Most annotations in Axon can be placed on other annotations, as so-called meta-annotation. When Axon scans for annotations, it will automatically scan meta-annotations as well. Annotations can override the properties defined on the meta-annotations, if desired.
For example, if you have a practice in your development team that payloads are always represented in JSON and you wish the command name to be explicitly configured, you could create your own annotation:
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.CONSTRUCTOR, ElementType.ANNOTATION_TYPE})
@CommandHandler(payloadType = JsonNode.class)
public @interface JsonCommandHandler {
String commandName;
String routingKey() default "";
}By specifying the payloadType on the @CommandHandler meta-annotation,
this becomes the value used for all Command Handlers annotated with JsonCommandHandler.
These command handlers may (and should) still provide a parameter for the payload,
but Axon will complain if it isn't a subclass of JsonNode.
The commandName attribute on the JsonCommandHandler annotation does not have a default value,
and will therefore force developers to specify the name of the command.
Note that, to override values, the attribute name must identical to the name on the @CommandHandler meta-annotation.
Lastly, the routingKey property is defined exactly as in the @CommandHandler annotation's specification to still
allow developers to choose to provide a Routing Key when using the JsonCommandHandler.
When writing custom logic to access properties of annotation that may be meta-annotated,
be sure to use the AnnotationUtils#findAnnotationAttributes(AnnotatedElement, String) method,
or the annotationAttributes on the MessageHandlingMember.
Using Java's annotation API will not take meta-annotations into consideration.