How can I get a list of all the implementations of an interface programmatically in Java?

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

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);
   

2. 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
   }
   

   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
   

3. package-level annotation. here is an example:

   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());
           }
       }
   }
   

   and the annotation definition:

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

   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;
   

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

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.