Configuration

This page describes the process when it comes to configuring a Query Handlers. Note, that a Query Handler is a (singleton) object containing @QueryHandler annotated functions.

Registering a query handler

When you register a Query Handler, that means you are registering a class containing annotated query handlers. Upon receiving such a class during configuration, Axon will scan its contents for all @QueryHandler annotated methods. In the registration process the following information defines a given query handling function:

The first parameter of the method is the query payload.
The method’s response type is the query’s response type.
The value of the queryName field in the annotation as the query’s name (this is optional and in its absence will default to the query payload).

Note that it is possible to register multiple query handlers for the same query payload, response type and name. Furthermore, when dispatching a query the client can indicate whether it wants the result from a single handler or the result from all handlers corresponding to the query payload, name and response type combination.

The following snippets point out how a Query Handler can be registered:

Configuration API
Spring Boot

Given the existence of the following query handler:

public class CardSummaryProjection {

    @QueryHandler
    public CardSummary handle(FetchCardSummaryQuery query) {
        CardSummary cardSummary;
        // Retrieve CardSummary instance, for example from a repository.
        return cardSummary;
    }

}

The following is needed to register a CardSummaryProjection as being a Query Handler:

Configurer axonConfigurer = DefaultConfigurer.defaultConfiguration()
    .registerQueryHandler(conf -> new CardSummaryProjection());

Or, a more general approach to registering all types of message handlers in a component can be used:

Configurer axonConfigurer = DefaultConfigurer.defaultConfiguration()
    .registerMessageHandler(conf -> new CardSummaryProjection());

When using Spring Boot, simply specifying the query handler as a bean is sufficient:

@Component
public class CardSummaryProjection {

    @QueryHandler
    public CardSummary handle(FetchCardSummaryQuery query) {
        CardSummary cardSummary;
        // Retrieve CardSummary instance, for example from a repository.
        return cardSummary;
    }

}

Identical Query Handling methods in a single Query Handler

A query handler class can currently contain several identical query handling methods. The outcome of which method will actually be called is however unspecified.

Note that this should be regarded as a extremely uncommon scenario, as typically identical query handling methods would be spread over several query handlers.