How to Create a Java Annotation That Only Applies to Classes Implementing a Specific Interface?

How can I define a custom Java annotation that is applicable only to classes implementing a certain interface?

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface MyCustomAnnotation {
    String value() default "";
}

Creating a Java annotation that can be applied only to classes implementing a specific interface involves using a combination of meta-annotations and runtime checks. Java doesn’t enforce such constraints at the annotation definition level, but you can enforce it through reflection.

// Define the annotation
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface MyCustomAnnotation {
    String value() default "";
}

// Usage of the annotation on a class
@MyCustomAnnotation(value="Example")
public class ExampleClass implements MyInterface {
    // Class implementation
}

// Runtime check
public static void validateAnnotation(Class<?> clazz) {
    if (clazz.isAnnotationPresent(MyCustomAnnotation.class)) {
        if (!MyInterface.class.isAssignableFrom(clazz)) {
            throw new IllegalArgumentException("Class must implement MyInterface.");
        }
    }
}

Causes

• Annotations in Java do not inherently validate the target types at compile-time.
• Java annotations are designed to provide metadata to classes, but they can't restrict their usage based on class hierarchies without additional checks.

Solutions

• Define the annotation with the @Target(ElementType.TYPE) meta-annotation to indicate it can only be applied to classes.
• At runtime, when retrieving annotations, use reflection to check if the annotated class implements the desired interface before performing any action.

Mistake: Not checking for interface implementation when processing the annotation.

Solution: Always validate that the class implements the desired interface within the logic handling the annotation.

Mistake: Assuming annotations automatically provide type safety or validation.

Solution: Understand that annotations are merely metadata; enforce business logic checks via reflection.