UMLResource res = (UMLResource) resourceSet.getResource(URI.createFileURI("NPModel.uml"), true); JsonResource resource = (JsonResource) resourceSet.createResource(URI.createURI("generated/diagram.json")); res.getAllContents().forEachRemaining(eObject -> resource.setID(eObject, res.getID(eObject))); resource.getContents().addAll(res.getContents());