C. Meta Annotations

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.