How to Retrieve All Classes with a Specific Annotation in Android and Store Them in a HashMap

How can I find all classes in an Android project that are annotated with a specific annotation and store them in a HashMap?

// Example annotation
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface MyAnnotation {}

// Example classes with the annotation
@MyAnnotation
public class ExampleClassOne {}

@MyAnnotation
public class ExampleClassTwo {}

public class AnotherClass {}

In Android development, you may need to gather all classes that are annotated with a specific annotation for processing or runtime behavior. This involves reflection and classpath scanning techniques.

import java.lang.annotation.Annotation;
import java.util.HashMap;
import java.util.Set;  
import org.reflections.Reflections;  

public class AnnotationProcessor {
    public static void main(String[] args) {
        HashMap<String, Class<?>> annotatedClasses = new HashMap<>();
        Reflections reflections = new Reflections("com.example.yourpackage");

        Set<Class<?>> classes = reflections.getTypesAnnotatedWith(MyAnnotation.class);
        for (Class<?> clazz : classes) {
            annotatedClasses.put(clazz.getSimpleName(), clazz);
        }

        // Outputting the results
        annotatedClasses.forEach((key, value) -> System.out.println(key + " -> " + value));
    }
}

Causes

• Lack of awareness of reflection capabilities in Java.
• Understanding how to manage annotations in Android projects.
• Challenges with classpath scanning in Android.

Solutions

• Utilize Java reflection to inspect classes and their annotations.
• Leverage a library like Reflections or similar for classpath scanning.
• Iterate through the classes in a package using the ClassLoader and check for annotations.

Mistake: Assuming all classes are loaded at runtime without using a proper scanning library.

Solution: Use a dedicated library like Reflections to scan classpath efficiently.

Mistake: Not correctly defining annotation retention policy (RUNTIME).

Solution: Ensure the annotation is defined with @Retention(RetentionPolicy.RUNTIME) to be accessible at runtime.