quarkusio/quarkus · error · IllegalArgumentException

PartType annotation is only supported on fields and (setter/

Error message

PartType annotation is only supported on fields and (setter/getter) methods for multipart responses, found one on " + target

What it means

When generating multipart client code, Quarkus resolves the part name from @PartType/@RestForm/@FormParam annotated fields or setter/getter methods. If the annotated target is a method parameter, constructor, class, or another element kind, no valid part name can be extracted and an IllegalArgumentException is thrown at build time.

Source

Thrown at extensions/resteasy-reactive/rest-client-jaxrs/deployment/src/main/java/io/quarkus/jaxrs/client/reactive/deployment/JaxrsClientReactiveProcessor.java:590

            constructor.returnValue(null);
        }
        return context.newInstance(dataClassName);
    }

    private String extractPartName(AnnotationTarget target, String fieldName) {
        AnnotationInstance restForm;
        AnnotationInstance formParam;
        switch (target.kind()) {
            case FIELD:
                restForm = target.asField().annotation(REST_FORM_PARAM);
                formParam = target.asField().annotation(FORM_PARAM);
                break;
            case METHOD:
                restForm = target.asMethod().annotation(REST_FORM_PARAM);
                formParam = target.asMethod().annotation(FORM_PARAM);
                break;
            default:
                throw new IllegalArgumentException(
                        "PartType annotation is only supported on fields and (setter/getter) methods for multipart responses, found one on "
                                + target);
        }
        return getAnnotationValueOrDefault(fieldName, restForm, formParam);
    }

    private String getAnnotationValueOrDefault(String fieldName, AnnotationInstance restForm, AnnotationInstance formParam) {
        if (restForm != null) {
            AnnotationValue restFormValue = restForm.value();
            return restFormValue == null ? fieldName : restFormValue.asString();
        } else if (formParam != null) {
            return formParam.value().asString();
        } else {
            return fieldName;
        }
    }

    private String createFieldFillerForSetter(AnnotationInstance partType, MethodInfo setter, String partName,

View on GitHub (pinned to e1c734241f)

Solutions

1. Move @PartType and @RestForm/@FormParam onto a field or a getter/setter method of the multipart response class
2. Ensure the multipart DTO uses standard JavaBean fields or accessor methods
3. Rebuild and verify the annotation targets with the client interface

Example fix

// before
public MultipartBody get(@PartType("text/plain") String id);
// after
class MultipartBody {
    @PartType("text/plain")
    @RestForm
    public String file;
}

for (var f : dto.getClass().getDeclaredFields()) { if (f.isAnnotationPresent(PartType.class) == false && (f.isAnnotationPresent(RestForm.class) || f.isAnnotationPresent(FormParam.class))) throw new IllegalStateException("@PartType must sit on a field or accessor: " + f); }

boolean validPartTarget(java.lang.reflect.AnnotatedElement el) { return el instanceof java.lang.reflect.Field || el instanceof java.lang.reflect.Method; }

try { client.post(body); } catch (IllegalArgumentException e) { if (e.getMessage().contains("PartType")) { log.error("Fix @PartType placement on multipart DTO"); } throw e; }

When it happens

Trigger: Placing @PartType (or having the multipart part-name extraction logic reach) an annotation target whose kind is not FIELD or METHOD inside an interface used as a REST client with multipart responses.

Common situations: Misplacing @PartType on a method parameter instead of a field/getter/setter of the multipart DTO; refactoring a field to a local variable; copy-pasting annotation placement between entity and client code.

Related errors

• Primitive types are not supported for multipart response map
• Unsupported field type for multipart response mapping: " + t
• Only a single argument can be annotated with '" + URL + "'.
• multipart responses can only be mapped to non-generic classe
• Unimplemented mode of use of 'io.quarkus.runtime.logging.Log

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/175a67757eb2f141. Report an issue: GitHub.