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

Querying Interface Implementations Programmatically in Java

Retrieving a list of classes implementing an interface can prove challenging. Luckily, Java offers several approaches to address this query:

Using the Reflections Library:

Reflections is a popular library that allows developers to inspect and manipulate metadata about classes and packages. Using Reflections, you can obtain a list of subclasses implementing a given interface:

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

Leveraging ServiceLoader:

ServiceLoader provides a platform-independent mechanism for discovering compatible implementations of a service provider interface. This approach requires defining your interface as a service provider interface and declaring its implementations. After setting up this infrastructure, you can obtain the implementations as follows:

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

Employing Package-Level Annotations:

Package-level annotations provide a way to annotate packages with metadata. This feature can be exploited to tag packages with information about the classes they contain. To utilize this approach:

1. Define a custom annotation to identify implementing classes:

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

2. Apply the annotation to the appropriate packages in the source code:

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

3. Query implemented classes:

   Package[] packages = Package.getPackages();
   for (Package p : packages) {
       MyPackageAnnotation annotation = p.getAnnotation(MyPackageAnnotation.class);
       if (annotation != null) {
           Class<?>[] implementations = annotation.implementationsOfPet();
           System.out.println("Found implementation: " + implementations[0].getSimpleName());
       }
   }

These techniques offer flexibility in obtaining a list of classes implementing a specified interface.

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