OpenAPITools/openapi-generator · error · IllegalArgumentException

useJackson3 with jvm-spring-restclient requires useSpringBoo

Error message

useJackson3 with jvm-spring-restclient requires useSpringBoot4=true.

What it means

Even on jvm-spring-restclient, Jackson 3 support is gated on Spring Boot 4: the tools.jackson artifacts line up with Boot 4's dependency set, so the guard requires the separate boolean useSpringBoot4 to be true. useSpringBoot3=true does NOT satisfy it — the check reads the useSpringBoot4 field directly, and useJackson3 does not imply it.

Source

Thrown at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinClientCodegen.java:504

            setSerializationLibrary((String) additionalProperties.get(CodegenConstants.SERIALIZATION_LIBRARY));
            additionalProperties.put(this.serializationLibrary.name(), true);
        } else {
            additionalProperties.put(this.serializationLibrary.name(), true);
        }

        if (isUseJackson3()) {
            if (this.serializationLibrary != SERIALIZATION_LIBRARY_TYPE.jackson) {
                throw new IllegalArgumentException("useJackson3 requires serializationLibrary=jackson");
            }
            if (additionalProperties.containsKey("openApiNullable")
                    && Boolean.parseBoolean(additionalProperties.get("openApiNullable").toString())) {
                throw new IllegalArgumentException("openApiNullable cannot be set with useJackson3");
            }
            if (!JVM_OKHTTP4.equals(getLibrary()) && !JVM_SPRING_RESTCLIENT.equals(getLibrary())) {
                throw new IllegalArgumentException("useJackson3 is only supported for the jvm-okhttp4 and jvm-spring-restclient libraries at this time.");
            }
            if (JVM_SPRING_RESTCLIENT.equals(getLibrary()) && !useSpringBoot4) {
                throw new IllegalArgumentException("useJackson3 with jvm-spring-restclient requires useSpringBoot4=true.");
            }
        }

        if (additionalProperties.containsKey(MAP_FILE_BINARY_TO_BYTE_ARRAY)) {
            setMapFileBinaryToByteArray(convertPropertyToBooleanAndWriteBack(MAP_FILE_BINARY_TO_BYTE_ARRAY));
        }
        additionalProperties.put(MAP_FILE_BINARY_TO_BYTE_ARRAY, mapFileBinaryToByteArray);
        if (mapFileBinaryToByteArray) {
            typeMapping.put("file", "kotlin.ByteArray");
            typeMapping.put("binary", "kotlin.ByteArray");
        }

        if (additionalProperties.containsKey(GENERATE_ONEOF_ANYOF_WRAPPERS)) {
            setGenerateOneOfAnyOfWrappers(convertPropertyToBooleanAndWriteBack(GENERATE_ONEOF_ANYOF_WRAPPERS));
        }

        if (additionalProperties.containsKey(FAIL_ON_UNKNOWN_PROPERTIES)) {
            setFailOnUnknownProperties(convertPropertyToBooleanAndWriteBack(FAIL_ON_UNKNOWN_PROPERTIES));

View on GitHub (pinned to fcec517be3)

Solutions

Add `useSpringBoot4=true` alongside useJackson3 on jvm-spring-restclient.
If the project is still on Boot 3, drop useJackson3 and stay on Jackson 2 — the combination is unsupported by design.

Example fix

# before
openapi-generator-cli generate -g kotlin --library=jvm-spring-restclient -i api.yaml -p useJackson3=true,useSpringBoot3=true
# after
openapi-generator-cli generate -g kotlin --library=jvm-spring-restclient -i api.yaml -p useJackson3=true,useSpringBoot4=true

Defensive patterns

Strategy: validation

Validate before calling

# shell: useJackson3 on spring-restclient needs Boot 4 explicitly
if [ "$USE_JACKSON_3" = "true" ] && [ "$LIBRARY" = "jvm-spring-restclient" ] && [ "$USE_SPRING_BOOT4" != "true" ]; then
  echo "useJackson3 with jvm-spring-restclient requires useSpringBoot4=true" >&2; exit 2
fi

Try / catch

try {
    DefaultGenerator().opts(clientOptInput).generate()
} catch (e: IllegalArgumentException) {
    throw IllegalStateException("useJackson3 needs useSpringBoot4=true on jvm-spring-restclient: ${e.message}", e)
}

Prevention

Pass useSpringBoot4=true (not useSpringBoot3) when enabling useJackson3.
Do not assume flags imply each other — boot version and Jackson version are independent booleans.
During Boot 3→4 migrations, update generator flags in the same commit.

When it happens

Trigger: `--library=jvm-spring-restclient -p useJackson3=true,useSpringBoot3=true` (Boot 3 only, no boot4 flag) → throws; also `useSpringBoot4` omitted entirely while useJackson3=true.

Common situations: Projects mid-migration that enable Boot 3 flags plus Jackson 3; assuming the newest serialization flag pulls in the newest Boot flag automatically.

Related errors

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/8ec670d767221fc5. Report an issue: GitHub.