Advanced customizations

Parameter Resolvers

You can configure additional ParameterResolvers by extending the ParameterResolverFactory class and creating a file named /META-INF/service/org.axonframework.common.annotation.ParameterResolverFactory containing the fully qualified name of the implementing class.

Caution

At this moment, OSGi support is limited to the fact that the required headers are mentioned in the manifest file. The automatic detection of ParameterResolverFactory instances works in OSGi, but due to classloader limitations, it might be necessary to copy the contents of the /META-INF/service/org.axonframework.common.annotation.ParameterResolverFactory file to the OSGi bundle containing the classes to resolve parameters for (i.e. the event handler).

Serializers

Serializers come in several flavors in the Axon Framework and are used for a variety of subjects. Currently you can choose between the XStreamSerializer, JacksonSerializer and JavaSerializer to serialize the messages (commands/queries/events), tokens, snapshots and sagas in an Axon application.

As there are several objects to be serialized, it is typically desired to tune which serializer handles which. To that end, the Configuration API allows you to define a default, message and event serializer, which lead to the following object-serialization break down:

1. The Event Serializer is in charge of de-/serializing Event messages. Events are typically stored in an Event Store for a long period of time. This is the main driver for choosing the Event Serializer implementation.

2. The Message Serializer is in charge of de-/serializing the Command and Query messages (used in a distributed application set up). Messages are shared between nodes and typically need to be interoperable and/or compact. Take this into account when choosing the Message Serializer.

3. The Default Serializer is in charge of de-/serializing the remainder, being the Tokens, Snapshots and Sagas. These objects are generally not shared between different applications, and most of these classes aren't expected to have some of the getters and setters that are, for example, typically required by Jackson based serializers. A flexible, general purpose serializer like XStream is quite suited for this purpose.

By default all three Serializer flavors are set to use the XStreamSerializer, which internally uses XStream to serialize objects to an XML format. XML is a verbose format to serialize to, but XStream has the major benefit of being able to serialize virtually anything. This verbosity is typically fine when storing tokens, sagas or snapshots, but for messages (and specifically events) XML might prove to cost too much due to its serialized size. Thus for optimization reasons you can configure different serializers for your messages. Another very valid reason for customizing Serializers is to achieve interoperability between different (Axon) applications, where the receiving end potentially enforces a specific serialized format.

There is an implicit ordering between the configurable serializer. If no Event Serializer is configured, the Event de-/serialization will be performed by the Message Serializer. In turn, if no Message Serializer is configured, the Default Serializer will take that role.

See the following example on how to configure each serializer specifically, were we use XStreamSerializer as the default and JacksonSerializer for all our messages:

Axon Configuration API

public class SerializerConfiguration {

    public void serializerConfiguration(Configurer configurer) {
        // Per default we want the XStreamSerializer
        XStreamSerializer defaultSerializer = new XStreamSerializer();
        // But for all our messages we'd prefer the JacksonSerializer due to JSON its smaller format
        JacksonSerializer messageSerializer = new JacksonSerializer();

        configurer.configureSerializer(configuration -> defaultSerializer)
                  .configureMessageSerializer(configuration -> messageSerializer)
                  .configureEventSerializer(configuration -> messageSerializer);
    }
}

Spring Boot AutoConfiguration - Properties file

# Possible values for these keys are `default`, `xstream`, `java`, and `jackson`.
axon.serializer.general
axon.serializer.events
axon.serializer.messages

Spring Boot AutoConfiguration - YML file

# Possible values for these keys are `default`, `xstream`, `java`, and `jackson`.
axon:
    serializer:
        general: 
        events: 
        messages:

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.

Customizing Message Handler behavior

Overriding annotations is very useful to implement best practices that you have established in your team, providing defaults or restrictions of how annotations may be used. However, they can also be very useful when special behavior needs to be added to message handlers based on the presence of an annotation.

HandlerEnhancers

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 HandlerEnhancers to intercept and perform checks on groups of @CommandHandlers or @EventHandlers.

To create a HandlerEnhancer 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 HandlerEnhancer to run you'll need to create a META-INF/services/org.axonframework.messaging.annotation.HandlerEnhancerDefinition file containing the fully qualified class name of the handler enhancer you have created.

Example:

public class MethodCommandHandlerDefinition implements HandlerEnhancerDefinition { // 1.

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

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

        private final String commandName;

        private MethodCommandMessageHandlingMember(MessageHandlingMember<T> delegate,
                                                   Map<String, Object> annotationAttributes) {
            super(delegate);

            if ("".equals(annotationAttributes.get("commandName"))) {
                commandName = delegate.payloadType().getName();
            } else {
                commandName = (String) annotationAttributes.get("commandName");
            }
        }

        @Override
        public boolean canHandle(Message<?> message) {
            return super.canHandle(message) && commandName.equals(((CommandMessage) message).getCommandName()); // 4.
        }

        public String commandName() {
            return commandName;
        }
    }
}

1. Implement the HandlerEnhancerDefinition interface

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

3. Sort out the types of messages you want to handle, for example any CommandHandlers or your own custom annotation even.

4. Handle the method inside of a MessageHandlingMember

5. If you would like to skip handling just return the original that was passed into the wrapHandler method.

To skip all handling of the handler then just throw an exception.

Filtering Event Storage Engine

In some scenarios you might want to chose which events to store. For this you can use the FilteringEventStorageEngine. One imaginable use case could be that we don't want to store non-domain events. FilteringEventStorageEngine uses a Predicate<? super EventMessage<?>> in order to filter events which get stored in the delegated engine. Let's try to configure a FilteringEventStorageEngine with the Configurer (if you are using Spring, it's enough to have a bean of type EventStorageEngine in your Application Context). The next example will only store domain events:

public class EventStorageEngineConfiguration {

    public void configureEventStorageEngine(Configurer configurer) {
        EventStorageEngine delegate = ...; // It does not matter for this example what storage engine you use

        configurer.configureEmbeddedEventStore(c -> new FilteringEventStorageEngine(delegate, em -> em instanceof DomainEventMessage));        
    }
}