quarkusio/quarkus · error · InternalServerErrorException

Could not find MessageBodyWriter for

Error message

Could not find MessageBodyWriter for 

What it means

TypeDelegatingEntityWriter walks the entity's class hierarchy looking for a delegate MessageBodyWriter registered for that class (or a superclass). If no writer is registered anywhere up the hierarchy, an InternalServerErrorException (500) is thrown.

Source

Thrown at independent-projects/resteasy-reactive/server/runtime/src/main/java/org/jboss/resteasy/reactive/server/core/serialization/TypeDelegatingEntityWriter.java:34

    private final Map<Class<?>, EntityWriter> typeMap;

    public TypeDelegatingEntityWriter(Map<Class<?>, EntityWriter> typeMap) {
        this.typeMap = typeMap;
    }

    @Override
    public void write(ResteasyReactiveRequestContext context, Object entity) throws IOException {
        Class<?> c = entity.getClass();
        while (c != null) {
            EntityWriter delegate = typeMap.get(c);
            if (delegate != null) {
                delegate.write(context, entity);
                return;
            }
            c = c.getSuperclass();
        }
        throw new InternalServerErrorException("Could not find MessageBodyWriter for " + entity.getClass(),
                Response.serverError().build());
    }
}

View on GitHub (pinned to e1c734241f)

Solutions

1. Add the JSON serialization extension (quarkus-rest-jackson or quarkus-rest-jsonb).
2. Return the concrete type so build-time wiring selects a writer for it.
3. For native builds add @RegisterForReflection to the entity (and relevant superclasses).
4. Register a custom MessageBodyWriter or a serializer mapper for the third-party class.

Example fix

// before
@GET
public Object get() { return new ThirdPartyDto(); }
// after
@RegisterForReflection
@GET @Produces(MediaType.APPLICATION_JSON)
public ThirdPartyDto get() { return new ThirdPartyDto(); }

try {
    return target.request().get(MyDto.class);
} catch (InternalServerErrorException e) {
    if (e.getMessage() != null && e.getMessage().contains("Could not find MessageBodyWriter")) {
        log.errorf("No writer registered in type hierarchy of %s", MyDto.class);
    }
    throw e;
}

When it happens

Trigger: Returning an entity whose class (and all superclasses) has no writer registered in the runtime type map — typically a plain POJO with no JSON serializer or no @RegisterForReflection-based registration when the declared return type was Object/dynamic.

Common situations: Missing quarkus-rest-jackson/quarkus-rest-jsonb dependency; entity class not registered for reflection in native mode; returning a third-party class the app never registered a writer for.

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/31c2d4e238acd715. Report an issue: GitHub.