有没有办法模拟C ++的“朋友”的概念在Java？ [英] Is there a way to simulate the C++ 'friend' concept in Java?

问题描述

我想能够在一个包中编写一个Java类，它可以访问另一个包中的类的非公共方法，而不必将其作为另一个类的子类。这是可能吗？

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

friend概念在Java中非常有用，例如，将API与其实现分离。通常实现类需要访问API类内部结构，但这些不应该暴露给API客户端。这可以使用Friend Accessor模式实现，如下所示：

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公开的类：

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();
        }
    }
}

'functionality：

The class providing the 'friend' functionality:

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friend实现包中的类访问示例：

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);
    }
}

这篇关于有没有办法模拟C ++的“朋友”的概念在Java？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！