How Can I Programmatically Find All Classes Implementing a Given Interface in Java?

Programmatically Retrieving Class Implementations of an Interface in Java

Java reflection offers flexible mechanisms for introspecting classes and interfaces. In this context, developers can leverage reflection to retrieve a list of all classes that implement a specific interface.

Reflection-Based Approach

Using the reflections library, developers can effortlessly retrieve subclasses of an interface as follows:

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

ServiceLoader Approach

The Java ServiceLoader provides an alternative approach for discovering interface implementations. This technique requires defining the interface as a Service Provider Interface (SPI) and declaring its implementations:

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

Package-Level Annotation Approach

With package-level annotations, we can declare the implementations of an interface within the package-info.java file:

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

Then, we can retrieve these implementations using reflection:

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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