How would you read custom annotations at runtime using reflection to implement annotation-driven behavior, such as a simple dependency injection or validation framework?

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Answer

Use java.lang.reflect classes (Class.getDeclaredMethods(), Method.getAnnotation(YourAnnotation.class), Field.isAnnotationPresent()) to inspect a class's structure at runtime, checking for the presence of your custom RUNTIME-retention annotation and extracting its element values to drive behavior -- this is the fundamental mechanism underlying frameworks like Spring's @Autowired or Hibernate Validator's @NotNull, where annotation metadata is read reflectively to wire dependencies or apply validation rules without generated boilerplate.

public class AnnotationProcessor {
    public static void invokeLoggable(Object obj) throws Exception {
        for (Method method : obj.getClass().getDeclaredMethods()) {
            if (method.isAnnotationPresent(Loggable.class)) {
                Loggable ann = method.getAnnotation(Loggable.class);
                System.out.println("Invoking logged method: " + ann.value());
                method.invoke(obj);
            }
        }
    }
}

Real-world example A lightweight in-house DI container scans classes for a custom @Inject annotation on fields, using reflection to instantiate and set the appropriate dependency, replicating a simplified version of what Spring's @Autowired does under the hood.

Common follow-ups: What's the performance cost of reflection-based annotation processing at runtime versus compile-time annotation processing?;How does Java's annotation processing API (APT) differ from reading annotations reflectively at runtime?

Reflection API;Design Patterns in Java

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