Have you ever written custom annotations or performed class scanning?

Yes, I have had to create custom annotations and implement class scanning. Custom annotations are useful for adding metadata to classes, methods, or fields, which can then be processed using reflection.

For example, you can create an annotation to mark methods that should be executed within a transaction:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Transactional {
}

Then, using class scanning and reflection, you can find all methods with this annotation and wrap their calls in a transaction.

Class scanning is usually implemented through libraries like Reflections or custom code that iterates over classes in a specified package and checks for the presence of the required annotations.