quarkusio/quarkus · error · InternalServerErrorException

Could not find MessageBodyWriter for

Error message

Could not find MessageBodyWriter for 

What it means

FixedEntityWriter wraps a single MessageBodyWriter chosen at build time. If ServerSerialisers.invokeWriter reports that the writer cannot (or is not applicable to) write the entity, an InternalServerErrorException (500) is thrown.

Source

Thrown at independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/core/serialization/FixedEntityWriter.java:29

/**
 * A fixed entity writer that can be used when we know the result will always be written
 * by a given provider.
 */
public class FixedEntityWriter implements EntityWriter {

    private final MessageBodyWriter writer;
    private final ServerSerialisers serialisers;

    public FixedEntityWriter(MessageBodyWriter writer, ServerSerialisers serialisers) {
        this.writer = writer;
        this.serialisers = serialisers;
    }

    @Override
    public void write(ResteasyReactiveRequestContext context, Object entity) throws IOException {
        if (!ServerSerialisers.invokeWriter(context, entity, writer, serialisers)) {
            throw new InternalServerErrorException("Could not find MessageBodyWriter for " + entity.getClass(),
                    Response.serverError().build());
        }
    }

}

View on GitHub (pinned to e1c734241f)

Solutions

1. Rebuild the application (mvn clean install) so build-time writer selection matches the current code.
2. Align the method's declared return type and @Produces with the actual entity being returned.
3. Ensure custom MessageBodyWriter.isWriteable accepts the runtime entity class.
4. Register the entity type explicitly with the writer/serializer configuration.

Example fix

// before
@Produces(MediaType.TEXT_PLAIN)
public MyDto get() { ... } // plain-text String writer can't handle MyDto
// after
@Produces(MediaType.APPLICATION_JSON)
public MyDto get() { ... }

try {
    return resource.call();
} catch (InternalServerErrorException e) {
    if (e.getMessage() != null && e.getMessage().contains("Could not find MessageBodyWriter")) {
        // fall back to explicit JSON serialization
        return Response.ok(jsonOf(entity)).type(MediaType.APPLICATION_JSON).build();
    }
    throw e;
}

When it happens

Trigger: A resource whose return type was mapped to a fixed writer at build time returns an entity that the writer's isWriteable rejects at runtime (mismatched generic type, changed entity class, null vs non-null media type mismatch).

Common situations: Return type changed after compilation (stale build); entity subclass not handled by the selected writer; application produced a media type the fixed writer doesn't support.

Related errors

• Could not find MessageBodyWriter for
• Could not find MessageBodyWriter for
• Could not find MessageBodyWriter for
• Failed to serialize content of type
• Could not find MessageBodyWriter for ${entityClass}

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/e4d13e1c5975b1f1. Report an issue: GitHub.