Testing repository which map objects from files

how can I test in memory repository which is reading objects from JSON files in resource folder?

Repository:

@Repository
public class MyRepository{

    private TreeMap<String, MyObject> objects;
    private ObjectMapper mapper;

    @Autowired
    public MyRepository(ObjectMapper objectMapper) throws IOException {
        // mapping objects from /resources to TreeMap
    }

    // some custom methods working on my objects

}

Now I want to test these methods, but I dont want to use "real" objects. What is the best way to test it? Should I create some constructor that would take some collection of objects as parameter?