Sending JSON array to String function causes IllegalArgumentException

[olegz]

Reproduced with RabbitMQ

Function signature

public Function<String, List<Message<String>>> listFunction() {..}

The relevant stack trace is

Caused by: java.lang.IllegalArgumentException: SimpleMessageConverter only supports String, byte[] and Serializable payloads, received: reactor.core.publisher.FluxMapFuseable
	at org.springframework.amqp.support.converter.SimpleMessageConverter.createMessage(SimpleMessageConverter.java:143)
	at org.springframework.amqp.support.converter.AbstractMessageConverter.createMessage(AbstractMessageConverter.java:88)
	at org.springframework.amqp.support.converter.AbstractMessageConverter.toMessage(AbstractMessageConverter.java:70)
	at org.springframework.amqp.support.converter.AbstractMessageConverter.toMessage(AbstractMessageConverter.java:58)
	at org.springframework.cloud.stream.binder.rabbit.RabbitMessageChannelBinder$SimplePassthroughMessageConverter.createMessage(RabbitMessageChannelBinder.java:1109)
	at org.springframework.amqp.support.converter.AbstractMessageConverter.createMessage(AbstractMessageConverter.java:88)
	at org.springframework.amqp.support.converter.AbstractMessageConverter.toMessage(AbstractMessageConverter.java:70)
	at org.springframework.amqp.support.converter.AbstractMessageConverter.toMessage(AbstractMessageConverter.java:58)

[olegz]

Actually, this one requires a bit more explanation. It is still a bug, yet it is also a feature.
First, if you set the content-type header to text/plain and input type of your function is String, all is good regardless of the input you're sending.
However, the way we determine if we should treat it as text or not is not compleat, since we "only" check for content type.
We should also check for input type which in this case is String and therefore we shoudl skip the attempt to convert it and send it "as is" as we do when input type is byte[].

[olegz]

I am going to reclassify this to the enhancement since the framework is behaving as expected (will explain below), but perhaps few extra log messages would be more helpful

When sending input like [{"id": "1"},{"id": "1"}] which represents a collection while function's input type represents a single value the framework will convert input to Flux and function will be invoked iteratively for each value in the collection represented by the Flux.
That is unless the caller sets the content-type of the message to text/plain at which point we will not be fluxinfying input