quarkusio/quarkus · error · IllegalStateException

Could not find MessageBodyWriter for ${entityClass} / ${enti

Error message

Could not find MessageBodyWriter for ${entityClass} / ${entityType} as ${mediaType}

What it means

Thrown by StreamingUtil.serialiseEntity when the RESTEasy Reactive server needs to write an entity to bytes (e.g. streaming a response part) but no registered MessageBodyWriter can serialize the given Java type for the requested media type. It is a server-side serialization failure, meaning the resource's return type has no writer mapping for the negotiated content type. The runtime cannot produce a response body, so it aborts.

Source

Thrown at independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/core/StreamingUtil.java:82

        MediaType mediaType = context.getResponseMediaType();
        // FIXME: this should belong somewhere else as it's generic
        @SuppressWarnings("unchecked")
        MessageBodyWriter<Object>[] writers = (MessageBodyWriter<Object>[]) serialisers
                .findWriters(null, entityClass, mediaType, RuntimeType.SERVER)
                .toArray(ServerSerialisers.NO_WRITER);
        StreamingOutputStream baos = new StreamingOutputStream();
        boolean wrote = false;
        for (MessageBodyWriter<Object> writer : writers) {
            if (writer.isWriteable(entityClass, entityType, context.getAllAnnotations(), mediaType)) {
                // FIXME: spec doesn't really say what headers we should use here
                writer.writeTo(entity, entityClass, entityType, context.getAllAnnotations(), mediaType,
                        new QuarkusMultivaluedHashMap<>(), baos);
                wrote = true;
                break;
            }
        }
        if (!wrote) {
            throw new IllegalStateException(
                    "Could not find MessageBodyWriter for " + entityClass + " / " + entityType + " as " + mediaType);
        }
        return baos.toByteArray();
    }

    public static void setHeaders(ResteasyReactiveRequestContext context, ServerHttpResponse response,
            List<PublisherResponseHandler.StreamingResponseCustomizer> customizers) {
        // FIXME: spec says we should flush the headers when first message is sent or when the resource method returns, whichever
        // happens first
        if (!response.headWritten()) {
            response.setStatusCode(Response.Status.OK.getStatusCode());
            response.setResponseHeader(HttpHeaders.CONTENT_TYPE, context.getResponseContentType().toString());
            response.setChunked(true);
            for (int i = 0; i < customizers.size(); i++) {
                customizers.get(i).customize(response);
            }
        }
    }

View on GitHub (pinned to e1c734241f)

Solutions

1. Add a MessageBodyWriter or the extension providing one (e.g. quarkus-resteasy-reactive-jackson for JSON) and make sure @Produces matches (e.g. application/json).
2. Check the entityClass in the message: if it's a wrapper like Multi/Uni or a raw type, return the unwrapped/parameterized type or register a custom writer.
3. Register a custom Writer via @Provider implementing ServerMessageBodyWriter for the failing type/media type.
4. If the type should never be serialized, fix the method signature so the correct body is returned instead of the internal object.

Example fix

// before
@GET @Produces(MediaType.APPLICATION_XML)
public MyPojo get() { return new MyPojo(); } // no XML writer
// after
@GET @Produces(MediaType.APPLICATION_JSON)
public MyPojo get() { return new MyPojo(); } // with quarkus-resteasy-reactive-jackson on classpath

if (!produces.contains(MediaType.APPLICATION_JSON)) { throw new ConfigurationException("No MessageBodyWriter for " + entity.getClass() + " with " + produces); }

try { return response; } catch (IllegalStateException e) { if (e.getMessage().startsWith("Could not find MessageBodyWriter")) { log.errorf("Add a writer for %s", e.getMessage()); } throw e; }

When it happens

Trigger: A resource method (or multipart/streaming output path) returns an entity whose class has no @Produces-compatible MessageBodyWriter, or the mediaType requested (e.g. application/xml) has no writer registered for that entityClass/entityType (including generic Type information).

Common situations: Returning a POJO without a JSON mapper when only application/octet-stream is producible; missing quarkus-resteasy-reactive-jackson dependency; custom types without a registered writer; wrong @Produces annotation vs actual returned type.

Related errors

• Unknown JSON Value
• Failed to serialize content of type
• Could not find MessageBodyWriter for ${entityClass}
• Could not find MessageBodyWriter for ${entityClass} as ${med
• Could not find MessageBodyWriter for

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/b7705c622e1cc9bc. Report an issue: GitHub.