Annotations

Annotations attach metadata to declarations and type uses. Meta-annotations control where metadata is legal and how long it remains.

A runtime annotation

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Audited {
    String action();
}

@Audited(action = "user.read")
public User find(long id) { return repository.find(id); }

SOURCE annotations support tooling, CLASS annotations remain in bytecode, and RUNTIME annotations are reflectively visible. Repeatable and inherited annotations have specific rules. Compile-time processors can validate models and generate sources without runtime reflection. Treat annotations as declarative metadata, not hidden control flow that makes behavior impossible to trace.