如何以编程方式在 Java 中获取接口的所有实现的列表? [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方法来做吗?

Can I do it with reflection or something like that?

推荐答案

找了好久，好像有不同的方法，总结如下:

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

1. reflections 库很受欢迎，如果你不这样做的话介意添加依赖项.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，要使其正常工作，您需要将 Pet 定义为 ServiceProviderInterface (SPI) 并声明其实现.您可以通过在 resources/META-INF/services 中创建一个名为 examples.reflections.Pet 的文件并声明 Pet 的所有实现来实现在里面

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());
        }
    }
}

和注解定义:

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

并且您必须在该包内名为 package-info.java 的文件中声明包级注释.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，只有在那时 ClassLoader 已知的包才会通过调用 Package.getPackages() 加载.

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

此外，还有其他基于 URLClassLoader 的方法将始终限于已加载的类，除非您进行基于目录的搜索.

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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