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

Question

Can I do it with reflection or something like that?

Answer 1

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.