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1   package org.apache.commons.digester3.plugins;
2   
3   /*
4    * Licensed to the Apache Software Foundation (ASF) under one
5    * or more contributor license agreements.  See the NOTICE file
6    * distributed with this work for additional information
7    * regarding copyright ownership.  The ASF licenses this file
8    * to you under the Apache License, Version 2.0 (the
9    * "License"); you may not use this file except in compliance
10   * with the License.  You may obtain a copy of the License at
11   *
12   *   http://www.apache.org/licenses/LICENSE-2.0
13   *
14   * Unless required by applicable law or agreed to in writing,
15   * software distributed under the License is distributed on an
16   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
17   * KIND, either express or implied.  See the License for the
18   * specific language governing permissions and limitations
19   * under the License.
20   */
21  
22  import java.util.Properties;
23  
24  import org.apache.commons.digester3.Digester;
25  
26  /**
27   * Each concrete implementation of RuleFinder is an algorithm for locating a source of digester rules for a plugin. The
28   * algorithm may use info explicitly provided by the user as part of the plugin declaration, or not (in which case the
29   * concrete RuleFinder subclass typically has Dflt as part of its name).
30   * <p>
31   * Instances of this class can also be regarded as a Factory for RuleLoaders, except that an instance of a RuleLoader is
32   * only created if the particular finder algorithm can locate a suitable source of rules given the plugin class and
33   * associated properties.
34   * <p>
35   * This is an abstract class rather than an interface in order to make it possible to enhance this class in future
36   * without breaking binary compatibility; it is possible to add methods to an abstract class, but not to an interface.
37   * 
38   * @since 1.6
39   */
40  public abstract class RuleFinder
41  {
42  
43      /**
44       * Apply the finder algorithm to attempt to locate a source of digester rules for the specified plugin class.
45       * <p>
46       * This method is invoked when a plugin is declared by the user, either via an explicit use of
47       * PluginDeclarationRule, or implicitly via an "inline declaration" where the declaration and use are simultaneous.
48       * <p>
49       * If dynamic rules for the specified plugin class are located, then the RuleFinder will return a RuleLoader object
50       * encapsulating those rules, and this object will be invoked each time the user actually requests an instance of
51       * the declared plugin class, to load the custom rules associated with that plugin instance.
52       * <p>
53       * If no dynamic rules can be found, null is returned. This is not an error; merely an indication that this
54       * particular algorithm found no matches.
55       * <p>
56       * The properties object holds any xml attributes the user may have specified on the plugin declaration in order to
57       * indicate how to locate the plugin rules.
58       * <p>
59       *
60       * @param d The digester instance where locating plugin classes
61       * @param pluginClass The plugin Java class
62       * @param p The properties object that holds any xml attributes the user may have specified on the plugin
63       *          declaration in order to indicate how to locate the plugin rules.
64       * @return a source of digester rules for the specified plugin class.
65       * @throws PluginException if the algorithm finds a source of rules, but there is something invalid
66       *         about that source.
67       */
68      public abstract RuleLoader findLoader( Digester d, Class<?> pluginClass, Properties p )
69          throws PluginException;
70  
71  }