code: ///////////////////////////////////////////////////////////////////////////////////////////////
  // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
  // Copyright (C) 2001-2023 the original author or authors.
  //
  // This library is free software; you can redistribute it and/or
  // modify it under the terms of the GNU Lesser General Public
  // License as published by the Free Software Foundation; either
  // version 2.1 of the License, or (at your option) any later version.
  //
  // This library is distributed in the hope that it will be useful,
  // but WITHOUT ANY WARRANTY; without even the implied warranty of
  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  // Lesser General Public License for more details.
  //
  // You should have received a copy of the GNU Lesser General Public
  // License along with this library; if not, write to the Free Software
  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  ///////////////////////////////////////////////////////////////////////////////////////////////
  
  package com.puppycrawl.tools.checkstyle.api;
  
  import java.util.Set;
  
  import com.puppycrawl.tools.checkstyle.Checker;
  
  /**
   * The following interface should be implemented by each module (inheritor of
   * {@link AbstractCheck}, implementor of {@link FileSetCheck}, or {@link Filter}) which uses
   * external resources of any kind for its configuration. Such modules must declare external
   * resource locations as a set of {@link String} which will be returned from
   * {@link #getExternalResourceLocations}. This allows Checkstyle to invalidate (clear) cache
   * when the content of at least one external configuration resource of the module is changed.
   *
   */
  @FunctionalInterface
  public interface ExternalResourceHolder {
  
      /**
       * Returns a set of external configuration resource locations which are used by the module.
       * ATTENTION!
       * If 'getExternalResourceLocations()' return null, there will be
       * {@link NullPointerException} in {@link Checker}.
       * Such behaviour will signal that your module (check or filter) is designed incorrectly.
       * It makes sense to return an empty set from 'getExternalResourceLocations()'
       * only for composite modules like {@link com.puppycrawl.tools.checkstyle.TreeWalker}.
       *
       * @return a set of external configuration resource locations which are used by the module.
       */
      Set<String> getExternalResourceLocations();
  
  }
  