在运行时，查找 Java 应用程序中扩展基类的所有类 [英] At runtime, find all classes in a Java application that extend a base class

问题描述

我想做这样的事情:

List<Animal> animals = new ArrayList<Animal>();

for( Class c: list_of_all_classes_available_to_my_app() )
   if (c is Animal)
      animals.add( new c() );

因此，我想查看应用程序 Universe 中的所有类，当我找到一个来自 Animal 的类时，我想创建一个该类型的新对象并将其添加到列表中.这使我无需更新事物列表即可添加功能.我可以避免以下情况:

So, I want to look at all of the classes in my application's universe, and when I find one that descends from Animal, I want to create a new object of that type and add it to the list. This allows me to add functionality without having to update a list of things. I can avoid the following:

List<Animal> animals = new ArrayList<Animal>();
animals.add( new Dog() );
animals.add( new Cat() );
animals.add( new Donkey() );
...

通过上述方法，我可以简单地创建一个扩展 Animal 的新类，它会自动被选中.

With the above approach, I can simply create a new class that extends Animal and it'll get picked up automatically.

更新:2008 年 10 月 16 日上午 9:00 太平洋标准时间:

UPDATE: 10/16/2008 9:00 a.m. Pacific Standard Time:

这个问题引起了很多很好的回应——谢谢.从回复和我的研究中，我发现我真正想做的事情在 Java 下是不可能的.有一些方法，例如 ddimitrov 的 ServiceLoader 机制可以工作——但它们对于我想要的东西来说非常繁重，我相信我只是将问题从 Java 代码转移到外部配置文件. 更新 5/10/19(11 年后！)根据@IvanNik 的 answer<，现在有几个库可以帮助解决这个问题/a> org.reflections 看起来不错.还有来自@Luke Hutchison 的 ClassGraph9648">答案 看起来很有趣.答案中还有更多的可能性.

This question has generated a lot of great responses -- thank you. From the responses and my research, I've found that what I really want to do is just not possible under Java. There are approaches, such as ddimitrov's ServiceLoader mechanism that can work -- but they are very heavy for what I want, and I believe I simply move the problem from Java code to an external configuration file. Update 5/10/19 (11 years later!) There are now several libraries that can help with this according to @IvanNik's answer org.reflections looks good. Also ClassGraph from @Luke Hutchison's answer looks interesting. There are several more possibilities in the answers as well.

另一种表达我想要的方式:我的 Animal 类中的静态函数查找并实例化从 Animal 继承的所有类——无需任何进一步的配置/编码.如果我必须配置，我不妨在 Animal 类中实例化它们.我理解这是因为 Java 程序只是 .class 文件的松散联合，所以它就是这样.

Another way to state what I want: a static function in my Animal class finds and instantiates all classes that inherit from Animal -- without any further configuration/coding. If I have to configure, I might as well just instantiate them in the Animal class anyway. I understand that because a Java program is just a loose federation of .class files that that's just the way it is.

有趣的是，这似乎是 C# 中的相当简单.

Interestingly, it seems this is fairly trivial in C#.

推荐答案

我使用 org.reflections:

Reflections reflections = new Reflections("com.mycompany");    
Set<Class<? extends MyInterface>> classes = reflections.getSubTypesOf(MyInterface.class);

另一个例子:

public static void main(String[] args) throws IllegalAccessException, InstantiationException {
    Reflections reflections = new Reflections("java.util");
    Set<Class<? extends List>> classes = reflections.getSubTypesOf(java.util.List.class);
    for (Class<? extends List> aClass : classes) {
        System.out.println(aClass.getName());
        if(aClass == ArrayList.class) {
            List list = aClass.newInstance();
            list.add("test");
            System.out.println(list.getClass().getName() + ": " + list.size());
        }
    }
}

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