How Can I Programmatically Get a Complete List of Interface Implementations in Java?

Getting a Comprehensive List of Interface Implementations via Programming in Java

In Java, you may often encounter a need to programmatically retrieve a collection of classes that implement a particular interface. This can be achieved through various approaches, some of which involve utilizing reflection or other specialized techniques.

Using a Reflection Library: Reflections

The Reflections library provides a convenient way to inspect class metadata and relationships. By utilizing the Reflections instance, you can obtain a set of classes that extend the desired interface, as demonstrated in the following example:

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

Employing ServiceLoader

ServiceLoader offers a mechanism for locating and loading service provider implementations at runtime. To leverage this technique for retrieving interface implementations, you define a ServiceProviderInterface (SPI) and declare its implementations. The following code illustrates the approach:

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

Leveraging Package-Level Annotation

Another alternative involves using a package-level annotation. This approach entails creating a custom annotation that specifies the interface implementations. The following code sample shows how to define the annotation:

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

To declare the annotation in a package, create a file named package-info.java and include the following contents:

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

Then, you can retrieve the interface implementations using the following code:

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());
        }
    }
}

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