SpringDocUtils logs Json Processing Exception occurred warning for every @Min-constrained Integer query parameter (regression since 3.1.0)

[fkreisEnbw]

Describe the bug

Starting with 3.1.0, any endpoint with an Integer/int query parameter carrying a Bean Validation constraint (e.g. @Min) causes a WARN log on every api-docs request:

Json Processing Exception occurred: Cannot construct instance of java.util.HashSet (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('integer')
at [Source: ...]; line: 1, column: 9] (through reference chain: io.swagger.v3.oas.models.media.JsonSchema["type"])

It also occurs with array-typed schemas (ArraySchema["type"], value 'array'). On a real API with many endpoints this produces dozens of these warnings per api-docs request.

To Reproduce

• Spring Boot 4.1.0
• springdoc-openapi-starter-webmvc-ui 3.1.0

I wrote a minimal Spring Boot app and a test that calls GET /v3/api-docs and asserts no SpringDocUtils WARN is logged. I ran it myself against both versions to confirm the regression:

@RestController
class HelloController {
    @GetMapping("/hello")
    String hello(@RequestParam(required = false) @Min(1) Integer limit) {
        return "hello";
    }
}

@SpringBootTest
@AutoConfigureMockMvc
class ApiDocsWarningTest {

    @Autowired
    MockMvc mockMvc;

    ListAppender<ILoggingEvent> appender;

    @BeforeEach
    void attachAppender() {
        appender = new ListAppender<>();
        appender.start();
        ((Logger) LoggerFactory.getLogger("org.springdoc.core.utils.SpringDocUtils"))
                .addAppender(appender);
    }

    @Test
    void apiDocsRequestLogsNoJsonProcessingWarning() throws Exception {
        mockMvc.perform(get("/v3/api-docs"))
                .andExpect(status().isOk());

        List<String> warnMessages = appender.list.stream()
                .filter(e -> e.getLevel() == Level.WARN)
                .map(ILoggingEvent::getFormattedMessage)
                .toList();

        assertThat(warnMessages)
                .noneMatch(m -> m.contains("Json Processing Exception occurred"));
    }
}

• With springdoc-openapi-starter-webmvc-ui:3.0.3 → test passes, no warning.
• With springdoc-openapi-starter-webmvc-ui:3.1.0, everything else unchanged → test fails, SpringDocUtils logs the Json Processing Exception occurred warning shown above.

Expected behavior

No warning should be logged when generating the OpenAPI docs for this endpoint.

Additional context

• Response is still 200 OK with an apparently valid spec, so functionally nothing is visibly broken — it's just log noise.
• Bisected against every version between 3.0.1 and 3.1.0 published on Maven Central (no releases exist between 3.0.3 and 3.1.0, it's a direct jump). 3.0.1, 3.0.2, 3.0.3 are all clean; 3.1.0 is the first version to show this.

[Mattias-Sehlstedt]

Edit: I found the issue with my test, it is specifically tied to resolving against OAS 3.1. After changing to that the issue occurs. Seems to be tied to how the type is serialized and later deserialized for swagger-core's Schema. Since 3.1 allows the type to be an array, but it is usually adjusted to a scalar type if there is only one item in the type array.

---

I have created what should be a replica of your test scenario here, but it does not fail when calling SpringDocUtils#cloneViaJson (that is the only method that logs, so I assume the WARN that you are raising stems from that). When turning on logging for tests too the test itself does not fail.

Could you provide additional details for how to reproduce this within the repo itself? It might be that the issue is on your end with regards to version incompatibility or mixins.

[Mattias-Sehlstedt]

The issue stems from the logic introduced here, and that a JsonSchema (swagger-core's OAS 3.1 representation) fails when it is cloned. Further investigation is needed to determine whether the issue is within springdoc or if the fix might be more suited for swagger-core.