I have an integration flow like this, using Spring Integration 4.3.x:
<int:gateway> => <int:transformer> => <int-http:outbound-gateway> => <int:transformer> => return to the gateway
The gateway is defined by an interface with multiple methods, some of which have a return type, some others don't. Indeed, I'm calling a REST API through the HTTP outbound gateway and some methods are returning something meaningful, others nothing (i.e.: an empty response + a status code that I just use to detect errors).
As soon as I call any gateway method with a return type, all works fine. When I call one with void return type, I get this error:
org.springframework.messaging.core.DestinationResolutionException: no output-channel or replyChannel header available
I searched a lot, read SI documentation and debugged and I ended up to this: org.springframework.integration.gateway.GatewayProxyFactoryBean.invokeGatewayMethod(MethodInvocation, boolean), which substantially does this: if a reply is expected, do a "sendAndReceive", otherwise do a simple "send". The former sets a replyChannel header, the second does not.
This makes sense, but... why not simply automatically setting a NullChannel replyChannel header in the latter case? What I don't fully understand is whether this behaviour (i.e.: getting an exception whenever the flow produces a response that can't be translated into a gateway response) is the desired one, because I can't find any mention of it in the documentation. The docs seem to suggest that any reply is simply ignored, but it is not the case and I now need to put a transformer in the chain that sets a NullChannel replyChannel header whenever I detect that no actual response is going to be returned... indeed, something that I would have expected the gateway to do for me. Indeed, it seems like void methods in gateways can work "out-of-the-box" only when you have an adapter as the final flow endpoint.
Perhaps I'm missing something, so this is the reason of this question.
That's an interesting suggestion (NullChannel in replyChannel) header.
We'd have to think about it as to whether there might be some unintended consequences.
In the meantime, you should be able to use
@Gateway(headers = @GatewayHeader(name = MessageHeaders.REPLY_CHANNEL,
expression = "@nullChannel"))
void foo(String bar);