Handler Enhancers
Handler Enhancers allow you to wrap handlers and add custom logic to the execution or eligibility of handlers for a specific message. Handler enhancers differ from message handler interceptors by the access they provide to the message handling component (e.g., the aggregate member) at the resolution time. Hence, handler enhancers allow for more fine-grained control. You can use handler enhancers to intercept and perform checks on groups of @MessageHandler
annotated methods, like a command, event, or query handler.
To create a handler enhancer, you implement the HandlerEnhancerDefinition
interface and override the wrapHandler()
method. All this method does is give you access to the MessageHandlingMember<T>
, which is an object representing any handler specified in the system.
You can then filter these handlers based on the type of Message
they handle by using the MessageHandlingMember.canHandleMessageType(Class<? extends Message>)
method. Doing so, you can specifically enhance message handlers dealing with, for example, the CommandMessage
.
For your handler enhancer 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 or register the enhancer explicitly in the Configurer
. Here's an example of a HandlerEnhancerDefinition
that filters messages based on an expected MetaData
key and value.
Implement the
HandlerEnhancerDefinition
interfaceOverride the
wrapHandler
method to perform your logic.Filter the types of handlers you want to wrap based on a specific attribute, for example, the
metaDataKey
attribute from theMyAnnotation
.Handle the method inside of a
MessageHandlingMember
. In this case, indicating the handler is only suitable if the meta-data key matches a value.For annotation-specific attributes to exist in the
MessageHandlingMember's
attribute collection, meta-annotation the custom annotation withHasHandlerAttributes
.If you are not interested in wrapping the handler, return the original passed into the
wrapHandler
method.
To configure your HandlerEnhancerDefintion
, you can (1) register it directly with the Configurer
or (2) make it a part of the Application Context when you are in a Spring environment.
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