Spring Cloud Stream - #StremListener condition - spring-boot

Question

According to documentation: https://cloud.spring.io/spring-cloud-static/spring-cloud-stream/3.0.3.RELEASE/reference/html/spring-cloud-stream.html#_using_streamlistener_for_content_based_routing
I can route the incoming message to a handler based on a condition like below:
#EnableBinding(MySink.class)
#EnableAutoConfiguration
public static class TestPojoWithAnnotatedArguments {
#StreamListener(target = MySink.INPUT, condition = "headers['type']=='bogey'")
public void receiveBogey(#Payload BogeyPojo bogeyPojo) {
// handle the message
}
#StreamListener(target = MySink.INPUT, condition = "headers['type']=='bacall'")
public void receiveBacall(#Payload BacallPojo bacallPojo) {
// handle the message
}
#StreamListener(target = MySink.ANOTHER_INPUT, condition = "headers['type']=='bacall'")
public void receiveBacall(#Payload BacallPojo bacallPojo) {
// handle the message
}
}
How do I provide a handler that's called when none of the conditions match?
If I have 2 handlers, first one with a condition and second one without any, both the handlers are called when the first one's condition matches. How do i avoid this?

Answer 1

We probably need to modify the section you're referring to as it is somewhat outdated.
Also, we can not (should not) do any kind of routing based on the payload type, since the data comes in from the wire in the serialised form such as byte[]. I discuss it in details in this old post.
But you can definitely use other parts of the incoming Message as routing condition. The recommended best practice is to rely on Message Headers.
So let's look at the sample:
#Bean
public Function<String, String> uppercase() {
return v -> v.toUpperCase();
}
#Bean
public Function<String, String> lowercase() {
return v -> v.toLowerCase();
}
#Bean
public Function<String, String> reverse() {
return v -> new StringBuilder(v).reverse().toString();
}
. . .and indeed a single routing-expression property. You only need one expression, since however complex or simple your condition is it can be encoded with standard Spring SpEL
--spring.cloud.function.routing-expression=headers['type'] == 'upper' ? 'uppercase' : (headers['type'] == 'lower' ? 'lowercase' : ''reverse)
What will happen is, the incoming Message's header with the name type will be evaluated. And if its value is 'upper' it will go to 'uppercase' function; if 'lower' to 'lowercase' function and default to 'reverse'.
Hope that helps.