Package org.axonframework.eventhandling

Annotation Interface EventHandler

@Documented @Retention(RUNTIME) @Target({METHOD,ANNOTATION_TYPE}) @MessageHandler(messageType=EventMessage.class) public @interface EventHandler
Annotation to be placed on methods that can handle events. The parameters of the annotated method are resolved using parameter resolvers.

Axon provides a number of parameter resolvers that allow you to use the following parameter types:

  • The first parameter is always the payload of the Event message
  • 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 Meta Data of an Event Message injected.
  • Parameters of type Instant annotated with @Timestamp will resolve to the timestamp of the EventMessage. This is the time at which the Event was generated.
  • 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 <axon:annotation-config/> is declared, any other parameters will resolve to autowired beans, if exactly one autowire candidate is available in the application context. This allows you to inject resources directly into @EventHandler annotated methods.

For each event, only a single method will be invoked per object instance with annotated methods. This method is resolved in the following order:

  1. First, the event handler methods of the actual class (at runtime) are searched
  2. If a method is found with a parameter that the domain event can be assigned to, it is marked as eligible
  3. After a class has been evaluated (but before any super class), the most specific event handler method is called. That means that if an event handler for a class A and one for a class B are eligible, and B is a subclass of A, then the method with a parameter of type B will be chosen
  4. If no method is found in the actual class, its super class is evaluated.
  5. If still no method is found, the event listener ignores the event

If you do not want any events to be ignored, but rather have some logging of the fact that an unhandled event came by, make an abstract superclass that contains an event handler method that accepts any Object.

Note: if there are two event handler methods accepting the same argument, the behavior is undefined.

Since:
0.1
Author:
Allard Buijze
See Also:

  • Optional Element Summary

    Optional Elements
    Modifier and Type
    Optional Element
    Description
    Class<?>
    payloadType
    The type of event this method handles.

  • Element Details

    • payloadType

      Class<?> payloadType
      The type of event this method handles. This handler will only be considered for invocation if the event message's payload is assignable to this type.

      Optional. If unspecified, the first parameter of the method defines the type of supported event.

      Returns:
      The type of the event this method handles.
      Default:
      java.lang.Object.class