Query handling components act on incoming query messages. They typically read data from the view models created by the Event listeners. Query handling components typically do not raise new Events or send Commands.
Defining Query Handlers
In Axon, an object may declare a number of Query Handler methods, by annotating them with @QueryHandler. The declared parameters of the method define which messages it will receive.
By default, @QueryHandler annotated methods allow the following parameter types:
The first parameter is the payload of the Query Message. It may also be of type Message or QueryMessage, if the @QueryHandler annotation explicitly defined the name of the Query the handler can process. By default, a Query name is the fully qualified class name of the Query's payload.
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 a QueryMessage injected.
Parameters of type UnitOfWork get the current Unit of Work injected. This allows query handlers to register actions to be performed at specific stages of the Unit of Work, or gain access to the resources registered with it.
Parameters of type Message, or QueryMessage will get the complete message, with both the payload and the Meta Data. This is useful if a method needs several meta data fields, or other properties of the wrapping Message.
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 query handler method is invoked per query handling instance. Axon will search for the most specific method to invoke, using following rules:
On the actual instance level of the class hierarchy (as returned by this.getClass()), all annotated methods are evaluated
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
If no methods are found on this level of the class hierarchy, the super type is evaluated the same way
When the top level of the hierarchy is reached, and no suitable query handler is found, this query handling instance is ignored.
Note that similar to command handling, and unlike event handling, query handling does not take the class hierarchy of the Query message into account.
// assume QueryB extends QueryA
// and QueryC extends QueryB
// and a single instance of SubHandler is registered
In the example above, the handler method of SubHandler will be invoked for queries for QueryB and result MyResult; the handler methods of TopHandler are invoked for queries for QueryA and QueryC and result MyResult.
Registering Query Handlers
It is possible to register multiple query handlers for the same query name and type of response. When dispatching queries, the client can indicate whether he wants a result from one or from all available query handlers.