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Home Interviews Processing Annotations Using Reflection in Java – MindStick
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ICSM Computer
ICSM Computer 24 Apr 2025
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Once you've got defined and carried out custom annotations, you may use Java reflection to system them at runtime. This is especially useful in frameworks where you scan metadata to drive behavior (for example, dependency injection, configuration, validation).

1. Custom Annotation Example

Let's say we have a custom annotation for marking methods to be executed:

import java.lang.annotation.*;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface RunMe {
    String message() default "Executing";
}

2. Class with Annotated Methods

public class TaskRunner {
    @RunMe(message = "Start Task")
    public void start() {
        System.out.println("Starting...");
    }

    @RunMe
    public void process() {
        System.out.println("Processing...");
    }

    public void skipThis() {
        System.out.println("Not annotated");
    }
}

3. Processing Annotations with Reflection

You can now write a class that scans for methods with @RunMe and invokes them.

import java.lang.reflect.Method;

public class AnnotationProcessor {
    public static void main(String[] args) {
        try {
            TaskRunner runner = new TaskRunner();
            Class<?> clazz = runner.getClass();

            for (Method method : clazz.getDeclaredMethods()) {
                if (method.isAnnotationPresent(RunMe.class)) {
                    RunMe annotation = method.getAnnotation(RunMe.class);
                    System.out.println("Invoking: " + method.getName());
                    System.out.println("  Message: " + annotation.message());

                    method.invoke(runner);  // Call the method dynamically
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Output:

Invoking: start
  Message: Start Task
Starting...
Invoking: process
  Message: Executing
Processing...

4. Key Reflection APIs Used

Method Description
Class.getDeclaredMethods() Gets all declared methods in the class
method.isAnnotationPresent() Checks if an annotation is present
method.getAnnotation() Retrieves the annotation object
method.invoke(obj) Dynamically calls the method on the object

5. Use Cases in Real Projects

  1. JUnit: Uses @Test to identify test methods.
  2. Spring: Uses annotations like @Autowired, @RequestMapping and processes them at runtime.
  3. Hibernate: Reads @Entity, @Table, etc., to map Java classes to database tables.
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