4.5
Getting Started
Axon Framework
C. Meta Annotations
Most annotations in Axon can be placed on other annotations as meta-annotations. 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 as JSON and you want the command name to be explicitly configured, you could create your own annotation:
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@Retention(RetentionPolicy.RUNTIME)
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@Target({ElementType.METHOD, ElementType.CONSTRUCTOR, ElementType.ANNOTATION_TYPE})
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@CommandHandler(payloadType = JsonNode.class)
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public @interface JsonCommandHandler {
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​
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String commandName;
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​
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String routingKey() default "";
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}
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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 annotations 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.Last modified 3mo ago
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