External Command Handlers

Command handling functions are most often directly placed on the Aggregate (as described in more detail here). There are situations however where it is not possible nor desired to route a command directly to an Aggregate instance. Message handling functions, like Command Handlers, can however be placed on any object. It is thus possible to instantiate a 'Command Handling Object'.

A Command Handling Object is a simple (regular) object, which has @CommandHandler annotated methods. Unlike with Aggregates, there is only a single instance of a Command Handling Object, which handles all commands of the types it declares in its methods:

import org.axonframework.commandhandling.CommandHandler;
import org.axonframework.modelling.command.Repository;

public class GiftCardCommandHandler {

    // 1.
    private final Repository<GiftCard> giftCardRepository;

    public void handle(RedeemCardCommand cmd) {
        giftCardRepository.load(cmd.getCardId()) // 2.
                          .execute(giftCard -> giftCard.handle(cmd)); // 3.

    // omitted constructor

In the above snippet we have decided that the RedeemCardCommand should no longer be directly handled on the GiftCard. Instead, we load the GiftCard manually and execute the desired method on it:

  1. The Repository for the GiftCard Aggregate, used for retrieval and storage of an Aggregate.

    If @CommandHandler methods are placed directly on the Aggregate, Axon will automatically know to call the Repository to load a given instance.

    It is thus not mandatory to directly access the Repository, but a design choice.

  2. To load the intended GiftCard Aggregate instance, the Repository#load(String) method is used.

    The provided parameter should be the Aggregate identifier.

  3. After that Aggregate has been loaded, the Aggregate#execute(Consumer) function should be invoked to perform an operation on the Aggregate.

    Using the execute function ensure that the Aggregate life cycle is correctly started.

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