jsonapi-8.x-2.x-dev/tests/modules/jsonapi_test_field_type/jsonapi_test_field_type.services.yml

services:
  serializer.normalizer.string.jsonapi_test_field_type:
    class: Drupal\jsonapi_test_field_type\Normalizer\StringNormalizer
    tags:
      # The priority must be higher than serialization.normalizer.field_item.
      - { name: normalizer , priority: 1000 }