@Input
  public List<String> getPathStrings() {
    // Gradle warns about @Input annotations on File objects, so we have to expose a getter for a
    // String to make them go away.
    return getPaths().stream()
        .map(extraDirectoryParameters -> extraDirectoryParameters.getFrom().toString())
        .collect(Collectors.toList());
  }
Object in {@link JibExtension} that configures the extra directories.
jib-gradle-plugin/src/main/java/com/google/cloud/tools/jib/gradle/ExtraDirectoriesParameters.java