@VisibleForTesting
  static AbsoluteUnixPath getAppRootChecked(
      RawConfiguration rawConfiguration, ProjectProperties projectProperties)
      throws InvalidAppRootException {
    String appRoot = rawConfiguration.getAppRoot();
    if (appRoot.isEmpty()) {
      appRoot =
          projectProperties.isWarProject()
              ? DEFAULT_JETTY_APP_ROOT
              : JavaContainerBuilder.DEFAULT_APP_ROOT;
    }
    try {
      return AbsoluteUnixPath.get(appRoot);
    } catch (IllegalArgumentException ex) {
      throw new InvalidAppRootException(appRoot, appRoot, ex);
    }
  }
Gets the value of the {@code appRoot} parameter. If the parameter is empty, returns {@code
/var/lib/jetty/webapps/ROOT} for WAR projects or {@link JavaContainerBuilder#DEFAULT_APP_ROOT}
for other projects.

@param rawConfiguration raw configuration data
@param projectProperties the project properties
@return the app root value
@throws InvalidAppRootException if {@code appRoot} value is not an absolute Unix path
jib-plugins-common/src/main/java/com/google/cloud/tools/jib/plugins/common/PluginConfigurationProcessor.java