According to the spec, it should be possible to declare a parameter like @FormParam("part3Name") EntityPart part3. But when I do, I get a 400 Bad Request. The full response is also not very helpful:

HTTP/1.1 400 Bad Request
Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: origin, content-type, accept, authorization
Access-Control-Allow-Methods: GET, POST, PUT, DELETE, OPTIONS, HEAD
Access-Control-Allow-Origin: *
Connection: keep-alive
Content-Length: 0
Date: Sun, 03 Mar 2024 09:28:33 GMT

OTOH, using List<EntityPart> parts and then:

var part3 = parts.stream().filter(part → "part3Name".equals(part.getName())).findFirst().orElseThrow();
works just fine.