor
    {
        System.setSecurityManager(new SecurityManager());
    }

    public  static synchronized   SingletonObject getSingletonObject()
    {
        if (ref == null)
            ref = new SingletonObject();
                return ref;
    }   

    public Object clone() throws CloneNotSupportedException
    {
        throw new CloneNotSupportedException ();
    }

}

为了打破单例，以下网址包含所需的信息 cracking singleton with other ways .

Answer 1

来自Joshua Bloch's Effective Java关于单例属性:

The private constructor is called only once, to initialize the public static final field (...). The lack of public or protected constructors guarantees a “monoinstance” universe: Exactly one (...) instance will exist once the Singleton class is initialized—no more, no less. Nothing that a client does can change this.

因此，如果您将 Singleton 构造函数设置为 protected ，那么您将公开您的构造函数，以便让同一包中的任何类将实例设为 SingletonObject 的实例。如果发生这种情况，你的单例模式就被破坏了。

以任何方式放宽构造函数的访问修饰符(使其 protected 、公开或默认)都是实例生成的大门，这在单例实现中是不希望的。