Why does Micronaut return invalid, plain JSON strings?

[saw303]

When returning a plain String within a Micronaut controller such as

@Get ("/")
public HttpResponse<?> hello() {
    return HttpResponse.notFound("Hello");
}

the controller returns

HTTP/1.1 404 Not Found
date: Mon, 26 Aug 2024 17:40:05 GMT
content-type: application/json
content-length: 5

Hello

or with this one

@Get("/hello")
public String hello2() {
    return "Hello";
}

the controller returns

HTTP/1.1 200 OK
date: Mon, 26 Aug 2024 17:40:55 GMT
content-type: application/json
content-length: 5

Hello

Syntactically Hello is not a valid JSON string, but "Hello" is. Why is Micronaut not returning a proper JSON string value with this implementation, but declares the response to be application/json? Is this a bug or am I missing something?

If I use an ObjectMapper to serialise Hello into a JSON string, everything works as expected and a proper JSON string is returned to the client.

@Get("hello3")
public String hello3() throws IOException {
    return objectMapper.writeValueAsString("Hello");
}

results in

HTTP/1.1 200 OK
date: Mon, 26 Aug 2024 17:44:06 GMT
content-type: application/json
content-length: 7

"Hello"

[dstepanov]

The string is meant to be a raw type representing already encoded JSON

[saw303]

I understand. I stumpled over it, because it the first time we return a plain string there. When providing an object instance, the proper JSON serialization kicks in.

@Get("hello4")
public HttpResponse<Hello> hello4() throws IOException {
    return HttpResponse.notFound(new Hello("Hello"));
}

@Serdeable
public record Hello(String name) {}

This results in

{
  "name": "Hello"
}

Is this inconsistent behaviour intentional?

[yawkat]

Yes. Returning string directly is considered a "raw" response, while other types are serialized.

[saw303]

Ok. Thank you for that clarification.