Spring WebFlux fails to decode json array into String collection

Spring Boot: 3.5.6 Spring Framework: 6.2.11

When attempting to decode a JSON payload like this

["hello", "world"]

into a String collection using WebFlux' default Codec configuration, the payload is decoded into a single String with value ["hello","world"].

Reproducer:

@RestController
@SpringBootApplication
public class Demo3Application {

    public static void main(String[] args) {
        SpringApplication.run(Demo3Application.class, args);
    }

    @GetMapping(path = "/not-empty", produces = MediaType.APPLICATION_JSON_VALUE)
    List<String> strings() {
        return List.of("hello", "world");
    }

    @GetMapping(path = "/empty", produces = MediaType.APPLICATION_JSON_VALUE)
    List<String> empty() {
        return List.of();
    }

}

Test:

@WebFluxTest(Demo3Application.class)
class Demo3ApplicationTests {

    @Autowired
    private WebTestClient c;

    @Test
    void empty() {
        c.get()
                .uri("/empty")
                .exchange()
                .expectStatus().isOk()
                .expectBody().json("[]"); // success

        c.get()
                .uri("/empty")
                .exchange()
                .expectStatus().isOk()
                .expectBodyList(String.class).hasSize(0); // failure, list actually contains a string with value "[]"
    }

    @Test
    void notEmpty() {
        c.get()
                .uri("/not-empty")
                .exchange()
                .expectStatus().isOk()
                .expectBody().json("""
                        ["hello", "world"]"""); // success

        c.get()
                .uri("/not-empty")
                .exchange()
                .expectStatus().isOk()
                .expectBodyList(String.class)
                .hasSize(2) // failure, actual size is 1
                .contains("hello", "world"); // failure, list actually contains a single string with value "[\"hello\",\"world\"]"
    }

}

While debugging this I found that StringDecoder is used to parse the response instead of Jackson2JsonDecoder. This is unexpected considering the application/json content type of the response and my requested target type...

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A similar issue happens when encoding a Flux<String> to application/json. Instead of a JSON array, the response is a plain text concatenation of the individual strings emitted by the Flux...

Comment From: SuganthiThomas

Hi, I’m interested in working on this issue. I have experience with Java and Spring Boot. Could I take this up?