Question

The question tells it all.

For the experts, is there a reason the SUN java 5 compiler accepts recursive annotations (contrary to the langspec), while the later compilers do not? I mean, what could be an argument against recursive annotations.

Edit: a recursive annotation is something like:

@Panel(layout=BorderLayout.class,     nested={         @Panel(region=NORTH, layout=FlowLayout.class, ...)         @Panel(region=SOUTH, layout=FlowLayout.class, ...)     } ) 

Answer 1

First — I’m not sure what you mean by recursive annotations. Do you mean annotations that can contain references to other annotations of the same type? Something like

@Panel(layout=BorderLayout.class,     nested={         @Panel(region=NORTH, layout=FlowLayout.class, ...)         @Panel(region=SOUTH, layout=FlowLayout.class, ...)     } ) 

(which would be an example of where I’d like to use it if it were possible…)

As for my use of custom annotations (and processors): code generation.

See http://code.google.com/p/javadude/wiki/Annotations

For example, JavaBean properties:

@Bean(     properties={           @Property(name='name'),       @Property(name='phone', bound=true),       @Property(name='friend', type=Person.class, kind=PropertyKind.LIST)     } ) public class Person extends PersonGen {     // generated superclass PersonGen will contain getters/setters     //    field definitions, property change support... } 

or a mix-in example

package sample;  import java.util.List;  public interface IFlightAgent {     List<IFlight> getFlight();     void reserve(IFlight flight); }  public interface ICarAgent {     List<ICar> getCars();     void reserve(ICar car); }  public interface IHotelAgent {     List<IHotel> getHotels();     void reserve(IHotel hotel); }  package sample;  import com.javadude.annotation.Bean; import com.javadude.annotation.Delegate;  @Bean(delegates = {     @Delegate(type = IHotelAgent.class,               property = 'hotelAgent',               instantiateAs = HotelAgentImpl.class),     @Delegate(type = ICarAgent.class,               property = 'carAgent',               instantiateAs = CarAgentImpl.class),     @Delegate(type = IFlightAgent.class,               property = 'flightAgent',               instantiateAs = FlightAgentImpl.class)     } ) public class TravelAgent extends TravelAgentGen     implements IHotelAgent, ICarAgent, IFlightAgent {     // generated superclass TravelAgentGen will create instances     //   of the 'instantiateAs' classes and delegate the interface     //   methods to them } 

See The drawbacks of annotation processing in Java? and my answer to it for some potential issues with their usage.