Introductions

Overview

The principles behind introductions and mixins when using annotations are exactly the same as when using XML, all that is different is the way you define it.

Introductions

You declare the introductions within a class that has been annotated with @Aspect or @InterceptorDef. MyAspect in this example serves no purpose beyond allowing you to define introductions (i.e. it contains no actual advices).

   import org.jboss.aop.Introduction;

   @Aspect
   public class MyAspect
   {
      @Introduction (target=POJO.class, interfaces={java.io.Serializable.class})
      public static Object noInterfacesPOJOIntro;
   
      ...
   }

Basically, you just annotate any field within the aspect with @Introduction. The target attribute takes the class you want to introduce interfaces into, and the interfaces attribute takes an array of the interfaces you want to add to the class.

Mixins

You declare the mixins in much the same way as you declare introductions. Again, it must be done within a class that has been annotated with @Aspect or @InterceptorDef.

   import org.jboss.aop.Mixin;

   @Aspect
   public class MyAspect
   {
      ...

      @Mixin (target=POJO2.class, interfaces={java.io.Externalizable.class})
      public static POJO2ExternalizableMixin createExternalizableMixin(POJO2 pojo) {
         return new POJO2ExternalizableMixin(pojo);
      }
   }

You annotate a public static method within the aspect with @Mixin. The target attribute takes the class you want to introduce interfaces into, and the interfaces attribute takes an array of the interfaces you want to add to the class. The method must take a class of the same type as the target as its parameter, and it must contain the logic to create and return an instance of the mixin class.

Type expressions

The target attribute for @Introduction and @Mixin takes a fixed class. You can use the typeExpression attribute in place of target to make the @Introduction or @Mixin apply to a wider range of classes.

   import org.jboss.aop.Mixin;

   @Aspect
   public class MyAspect
   {
      ...
      
      @Introduction (typeExpression="class(POJO3) OR class(POJO4)", interfaces={java.io.Serializable.class})
      public static Object withTypeExpression;
   }

Running the example

$ ant

The output should be:

run:
     [java] --- POJO ---
     [java] deserialized pojo.stuff: hello world
     [java] --- POJO2 ---
     [java] deserialized pojo2.stuff2: hello world
     [java] --- POJO3 ---
     [java] pojo3 introduction expression worked
     [java] --- POJO4 ---
     [java] pojo4 introduction expression worked