How to get reactive streams in Micronaut?

In Spring Boot, for Webflux projects, we request a stream of data by sending a header - "Accept: application/stream+json" in the HTTP Request.

If we send, "Accept: application/json", we get get a valid Json.

In Micronaut, however, if I send "Accept: application/stream+json", it throws an error.

{
    "message": "Specified Accept Types [application/x-json-stream] not supported. Supported types: [application/json]",
    "_links": {
        "self": {
            "href": "/carts/reactive",
            "templated": false
        }
    }
}

What is the equivalent of "Accept: application/stream+json" in Micronaut?

What is the equivalent of "Accept: application/stream+json" in Micronaut?

As already mentioned in the comments, it's application/x-json-stream. (Sadly there's no one established standard for the content type for streaming json, at least not yet.)

The question here is to how the client can control the response type - Json/stream. You are using produces = {MediaType.APPLICATION_JSON_STREAM, which means the return type is always stream. In spring boot, we can use Accept header to control what response type we want. I was expecting the same behaviour from Micronaut.

Micronaut can do that too - you can pass more than one value to the produces parameter, and it will return either streamed or standard JSON accordingly:

@Get(value = "/", produces = {MediaType.APPLICATION_JSON_STREAM, MediaType.APPLICATION_JSON})
public Flux<Foo> getFoos() {
    return Flux.range(1, 3).map(i -> new Foo("Number " + i));
}

@Value
class Foo {
    private String content;
}

Then we can query the endpoint with either application/x-json-stream to retrieve a JSON stream:

 > curl localhost:8080 -H "Accept: application/x-json-stream"
{"content":"Number 1"}{"content":"Number 2"}{"content":"Number 3"}

...or plain application/json to retrieve a standard JSON array:

 > curl localhost:8080 -H "Accept: application/json"
[{"content":"Number 1"},{"content":"Number 2"},{"content":"Number 3"}]

If you want more custom control over the behaviour for each type of accept header, then you can just define separate controller methods entirely:

@Get(value = "/", produces = MediaType.APPLICATION_JSON_STREAM)
public Flux<Foo> getFoosStream() {
    return Flux.range(1, 3).map(i -> new Foo("Reactive " + i));
}

@Get(value = "/", produces = MediaType.APPLICATION_JSON)
public List<Foo> getFoosStandard() {
    return List.of(new Foo("Standard 1"), new Foo("Standard 2"), new Foo("Standard 3"));
}

...and depending on the header you send, a different method will be executed (as above you'll be able to see the difference with a standard curl command.)