Axon Framework provides extensive support for Spring, but does not require you to use Spring in order to use Axon. All components can be configured programmatically and do not require Spring on the classpath. However, if you do use Spring, much of the configuration is made easier with the use of Spring's annotation support. Axon provides Spring Boot starters on the top of that, so you can benefit from auto-configuration as well.

Auto-configuration

Axon's Spring Boot auto-configuration is by far the easiest option to get started configuring your Axon components. By simply declaring dependency to axon-spring-boot-starter, Axon will automatically configure the infrastructure components (command bus, event bus, query bus), as well as any component required to run and store aggregates and sagas.

JPA & Persistence Contexts

With Spring Data-JPA, a JPA Persistence Context is automatically configured. Axon's Spring Boot Autoconfiguration module will make sure Axon's JPA Entities are automatically registered with this default context.

However, when you explicitly include certain packages, for example using an @EntityScan annotation, this autoconfiguration will not happen anymore. If you then wish to use JPA based components from Axon, you will need to make sure the right Entities are registered.

To register Axon's JPA Entities, include the relevant packages, as described below:

• org.axonframework.eventhandling.tokenstore contains the entities necessary for the TokenStore used by Event

  Processors.

• org.axonframework.modelling.saga.repository.jpa contains the entities necessary to persist Sagas

• org.axonframework.eventsourcing.eventstore.jpa contains the entities necessary for the JPA Event Storage engine,

  when using a relational database as the Event Store.

Demystifying Axon Spring Boot Starter

With a lot of things happening in the background, it sometimes becomes difficult to understand how an annotation or just including a dependency enables so many features.

axon-spring-boot-starter follows general Spring boot convention in structuring the starter. It depends on axon-spring-boot-autoconfigure which holds concrete implementation of Axon auto-configuration. When Axon Spring Boot application starts up, it looks for a file named spring.factories in the classpath. This file is located in the META-INF directory of axon-spring-boot-autoconfigure module:

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
    org.axonframework.springboot.autoconfig.MetricsAutoConfiguration,\
    org.axonframework.springboot.autoconfig.EventProcessingAutoConfiguration,\
    org.axonframework.springboot.autoconfig.AxonAutoConfiguration,\
    org.axonframework.springboot.autoconfig.JpaAutoConfiguration,\
    org.axonframework.springboot.autoconfig.JpaEventStoreAutoConfiguration,\
    org.axonframework.springboot.autoconfig.JdbcAutoConfiguration,\
    org.axonframework.springboot.autoconfig.TransactionAutoConfiguration,\
    org.axonframework.springboot.autoconfig.NoOpTransactionAutoConfiguration,\
    org.axonframework.springboot.autoconfig.InfraConfiguration,\
    org.axonframework.springboot.autoconfig.ObjectMapperAutoConfiguration,\
    org.axonframework.springboot.autoconfig.AxonServerAutoConfiguration

This file maps different configuration classes which Axon Spring boot application will try to apply. So, as per this snippet, Spring Boot will try to apply all the configuration classes for AxonServerAutoConfiguration, AxonAutoConfiguration, ...

Whether these configuration classes will be applied or not, it will depend on conditions defined on this classes:

• AxonServerAutoConfiguration configures Axon Server as implementation for the Command Bus, Query Bus and Event Store. It will be applied before AxonAutoConfiguration, and it will be applied only if the org.axonframework.axonserver.connector.AxonServerConfiguration class is available in the classpath. Axon Server auto configuration can be disabled by setting the axon.axonserver.enabled property to false in the .properties/.yml file.

• AxonAutoConfiguration configures a 'non-axon-server' implementation of Command Bus, Query Bus, Event Store/Event Bus and other Axon components. These components will be initialized only if they are not in the Spring Application context already, eg. @ConditionalOnMissingBean(EventBus.class). As AxonAutoConfiguration will be applied after AxonServerAutoConfiguration these Axon components will be in the Spring Application Context already, and therefore Axon Server's implementation of Command Bus, Query Bus and Event Store/Event Bus will win.

Axon Spring Boot auto-configuration is not intrusive. It will define only Spring components that you haven't already explicitly defined in the application context. This allow you to completely override the auto-configured beans by defining your own in one of the @Configuration classes.

Specific Axon (Spring) component configurations will be explained in detail in the following sections of this guide.