Event listeners are the components that act on incoming events. They typically execute logic based on decisions that have been made by the command model. Usually, this involves updating view models or forwarding updates to other components, such as 3rd party integrations. In some cases Event Handlers will throw Events themselves based on (patterns of) Events that they received, or even send Commands to trigger further changes.

Defining Event Handlers

In Axon, an object may declare a number of Event Handler methods, by annotating them with @EventHandler. The declared parameters of the method define which events it will receive.

Axon provides out-of-the-box support for the following parameter types:

• The first parameter is always the payload of the Event Message. In the case the Event Handlers doesn't need access to the payload of the message, you can specify the expected payload type on the @EventHandler annotation. When specified, the first parameter is resolved using the rules specified below. Do not configure the payload type on the annotation if you want the payload to be passed as a parameter.

• Parameters annotated with @MetaDataValue will resolve to the Meta Data value with the key as indicated on the annotation. If required is false (default), null is passed when the meta data value is not present. If required is true, the resolver will not match and prevent the method from being invoked when the meta data value is not present.

• Parameters of type MetaData will have the entire MetaData of an EventMessage injected.

• Parameters annotated with @Timestamp and of type java.time.Instant (or java.time.temporal.Temporal) will resolve to the timestamp of the EventMessage. This is the time at which the Event was generated.

• Parameters annotated with @SequenceNumber and of type java.lang.Long or long will resolve to the sequenceNumber of a DomainEventMessage. This provides the order in which the Event was generated (within the scope of the Aggregate it originated from).

• Parameters assignable to Message will have the entire EventMessage injected (if the message is assignable to that parameter). If the first parameter is of type message, it effectively matches an Event of any type, even if generic parameters would suggest otherwise. Due to type erasure, Axon cannot detect what parameter is expected. In such case, it is best to declare a parameter of the payload type, followed by a parameter of type Message.

• When using Spring and the Axon Configuration is activated (either by including the Axon Spring Boot Starter module, or by specifying @EnableAxon on your @Configuration file), any other parameters will resolve to autowired beans, if exactly one injectable candidate is available in the application context. This allows you to inject resources directly into @EventHandler annotated methods.

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

In all circumstances, at most one event handler method is invoked per listener instance. Axon will search for the most specific method to invoke, using following rules:

1. On the actual instance level of the class hierarchy (as returned by this.getClass()), all annotated methods are evaluated

2. If one or more methods are found of which all parameters can be resolved to a value, the method with the most specific type is chosen and invoked

3. If no methods are found on this level of the class hierarchy, the super type is evaluated the same way

4. When the top level of the hierarchy is reached, and no suitable event handler is found, the event is ignored.

// assume EventB extends EventA 
// and    EventC extends EventB
// and    a single instance of SubListener is registered
​
public class TopListener {
​
    @EventHandler
    public void handle(EventA event) {
    }
​
    @EventHandler
    public void handle(EventC event) {
    }
}
​
public class SubListener extends TopListener {
​
    @EventHandler
    public void handle(EventB event) {
    }
}

In the example above, the handler methods of SubListener will be invoked for all instances of EventB as well as EventC (as it extends EventB). In other words, the handler methods of TopListener will not receive any invocations for EventC at all. Since EventA is not assignable to EventB (it's its superclass), those will be processed by the handler method in TopListener.

Registering Event Handlers

Event Handling components are defined using an EventHandlingConfiguration class, which is registered as a module with the global Axon Configurer. Typically, an application will have a single EventHandlingConfiguration defined, but larger more modular applications may choose to define one per module.

To register objects with @EventHandler methods, use the registerEventHandler method on the EventHandlingConfiguration:

// define an EventHandlingConfiguration
EventHandlingConfiguration ehConfiguration = new EventHandlingConfiguration()
    .registerEventHandler(conf -> new MyEventHandlerClass());
​
// the module needs to be registered with the Axon Configuration
Configurer axonConfigurer = DefaultConfigurer.defaultConfiguration()
    .registerModule(ehConfiguration);

@Component
public class MyEventHandlerClass {
    // contains @EventHandler(s)
}

See Event Handling Configuration for details on registering event handlers using Spring AutoConfiguration.