Support Optional in PayloadMethodArgumentResolver

[artembilan]

Sometime the data for payload method argument is optional and we cover that with a @Payload(required = false).
It would be nice if this PayloadMethodArgumentResolver would treat a @Nullable payload same way as that required = false.

Another improvement is around an Optional: the isEmptyPayload(@Nullable Object payload) should add a check for Optional.isEmpty().
Technically we cannot transfer a null in messaging at the moment, so the Optional.empty() is good compromise for us.
However this one probably should have an effect only if method param type is not an Optional, so no checks no conversions necessary. I assume this is an existing behavior.
What we need is just an automatic Optional resolution to null if @Nullable or @Payload(required = false).

Thanks

[rstoyanchev]

I think you're suggesting to turn Optional into null if the method parameter is not Optional, and is not required. I think we could do that. Wouldn't we want the opposite too, i.e. wrap with Optional if the method parameter is Optional?

As for @Nullable @Payload payload this has to be decided more broadly for such annotated handler methods, including web where we also don't support that currently.

[artembilan]

Hi @rstoyanchev !

Right, I concur with all your arguments.
Plus we'd like to have that check for Optional.isEmpty() in the isEmptyPayload(). In Spring Integration we are going to implement a Null payload feature: some users are requesting to be able to produce nulls from their POJO methods as valid result for a reply message.
Yes, we can revise @Nullable payload in the separate issue.

[rstoyanchev]

I've scheduled it for the Optional support improvements but I'll discuss the @Nullable and provide and update or create another issue if necessary.