OpenAPITools/openapi-generator · error · IllegalArgumentException

useJackson3 is only supported for the jvm-okhttp4 and jvm-sp

Error message

useJackson3 is only supported for the jvm-okhttp4 and jvm-spring-restclient libraries at this time.

What it means

The useJackson3 codegen paths only exist for two libraries: jvm-okhttp4 and jvm-spring-restclient. The check compares getLibrary() against exactly those two constants; with any other --library value (jvm-ktor, jvm-okhttp/jvm-okhttp3, jvm-retrofit2, jvm-volley, jvm-vertx, multiplatform, and notably jvm-spring-webclient) it throws IllegalArgumentException.

Source

Thrown at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinClientCodegen.java:501

        }

        if (additionalProperties.containsKey(CodegenConstants.SERIALIZATION_LIBRARY)) {
            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));
        }

View on GitHub (pinned to fcec517be3)

Solutions

  1. Switch `--library` to `jvm-okhttp4` or `jvm-spring-restclient` if you need Jackson 3.
  2. Or drop `useJackson3` and stay on Jackson 2 for the current library.
  3. Use the exact strings 'jvm-okhttp4' and 'jvm-spring-restclient' — close spellings silently pick a different library or fail.

Example fix

# before
openapi-generator-cli generate -g kotlin --library=jvm-spring-webclient -i api.yaml -p useJackson3=true
# after
openapi-generator-cli generate -g kotlin --library=jvm-spring-restclient -i api.yaml -p useJackson3=true
Defensive patterns

Strategy: validation

Validate before calling

# shell: gate useJackson3 on the two supported libraries
if [ "$USE_JACKSON_3" = "true" ]; then
  case "$LIBRARY" in
    jvm-okhttp4|jvm-spring-restclient) ;;
    *) echo "useJackson3 only supports jvm-okhttp4 / jvm-spring-restclient, got library: $LIBRARY" >&2; exit 2 ;;
  esac
fi

Type guard

// Kotlin
val JACKSON3_LIBRARIES = setOf("jvm-okhttp4", "jvm-spring-restclient")
fun supportsJackson3(library: String?): Boolean = library in JACKSON3_LIBRARIES

Try / catch

try {
    DefaultGenerator().opts(clientOptInput).generate()
} catch (e: IllegalArgumentException) {
    throw IllegalStateException("useJackson3 unsupported for the selected library: ${e.message}", e)
}

Prevention

When it happens

Trigger: `-g kotlin --library=jvm-ktor -p useJackson3=true`; `--library=jvm-spring-webclient -p useJackson3=true` (webclient is NOT in the allowlist — only restclient); `--library=jvm-retrofit2 -p useJackson3=true`.

Common situations: Assuming 'jvm-spring-webclient' counts as a Spring library for Jackson 3; retrofit2 users wanting to modernize to Jackson 3; option presets shared across different --library values.

Related errors

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/38369a7fa15be6f8. Report an issue: GitHub.