# Marker Annotation - for marking concepts, ideas and patterns

During yesterday's smoke test, I checked in the Marker Annotation "Pattern" to p4j5. I borrowed the name from the "Marker Interface" Pattern.However, I use Marker Annotations to annotate project-wide concepts and ideas in a lean and convenient way. Instead of writing JavaDoc (which is not typesafe - you can mispell the tags), I use lean annotations to mark patterns/idioms/concepts, and JavaDoc for the remaining (parameters, returnvalues etc.) tasks.Defining an annotation isn't really hard. For e.g. the following snippets is used to mark the "Service" pattern from p4j5:@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.TYPE)@Documentedpublic @interface Service {/*** * @return The main responsibility of the documented concept. */  String essentialResponsibility();   /**    *     * @return The difference between the concept from MAD and the realization     */   String delta() default "";}All such annotations has to be defined outside the project and deployed as a jar. Then they can be easily used:@Service( essentialResponsibility="CRUD")public class SeviceBean implemens Service {}Some benefits:Less documentation to write (actually a huge one).The meta-information is accessible in bytecode and can be processed with e.g. apt.The architecture can be validated afterwards (e.g. searching for unallowed dependencies between patterns) - the tools are able to access the annotations.The meaning of the annotation can be kept in the annotation itself and e.g. in wiki. Especially information like:TransactionalityConcurrencyRemoting (@Local, @Remote)can be stored in one place. Redundant documentation is nasty to write and maintain...