OpenAPITools/openapi-generator · error · RuntimeException

This library currently only supports jackson serialization.

Error message

This library currently only supports jackson serialization. Try adding '--additional-properties serializationLibrary=jackson' to your command.

What it means

processJvmSpring — shared by the jvm-spring-webclient and jvm-spring-restclient libraries (KotlinClientCodegen.java:886, 896) — hard-requires serializationLibrary=jackson because the Spring stack templates are Jackson-based. Since the default serializationLibrary is moshi, selecting either Spring library without the explicit flag throws this RuntimeException.

Source

Thrown at modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/KotlinClientCodegen.java:875

        additionalProperties.put(JVM, true);
        additionalProperties.put(JVM_OKHTTP, true);

        if (JVM_OKHTTP4.equals(getLibrary())) {
            additionalProperties.put(JVM_OKHTTP4, true);
        }

        supportedLibraries.put(JVM_OKHTTP, "A workaround to use the same template folder for both 'jvm-okhttp3' and 'jvm-okhttp4'.");
        setLibrary(JVM_OKHTTP);

        // jvm specific supporting files
        supportingFiles.add(new SupportingFile("infrastructure/Errors.kt.mustache", infrastructureFolder, "Errors.kt"));
        supportingFiles.add(new SupportingFile("infrastructure/ResponseExtensions.kt.mustache", infrastructureFolder, "ResponseExtensions.kt"));
        supportingFiles.add(new SupportingFile("infrastructure/ApiResponse.kt.mustache", infrastructureFolder, "ApiResponse.kt"));
    }

    private void processJvmSpring(final String infrastructureFolder) {
        if (getSerializationLibrary() != SERIALIZATION_LIBRARY_TYPE.jackson) {
            throw new RuntimeException("This library currently only supports jackson serialization. Try adding '--additional-properties serializationLibrary=jackson' to your command.");
        }

        commonJvmMultiplatformSupportingFiles(infrastructureFolder);
        addSupportingSerializerAdapters(infrastructureFolder);

        additionalProperties.put(JVM_SPRING, true);
        additionalProperties.put(JVM, true);
    }

    private void processJvmSpringWebClientLibrary(final String infrastructureFolder) {
        processJvmSpring(infrastructureFolder);
        additionalProperties.put(JVM_SPRING_WEBCLIENT, true);
    }

    private void processJvmSpringRestClientLibrary(final String infrastructureFolder) {
        if (additionalProperties.getOrDefault(USE_SPRING_BOOT3, false).equals(false)
                && additionalProperties.getOrDefault(USE_SPRING_BOOT4, false).equals(false)) {
            throw new RuntimeException("This library requires Spring Boot 3 or 4. Try adding '--additional-properties useSpringBoot3=true' or '--additional-properties useSpringBoot4=true' to your command.");

View on GitHub (pinned to fcec517be3)

Solutions

1. Add `-p serializationLibrary=jackson` when using jvm-spring-webclient or jvm-spring-restclient.
2. Remember the pairing rules: spring libraries ⇒ jackson; jvm-volley ⇒ gson; other libraries default to moshi.

Example fix

# before
openapi-generator-cli generate -g kotlin --library=jvm-spring-webclient -i api.yaml
# after
openapi-generator-cli generate -g kotlin --library=jvm-spring-webclient -i api.yaml -p serializationLibrary=jackson

# shell: spring libraries demand jackson
case "$LIBRARY" in
  jvm-spring-webclient|jvm-spring-restclient)
    [ "${SERIALIZATION_LIBRARY:-moshi}" = "jackson" ] || { echo "$LIBRARY requires serializationLibrary=jackson" >&2; exit 2; } ;;
esac

try {
    DefaultGenerator().opts(clientOptInput).generate()
} catch (e: RuntimeException) {
    throw IllegalStateException("Spring library serialization mismatch: ${e.message}", e)
}

When it happens

Trigger: `-g kotlin --library=jvm-spring-webclient` (default moshi → throw); `--library=jvm-spring-restclient -p serializationLibrary=gson|moshi|kotlinx_serialization`.

Common situations: Switching a kotlin client from okhttp4 (moshi default worked) to a Spring library by changing only --library; option presets that omit serializationLibrary.

Related errors

• {enumSerializationLibrary} is an invalid enum property namin
• useJackson3 requires serializationLibrary=jackson
• This library currently only supports gson serialization. Try
• useJackson3 is only available with Spring Boot >= 4
• Unexpected serializationLibrary value: {serializationLibrary

AI-assisted analysis of OpenAPITools/openapi-generator@fcec517be3 (2026-08-22). Data as JSON: /api/errors/56e2d7a2d4e24ad0. Report an issue: GitHub.