How to Retrieve Annotation Class Names and Attribute Values Using Reflection in Java

How can I retrieve the class name of an annotation and its attribute values using reflection in Java?

import java.lang.annotation.*;
import java.lang.reflect.*;

In Java, annotations are a powerful feature used to provide metadata about the code. Using the Reflection API, we can obtain not only the class name of an annotation but also the values of its attributes. This guide outlines the steps to achieve this.

@Retention(RetentionPolicy.RUNTIME)
@interface SampleAnnotation {
    String name();
    int value();
}

@SampleAnnotation(name="Example", value=10)
class SampleClass {}

public class Main {
    public static void main(String[] args) throws Exception {
        // Accessing the annotation
        Class<SampleClass> obj = SampleClass.class;
        if (obj.isAnnotationPresent(SampleAnnotation.class)) {
            SampleAnnotation annotation = obj.getAnnotation(SampleAnnotation.class);
            System.out.println("Annotation Class Name: " + annotation.annotationType().getSimpleName());
            System.out.println("Name: " + annotation.name());
            System.out.println("Value: " + annotation.value());
        }
    }
}

Causes

• Annotations are often not easily accessible without reflection.
• Understanding the structure of annotations and their attributes is essential.

Solutions

• Use the `Class.getAnnotations()` method to retrieve all annotations on a class.
• Loop through the annotations and obtain their class names with `annotation.annotationType().getName()`.
• Access attribute values using reflection methods like `Method.invoke()`.
• Here’s a simple example for clarity.

Mistake: Not using the correct retention policy for annotations.

Solution: Ensure the annotation has a retention policy of RUNTIME.

Mistake: Forgetting to check if the annotation is present before accessing it.

Solution: Always use `isAnnotationPresent()` method to verify presence.