Handler Enhancers

Handler Enhancers allow you to wrap handlers and add custom logic to the execution, or eligibility of handlers for a certain message. This differs from HandlerInterceptors in that you have access to the aggregate member at the time of resolving, and it allows for more fine-grained control. You can use handler enhancers to intercept and perform checks on groups of @CommandHandlers or @EventHandlers.

To create a handler enhancer you start by implementing HandlerEnhancerDefinition and overriding the wrapHandler() method. All this method does is give you access to the MessageHandlingMember<T> which is an object representing any handler that is specified in the system.

You can then sort these handlers based on their annotations by using the annotationAttributes(Annotation annotation) method. This will filter out only those handlers that use that Annotation.

For your handler enhancer to run you'll need to either create a META-INF/services/org.axonframework.messaging.annotation.HandlerEnhancerDefinition file containing the fully qualified class name of the handler enhancer you have created, or register it explicitly in the Configurer.

Example of a Handler Enhancer that filters messages based on an expected Meta-Data key and value.

// 1
public class ExampleHandlerDefinition implements HandlerEnhancerDefinition { 

    @Override // 2
    public <T> MessageHandlingMember<T> wrapHandler(MessageHandlingMember<T> original) {
        return original.annotationAttributes(MyAnnotation.class) // 3
                .map(attr -> (MessageHandlingMember<T>) 
                             new ExampleMessageHandlingMember<>(original, attr))
                .orElse(original); // 5
    }

    private static class ExampleMessageHandlingMember<T> 
                             extends WrappedMessageHandlingMember<T>{

        private final String metaDataKey;
        private final String expectedValue;

        private ExampleMessageHandlingMember(
                             MessageHandlingMember<T> delegate,
                             Map<String, Object> annotationAttributes) {
            super(delegate);
            metaDataKey = (String) annotationAttributes.get("metaDataKey");
            expectedValue = (String) annotationAttributes.get("expectedValue");
        }

        @Override
        public boolean canHandle(Message<?> message) {
            return super.canHandle(message) && expectedValue.equals(message.getMetaData().get(metaDataKey)); // 4
        }
    }
}

1. Implement the HandlerEnhancerDefinition interface

2. Override the wrapHandler method to perform your own logic.

3. Sort out the types of handlers you want to wrap, for example any handlers with a MyAnnotation.

4. Handle the method inside of a MessageHandlingMember, in this case, indicating the handler is only suitable if the meta-data key matches a value.

5. If you are not interested in wrapping the handler, just return the original that was passed into the wrapHandler method.

It is possible to configure HandlerDefinition with Axon Configuration. If you are using Spring Boot defining HandlerDefintions and HandlerEnhancerDefinitions as beans is sufficient (Axon autoconfiguration will pick them up and configure within Axon Configuration).

Axon Configuration API

Configurer configurer = DefaultConfigurer.defaultConfiguration();
configurer.registerHandlerDefinition((c, clazz) ->
                                             MultiHandlerDefinition.ordered(
                                                     MultiHandlerEnhancerDefinition.ordered(
                                                             ClasspathHandlerEnhancerDefinition.forClass(clazz),
                                                             new MyCustomEnhancerDefinition()
                                                     ),
                                                     new MyCustomHandlerDefinition(),
                                                     ClasspathHandlerDefinition.forClass(clazz)
                                             ));

Spring Boot AutoConfiguration