In Axon, all aggregate roots must implement the AggregateRoot interface. This interface describes the basic operations needed by the Repository to store and publish the generated domain events. However, Axon Framework provides a number of abstract implementations that help you writing your own aggregates.

The AbstractAggregateRoot is a basic implementation that provides a registerEvent(DomainEvent) method that you can call in your business logic method to have an event added to the list of uncommitted events. The AbstractAggregateRoot will keep track of all uncommitted registered events and make sure they are forwarded to the event bus when the aggregate is saved to a repository.

The interface EventSourcedAggregateRoot defines an extra method, initializeState(), on top of the AggregateRoot interface. This method initializes an aggregate's state based on an event stream.

Implementations of this interface must always have a default no-arg constructor. Axon Framework uses this constructor to create an empty Aggregate instance before initialize it using past Events. Failure to provide this constructor will result in an Exception when loading the Aggregate from a Repository.

The AbstractEventSourcedAggregateRoot implements all methods on the EventSourcedAggregateRoot interface. It defines an abstract handle() method, which you need to implement with the actual logic to apply state changes based on domain events. When you extend the AbstractEventSourcedAggregateRoot, you can register new events using apply(). This method will register the event to be committed when the aggregate is saved, and will call the handle() method with the event as parameter. You need to implement this handle() method to apply the state changes represented by that event. Below is a sample implementation of an aggregate.

public class MyAggregateRoot extends AbstractEventSourcedAggregateRoot {

    private String aggregateIdentifier;
    private String someProperty;

    public MyAggregateRoot(String id) {
        apply(new MyAggregateCreatedEvent(id));
    }

    // constructor required for reconstruction
    protected MyAggregateRoot() {
    }

    public void handle(DomainEvent event) {
        if (MyAggregateCreatedEvent.class.isAssignableFrom(event.getPayloadType())) {
            // make sure to always initialize the aggregate identifier
            this.aggregateIdentifier = ((MyAggregateCreatedEvent) this.getPayload()).getMyIdentifier();
            // do something with someProperty
        }
        // and more if-else-if logic here
    }

    public Object getAggregateIdentifier() {
        return aggregateIdentifier;
    }
}                

As you see in the example above, the implementation of the handle() method can become quite verbose and hard to read. The AbstractAnnotatedAggregateRoot can help. The AbstractAnnotatedAggregateRoot is a specialization of the AbstractAggregateRoot that provides @EventHandler annotation support to your aggregate. Instead of a single handle() method, you can split the logic in separate methods, with names that you may define yourself. Just annotate the event handler methods with @EventHandler, and the AbstractAnnotatedAggregateRoot will invoke the right method for you.

public class MyAggregateRoot extends AbstractAnnotatedAggregateRoot {

    @AggregateIdentifier
    private String aggregateIdentifier;
    private String someProperty;

    public MyAggregateRoot(String id) {
        apply(new MyAggregateCreatedEvent(id));
    }

    // constructor needed for reconstruction
    protected MyAggregateRoot() {
    }

    @EventHandler
    private void handleMyAggregateCreatedEvent(MyAggregateCreatedEvent event) {
        // make sure identifier is always initialized properly
        this.aggregateIdentifier = event.getMyAggregateIdentifier();
        // do something with someProperty
    }
}                

@EventHandler annotated methods are resolved using specific rules. These rules are thorougly explained in Section 6.2.2, “Annotated Event Handler”.

The AbstractAnnotatedAggregateRoot also requires that the field containing the aggregate identifier is annotated with @AggregateIdentifier. If you use JPA and have JPA annotations on the aggregate, Axon can also use the @Id annotation provided by JPA.