如何在Java中以编程方式获取接口的所有实现列表？ [英] How can I get a list of all the implementations of an interface programmatically in Java?

问题描述

我可以用反射或类似的东西来做吗？

Can I do it with reflection or something like that?

推荐答案

我一直在寻找一段时间似乎这是一个总结：

I have been searching for a while and there seems to be different approaches, here is a summary:

1. 反射 如果您不介意添加依赖项，那么库很受欢迎。它看起来像这样：

1. reflections library is pretty popular if u don't mind adding the dependency. It would look like this:

Reflections reflections = new Reflections("firstdeveloper.examples.reflections");
Set<Class<? extends Pet>> classes = reflections.getSubTypesOf(Pet.class);

ServiceLoader （根据erickson的回答）它会看起来像这样：

ServiceLoader (as per erickson answer) and it would look like this:

ServiceLoader<Pet> loader = ServiceLoader.load(Pet.class);
for (Pet implClass : loader) {
    System.out.println(implClass.getClass().getSimpleName()); // prints Dog, Cat
}

请注意，为此，您需要定义 Pet 作为ServiceProviderInterface（SPI）并声明其实现。你可以通过在 resources / META-INF / services 中创建一个名为 examples.reflections.Pet 的文件来做到这一点。声明其中 Pet 的所有实现

Note that for this to work you need to define Petas a ServiceProviderInterface (SPI) and declare its implementations. you do that by creating a file in resources/META-INF/services with the name examples.reflections.Pet and declare all implementations of Pet in it

examples.reflections.Dog
examples.reflections.Cat

包级注释。这是一个例子：

Package[] packages = Package.getPackages();
for (Package p : packages) {
    MyPackageAnnotation annotation = p.getAnnotation(MyPackageAnnotation.class);
    if (annotation != null) {
        Class<?>[]  implementations = annotation.implementationsOfPet();
        for (Class<?> impl : implementations) {
            System.out.println(impl.getSimpleName());
        }
    }
}

和注释定义：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.PACKAGE)
public @interface MyPackageAnnotation {
    Class<?>[] implementationsOfPet() default {};
}

您必须在名为<$ c的文件中声明包级注释该包中的$ c> package-info.java 。以下是示例内容：

and you must declare the package-level annotation in a file named package-info.java inside that package. here are sample contents:

@MyPackageAnnotation(implementationsOfPet = {Dog.class, Cat.class})
package examples.reflections;

请注意，只有当时ClassLoader已知的软件包才能通过调用 Package.getPackages（）。

Note that only packages that are known to the ClassLoader at that time will be loaded by a call to Package.getPackages().

此外，还有基于URLClassLoader的其他方法将始终限制为已经加载的类，除非您执行基于目录的搜索。

In addition, there are other approaches based on URLClassLoader that will always be limited to classes that have been already loaded, Unless you do a directory-based search.

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