SPR-16166: Handle top level json value in Jackson2JsonTokenizer

[madgnome]

This commit introduces support for top level Json value
String, true, false, null, number) to Jackson2JsonTokenizer.

Before this change Jackson2JsonTokenizer would only consider token in an object or array and this scenario will fail:

@RestController
public class JsonValueResource {
    @GetMapping("/fail")
    public Mono<Boolean> fail() {
        return WebClient.builder().baseUrl("http://localhost:8000")
                .build().get().uri("/")
                .accept(MediaType.APPLICATION_JSON)
                .retrieve()
                .bodyToMono(Boolean.class)
                .switchIfEmpty(Mono.error(new RuntimeException("No response")));
    }

    @GetMapping
    Mono<Boolean> jsonValueBoolean() {
        return Mono.just(true);
    }

}

Issue: SPR-16166

[madgnome]

Codacy/PMD complains about missing asserts in the new tests but I followed the existing pattern in the test. is there a way to ignore?

[poutsma]

Thank you for providing a PR for this issue! However, I decided to fix it in an alternative, arguably cleaner way. See 8e253a3

[madgnome]

Thanks a lot for fixing it @poutsma, your solution is cleaner indeed!

Could you double check that this use case is working?

testTokenizeAsJsonToken(
				singletonList("true"),
 				singletonList(JsonToken.VALUE_TRUE));

[poutsma]

Unfortunately, the JSON assert framework we use does not support null, true, or false as top-level elements (see this issue). I did test those values in a test project, though, and they worked for me.