Retrieve all the implementations of an interface in Java

Question

Can someone explain how to retrieve the entire list of the implementations of an interface in Java?

Answer 1

Hi...I think you can achieve this by using the following:

1. Reflections: As Reflections help you to perform the following actions:
   • get all subtypes of some type
   • get all types annotated with some annotation 
   • get all types annotated with some annotation, including annotation parameters matching
   • get all methods annotated with some

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

2. ServiceLoader 

   ServiceLoader<Fruit> loader = ServiceLoader.load(Fruit.class);
   for (Fruit implClass : loader) {
       System.out.println(implClass.getClass().getSimpleName()); ​
   // prints Apple, Mango
   }

   Do remember, you need to define Fruit as a ServiceProviderInterface (SPI) and declare its implementations. you do that by creating a file in resources/META-INF/services with the name examples.reflections.Fruit and declare all implementations of Fruit in it.
   examples.reflections.Apple
   examples.reflections.Mango

3. package-level annotation
    

   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 need to declare the package-level annotation in a file named package-info.java inside that package. here are sample contents:

   @MyPackageAnnotation(implementationsOfPet = {Apple.class, Mango.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 case you need to know more about Java interface, join our Java training class today.

Answer 2

Take a look to this example: https://github.com/burningwave/core/wiki/How-retrieve-all-classes-that-implement-one-or-more-interfaces

Comments

Hi @roberto, there is an option to edit your answer. Please edit the answer with the updated link.

Thank you