@Override
  @NonNull
  public Map<String, ExternalTransformBuilder<?, ?, ?>> knownBuilderInstances() {
    return ImmutableMap.<String, ExternalTransformBuilder<?, ?, ?>>builder()
        .put(INSERT_URN, new InsertBuilder())
        .put(UPDATE_URN, new UpdateBuilder())
        .put(REPLACE_URN, new ReplaceBuilder())
        .put(INSERT_OR_UPDATE_URN, new InsertOrUpdateBuilder())
        .put(DELETE_URN, new DeleteBuilder())
        .put(READ_URN, new ReadBuilder())
        .build();
  }
@Test
  public void testKnownBuilderInstances() {
    Map<String, ExternalTransformBuilder<?, ?, ?>> builderInstancesMap =
        spannerTransformRegistrar.knownBuilderInstances();
    assertEquals(6, builderInstancesMap.size());
    assertThat(builderInstancesMap, IsMapContaining.hasKey(SpannerTransformRegistrar.INSERT_URN));
    assertThat(builderInstancesMap, IsMapContaining.hasKey(SpannerTransformRegistrar.UPDATE_URN));
    assertThat(builderInstancesMap, IsMapContaining.hasKey(SpannerTransformRegistrar.REPLACE_URN));
    assertThat(
        builderInstancesMap,
        IsMapContaining.hasKey(SpannerTransformRegistrar.INSERT_OR_UPDATE_URN));
    assertThat(builderInstancesMap, IsMapContaining.hasKey(SpannerTransformRegistrar.DELETE_URN));
    assertThat(builderInstancesMap, IsMapContaining.hasKey(SpannerTransformRegistrar.READ_URN));
  }