有没有办法在 Java 中模拟 C++“朋友"概念? [英] Is there a way to simulate the C++ 'friend' concept in Java?

问题描述

我希望能够在一个包中编写一个 Java 类，该类可以访问另一个包中某个类的非公共方法，而不必使其成为另一个类的子类.这可能吗?

I would like to be able to write a Java class in one package which can access non-public methods of a class in another package without having to make it a subclass of the other class. Is this possible?

推荐答案

朋友"概念在 Java 中很有用，例如，用于将 API 与其实现分开.实现类通常需要访问 API 类内部，但这些不应暴露给 API 客户端.这可以使用如下详述的朋友访问器"模式来实现:

The 'friend' concept is useful in Java, for example, to separate an API from its implementation. It is common for implementation classes to need access to API class internals but these should not be exposed to API clients. This can be achieved using the 'Friend Accessor' pattern as detailed below:

通过API暴露的类:

package api;

public final class Exposed {
    static {
        // Declare classes in the implementation package as 'friends'
        Accessor.setInstance(new AccessorImpl());
    }

    // Only accessible by 'friend' classes.
    Exposed() {

    }

    // Only accessible by 'friend' classes.
    void sayHello() {
        System.out.println("Hello");
    }

    static final class AccessorImpl extends Accessor {
        protected Exposed createExposed() {
            return new Exposed();
        }

        protected void sayHello(Exposed exposed) {
            exposed.sayHello();
        }
    }
}

提供朋友"功能的类:

package impl;

public abstract class Accessor {

    private static Accessor instance;

    static Accessor getInstance() {
        Accessor a = instance;
        if (a != null) {
            return a;
        }

        return createInstance();
    }

    private static Accessor createInstance() {
        try {
            Class.forName(Exposed.class.getName(), true, 
                Exposed.class.getClassLoader());
        } catch (ClassNotFoundException e) {
            throw new IllegalStateException(e);
        }

        return instance;
    }

    public static void setInstance(Accessor accessor) {
        if (instance != null) {
            throw new IllegalStateException(
                "Accessor instance already set");
        }

        instance = accessor;
    }

    protected abstract Exposed createExposed();

    protected abstract void sayHello(Exposed exposed);
}

从朋友"实现包中的类访问的示例:

Example access from a class in the 'friend' implementation package:

package impl;

public final class FriendlyAccessExample {
    public static void main(String[] args) {
        Accessor accessor = Accessor.getInstance();
        Exposed exposed = accessor.createExposed();
        accessor.sayHello(exposed);
    }
}

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