quarkusio/quarkus · error · IllegalStateException

Failed to find multipart data for class " + responseClass +

Error message

Failed to find multipart data for class " + responseClass + ". If it's meant to be used as multipart response type, consider annotating it with @MultipartForm

What it means

The client received a multipart response whose target class is not registered in the build-time-generated multipartDataMap. Multipart DTO classes must be discovered at build time (annotated with @MultipartForm so the processor generates MultipartResponseData for them); at runtime the lookup by class fails and mapToResponse throws IllegalStateException.

Source

Thrown at independent-projects/resteasy-reactive/client/runtime/src/main/java/org/jboss/resteasy/reactive/client/handlers/ClientResponseCompleteRestHandler.java:86

                GenericType<?> responseType = context.getResponseType();

                if (EntityPartImpl.isEntityPartList(responseType.getType())) {
                    Serialisers serialisers = context.getRestClient().getClientContext().getSerialisers();
                    List<EntityPart> entityParts = new ArrayList<>();
                    for (InterfaceHttpData httpData : context.getResponseMultipartParts()) {
                        entityParts.add(nettyPartToEntityPart(httpData, serialisers));
                    }
                    builder.entity(entityParts);
                } else {

                    if (!(responseType.getType() instanceof Class)) {
                        throw new IllegalArgumentException("Not supported return type for a multipart message, " +
                                "expected a non-generic class got : " + responseType.getType());
                    }
                    Class<?> responseClass = (Class<?>) responseType.getType();
                    MultipartResponseData multipartData = multipartDataMap.get(responseClass);
                    if (multipartData == null) {
                        throw new IllegalStateException("Failed to find multipart data for class " + responseClass + ". " +
                                "If it's meant to be used as multipart response type, consider annotating it with @MultipartForm");
                    }
                    Object result = multipartData.newInstance();
                    builder.entity(result);
                    List<InterfaceHttpData> parts = context.getResponseMultipartParts();
                    for (InterfaceHttpData httpData : parts) {
                        FieldFiller fieldFiller = null;
                        // find the correct filler
                        for (FieldFiller ff : multipartData.getFieldFillers()) {
                            if (ff.getPartName().equals(httpData.getName())) {
                                fieldFiller = ff;
                                break;
                            }
                        }
                        if (fieldFiller == null) {
                            continue;
                        }
                        if (httpData instanceof Attribute at) {

View on GitHub (pinned to e1c734241f)

Solutions

1. Annotate the response POJO with @MultipartForm and annotate its fields appropriately (@RestForm, @RestForm(FileUpload) etc.).
2. Rebuild/restart the application so the multipart processor regenerates MultipartResponseData for the class.
3. Ensure the DTO class is part of the application's Jandex index (in-app class, or indexed dependency).
4. Handle the response manually as a List<EntityPart> or raw body if you cannot annotate the class.

Example fix

// before
class FormData { String name; File file; }
// after
@MultipartForm
class FormData {
  @RestForm String name;
  @RestForm File file;
}

// Multipart response DTO must be annotated
static void checkMultipartDto(Class<?> dto) {
  if (dto.getAnnotation(MultipartForm.class) == null) {
    throw new IllegalStateException(dto + " must be annotated with @MultipartForm");
  }
}

When it happens

Trigger: Declaring a client method with @Produces("multipart/form-data") returning a POJO that is not annotated with @MultipartForm (or the annotation is missing on the client-side signature), so no MultipartResponseData was generated for the class.

Common situations: Sharing response DTOs from a library without the annotation; adding a new multipart endpoint and forgetting the annotation; running against a stale build where the generated multipart data predates the new DTO class.

Related errors

• Not supported return type for a multipart message, expected
• Unsupported multipart message element type. Expected FileAtt
• PartType annotation is only supported on fields and (setter/
• Primitive types are not supported for multipart response map
• Unsupported field type for multipart response mapping: " + t

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/baf3733f8f1ed51d. Report an issue: GitHub.