This is an example for an ExtensionService and the updated ExtensionController with the changes from here: https://gist.github.com/dacrome/a04f2377f70676fc5230

ExtensionService

// sorry, no imports

@Service
@Transactional // I know this should be in the respository, but one layer deeper than controller. yay!
public class ExtensionService {

    @Inject
    private ExtensionDao dao;

    public List<ExtensionMap> getAllExtensions() {
        List<Extension> extensions = fromEntity(dao.getAllExtensions());
        final List<ExtensionMap> extensionMaps = new ArrayList<>();
        for(Extension extension : extensions) {
            final ExtensionMap extensionMap = new ExtensionMap(extension.getUrn());
            for(Map.Entry<String, Extension.Field> fieldEntry : extension.getFields().entrySet()){
                final String field = fieldEntry.getKey();
                final ExtensionFieldType type = fieldEntry.getValue().getType();
                extensionMap.addPair(field, type);
            }
            extensionMaps.add(extensionMap);
        }
        return extensionMaps;
    }

    public List<Extension> fromEntity(List<ExtensionEntity> entities){
        List<Extension> result = new ArrayList<Extension>();
        for(ExtensionEntity entity : entities){
            Extension.Builder builder = new Extension.Builder(entity.getUrn());
            setField(entity, builder);
            result.add(builder.build());
        }
        return result;
    }

    private void setField(ExtensionEntity entity, Extension.Builder builder) {
        for(ExtensionFieldEntity fieldEntity : entity.getFields()){
            switch (fieldEntity.getType().getName()) {
                case "STRING":
                    builder.setField(fieldEntity.getName(), "null");
                    break;
                case "INTEGER":
                    builder.setField(fieldEntity.getName(), BigInteger.ZERO);
                    break;
                case "DECIMAL":
                    builder.setField(fieldEntity.getName(), BigDecimal.ZERO);
                    break;
                case "BOOLEAN":
                    builder.setField(fieldEntity.getName(), false);
                    break;
                case "DATE_TIME":
                    builder.setField(fieldEntity.getName(), new Date(0L));
                    break;
                case "BINARY":
                    builder.setField(fieldEntity.getName(), ByteBuffer.wrap(new byte[]{}));
                    break;
                case "REFERENCE":
                    try {
                        builder.setField(fieldEntity.getName(), new URI("http://www.osiam.org"));
                    } catch (URISyntaxException e) {
                        throw new IllegalStateException(e);
                    }
                    break;
                default:
                    throw new IllegalArgumentException("Type " + fieldEntity.getType().getName() + " does not exist");
            }
        }
    }
}

OsiamController

@Controller
@RequestMapping(value = "/osiam")
public class OsiamController {

    @Autowired
    private ExtensionService extensionService;

    @RequestMapping(value="/Extensiontypes", method = RequestMethod.GET)
    @ResponseStatus(HttpStatus.OK)
    @ResponseBody
    public List<ExtensionMap> getAllExtensions() {
        return extensionService.getExtensions();
    }
}