@Input
  @Optional
  public Set<String> getTags() {
    String property = System.getProperty(PropertyNames.TO_TAGS);
    Set<String> tagsValue;
    if (property != null) {
      tagsValue = ImmutableSet.copyOf(ConfigurationPropertyValidator.parseListProperty(property));
    } else {
      try {
        tagsValue = tags.get();
      } catch (NullPointerException ex) {
        throw new IllegalArgumentException("jib.to.tags contains null tag");
      }
    }
    if (tagsValue.stream().anyMatch(str -> str.isEmpty())) {
      throw new IllegalArgumentException("jib.to.tags contains empty tag");
    }
    return tagsValue;
  }
Object in {@link JibExtension} that configures the target image.
jib-gradle-plugin/src/main/java/com/google/cloud/tools/jib/gradle/TargetImageParameters.java