How Can I Dynamically Retrieve Java Interface Implementations?

Dynamically Retrieving Implementations of an Interface in Java

In object-oriented programming, it can be beneficial to obtain a list of implementations for a given interface programmatically. In Java, this can be achieved through various techniques, such as reflections.

One approach is to use the Reflections library, which provides a convenient mechanism for introspecting classes and extracting class information. Here's how you can accomplish this using Reflections:

import org.reflections.Reflections;
import org.reflections.SubTypesScanner;

// Define the interface of interest
interface Pet {}

// Implementations of the Pet interface
class Dog implements Pet {}
class Cat implements Pet {}

// Use Reflections to get a list of Pet implementations
Reflections reflections = new Reflections("package.containing.implementations");
Set<Class<? extends Pet>> petImplementations = reflections.getSubTypesOf(Pet.class);

// Iterate over and display the implementations
for (Class<? extends Pet> implementation : petImplementations) {
    System.out.println(implementation.getSimpleName()); // prints Dog, Cat
}

Alternatively, you can utilize Java's ServiceLoader facility to discover implementations for a service provider interface (SPI).

import java.util.ServiceLoader;

// Define the Pet interface as an SPI
interface Pet {}

// Implementations of the Pet interface
class Dog implements Pet {}
class Cat implements Pet {}

// Use ServiceLoader to load implemented services
ServiceLoader<Pet> loader = ServiceLoader.load(Pet.class);

// Iterate over and display the implementations
for (Pet implementation : loader) {
    System.out.println(implementation.getClass().getSimpleName()); // prints Dog, Cat
}

Note that for this approach, you must specify your interface as an SPI by creating a file in the resources/META-INF/services directory with the interface's fully qualified name and listing the implementations within.

Another option involves defining a package-level annotation.

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

// Package-level annotation to define implementations
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.PACKAGE)
public @interface MyPackageAnnotation {
    Class<?>[] implementationsOfPet() default {};
}

// Implementations of the Pet interface
class Dog implements Pet {}
class Cat implements Pet {}

// Define the package-level annotation in a package-info.java file
@MyPackageAnnotation(implementationsOfPet = {Dog.class, Cat.class})
package package.containing.implementations;

// Iterate over and display the implementations
Package[] packages = Package.getPackages();
for (Package p : packages) {
    MyPackageAnnotation annotation = p.getAnnotation(MyPackageAnnotation.class);
    if (annotation != null) {
        Class<?>[] implementations = annotation.implementationsOfPet();
        for (Class<?> implementation : implementations) {
            System.out.println(implementation.getSimpleName());
        }
    }
}

Keep in mind that this approach will only work for packages already known to the ClassLoader. For a more exhaustive search, consider URLClassLoaders, but be mindful of limitations specific to loaded classes.

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