CommandHandlerWithReplyBuilder "state" is null error even if using forNullState

[aklikic]

Reproducable:

public class TestEntity extends EventSourcedBehaviorWithEnforcedReplies<TestEntity.Command, TestEntity.Event, TestEntity.State> {

    public interface Command extends CborSerializable {}
    record CreateCommand(ActorRef<Response> replyTo) implements Command {}
    record StartCommand(ActorRef<Response> replyTo) implements Command {}
    record Response() implements CborSerializable {}

    public interface Event extends CborSerializable{}
    record CreatedEvent(Instant timestamp) implements Event {}
    record StartedEvent(Instant timestamp) implements Event {}

    public enum CurrentStatus {
        Created,
        Started
    }
    record State(CurrentStatus status) implements CborSerializable{
        public static State onCreated(CreatedEvent event){
            return new State(CurrentStatus.Created);
        }
        public State onStarted(StartedEvent event){
            return new State(CurrentStatus.Started);
        }
    }

    public TestEntity(String entityId) {
        super(persistenceId(entityId));
    }
    public static final PersistenceId persistenceId(String entityId) {
        return PersistenceId.of(ENTITY_TYPE_KEY.name(), entityId);
    }


    public static final EntityTypeKey<TestEntity.Command> ENTITY_TYPE_KEY =
            EntityTypeKey.create(TestEntity.Command.class, "TestEntity");
    public static Behavior<TestEntity.Command> create(String entityId) {
        return Behaviors.setup(context -> new TestEntity(entityId));
    }

    public static void initSharding(ActorSystem<?> system) {
        ClusterSharding.get(system).init(
                Entity.of(
                        ENTITY_TYPE_KEY,
                        entityContext -> TestEntity.create(entityContext.getEntityId())));
    }

    @Override
    public TestEntity.State emptyState() {
        return null;
    }

    @Override
    public CommandHandlerWithReply<TestEntity.Command, TestEntity.Event, TestEntity.State> commandHandler() {
        CommandHandlerWithReplyBuilder<TestEntity.Command, TestEntity.Event, TestEntity.State> b = newCommandHandlerWithReplyBuilder();
        b.forNullState().onCommand(TestEntity.CreateCommand.class, this::onCreate);
        b.forState(state -> state.status() == TestEntity.CurrentStatus.Created)
                .onCommand(TestEntity.StartCommand.class,this::onStart);
        b.forAnyState()
                .onAnyCommand(() -> Effect().unhandled().thenNoReply());
        return b.build();
    }

    private ReplyEffect<TestEntity.Event, TestEntity.State> onCreate(TestEntity.State state, TestEntity.CreateCommand cmd){
        return Effect().persist(new TestEntity.CreatedEvent(Instant.now()))
                .thenReply(cmd.replyTo,updatedState -> new TestEntity.Response());
    }

    private ReplyEffect<TestEntity.Event, TestEntity.State> onStart(TestEntity.State state, TestEntity.StartCommand cmd){
        return Effect().persist(new TestEntity.StartedEvent(Instant.now()))
                .thenReply(cmd.replyTo,updatedState -> new TestEntity.Response());
    }

    public EventHandler<TestEntity.State, TestEntity.Event> eventHandler() {
        EventHandlerBuilder<TestEntity.State, TestEntity.Event> b = newEventHandlerBuilder();
        b.forNullState()
                .onEvent(TestEntity.CreatedEvent.class, TestEntity.State::onCreated);
        b.forAnyState()
                .onEvent(TestEntity.StartedEvent.class, TestEntity.State::onStarted);
        return b.build();
    }
}

public class TestEntityTest {

    @Test
    public void happyPath(){
        String entityId = "1";
        UnpersistentBehavior<TestEntity.Command,TestEntity.Event,TestEntity.State> test =
                UnpersistentBehavior.fromEventSourced(TestEntity.create(entityId));

        BehaviorTestKit<TestEntity.Command> testkit = test.getBehaviorTestKit();
        ReplyInbox<TestEntity.Response> replyInbox = testkit.runAsk(replyTo -> new TestEntity.StartCommand(replyTo));

        replyInbox = testkit.runAsk(replyTo -> new TestEntity.StartCommand(replyTo));
        replyInbox.expectReply(new TestEntity.Response());
        test.getEventProbe().expectPersistedClass(TestEntity.StartedEvent.class);
        assertFalse(test.getSnapshotProbe().hasEffects());

    }
}

Error:

java.lang.NullPointerException: Cannot invoke "TestEntity$State.status()" because "state" is null
	at TestEntity.lambda$commandHandler$0(TestEntity.java:74)

[aklikic]

Akka version 2.8.4
Java 17

[johanandren]

Thanks, I think there are two things we need to investigate/discuss. Not 100% sure :

1. Should null ever be fed to the forState predicate?
2. Is null one of the possible forAnyState's or should it mean any non-null state?

I think a possible workaround with current behavior would be to end the forNullState set of handlers with an onAnyCommand to catch all other commands and return Effect().unhandled() to keep the commands from falling through to the other two state handlers.

[patriknw]

I would say that it works as designed. null has been chosen to be a valid state and can therefore be passed to the predicate. If there is no matching command handler in the forNullState() it would continue with next forState. It would be the same if you add several forState and the first didn't have a matching command handler, then it would continue with next forState.

Johan's suggestion of capturing all with onAnyCommand is good.

That said, documentation should clarify this.