class BlobsServiceImpl implements BlobsService {

    @Override
    public List<Entity> extractEntitiesFromBlob(BlobKey blobKey) {
        JsonReader jsonReader = jsonBlobReaderFactory.create(blobKey);

        return gson.fromJson(jsonReader, new TypeToken<List<Entity>>() {
        }.getType());
    }

}

class BlobsServiceImplTest  {

    @Inject BlobsService blobsService;
    @Inject BlobGenerator blobGenerator;

    @Test
    public void extractEntitiesFromBlob() throws Exception {

        // given
        BlobKey blobKey = createCarBlob();

        // when
        List<Entity> entities = blobsService.extractEntitiesFromBlob(blobKey);

        // then
        assertEquals(1, entities.size());
    }

}