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1   ////////////////////////////////////////////////////////////////////////////////
2   // checkstyle: Checks Java source code for adherence to a set of rules.
3   // Copyright (C) 2001-2017 the original author or authors.
4   //
5   // This library is free software; you can redistribute it and/or
6   // modify it under the terms of the GNU Lesser General Public
7   // License as published by the Free Software Foundation; either
8   // version 2.1 of the License, or (at your option) any later version.
9   //
10  // This library is distributed in the hope that it will be useful,
11  // but WITHOUT ANY WARRANTY; without even the implied warranty of
12  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
13  // Lesser General Public License for more details.
14  //
15  // You should have received a copy of the GNU Lesser General Public
16  // License along with this library; if not, write to the Free Software
17  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
18  ////////////////////////////////////////////////////////////////////////////////
19  
20  package com.puppycrawl.tools.checkstyle.api;
21  
22  import java.util.Set;
23  
24  import com.puppycrawl.tools.checkstyle.Checker;
25  
26  /**
27   * The following interface should be implemented by each module (inheritor of
28   * {@link AbstractCheck}, implementor of {@link FileSetCheck}, or {@link Filter}) which uses
29   * external resources of any kind for its configuration. Such modules must declare external
30   * resource locations as a set of {@link String} which will be returned from
31   * {@link #getExternalResourceLocations}. This allows Checkstyle to invalidate (clear) cache
32   * when the content of at least one external configuration resource of the module is changed.
33   *
34   * @author Andrei Selkin
35   */
36  @FunctionalInterface
37  public interface ExternalResourceHolder {
38  
39      /**
40       * Returns a set of external configuration resource locations which are used by the module.
41       * ATTENTION!
42       * If 'getExternalResourceLocations()' return null, there will be
43       * {@link NullPointerException} in {@link Checker}.
44       * Such behaviour will signal that your module (check or filter) is designed incorrectly.
45       * It make sense to return an empty set from 'getExternalResourceLocations()'
46       * only for composite modules like {@link com.puppycrawl.tools.checkstyle.TreeWalker}.
47       * @return a set of external configuration resource locations which are used by the module.
48       */
49      Set<String> getExternalResourceLocations();
50  
51  }