Java – How to get a list of all the implementations of an interface programmatically in Java?

interfacejava

Can I do it with reflection or something like that?

Best Solution

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.