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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements. See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache license, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License. You may obtain a copy of the License at
8    *
9    *      http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the license for the specific language governing permissions and
15   * limitations under the license.
16   */
17  package org.apache.logging.log4j.core.lookup;
18  
19  import org.apache.logging.log4j.core.LogEvent;
20  
21  /**
22   * Lookup a String key to a String value.
23   * <p>
24   * This class represents the simplest form of a string to string map.
25   * It has a benefit over a map in that it can create the result on
26   * demand based on the key.
27   * <p>
28   * This class comes complete with various factory methods.
29   * If these do not suffice, you can subclass and implement your own matcher.
30   * <p>
31   * For example, it would be possible to implement a lookup that used the
32   * key as a primary key, and looked up the value on demand from the database
33   *
34   * @version $Id$
35   */
36  public interface StrLookup {
37      /**
38       * Looks up a String key to a String value.
39       * <p>
40       * The internal implementation may use any mechanism to return the value.
41       * The simplest implementation is to use a Map. However, virtually any
42       * implementation is possible.
43       * <p>
44       * For example, it would be possible to implement a lookup that used the
45       * key as a primary key, and looked up the value on demand from the database
46       * Or, a numeric based implementation could be created that treats the key
47       * as an integer, increments the value and return the result as a string -
48       * converting 1 to 2, 15 to 16 etc.
49       * <p>
50       * The {@link #lookup(String)} method always returns a String, regardless of
51       * the underlying data, by converting it as necessary. For example:
52       * <pre>
53       * Map<String, Object> map = new HashMap<String, Object>();
54       * map.put("number", new Integer(2));
55       * assertEquals("2", StrLookup.mapLookup(map).lookup("number"));
56       * </pre>
57       * @param key  the key to be looked up, may be null
58       * @return the matching value, null if no match
59       */
60      String lookup(String key);
61  
62      /**
63       * Looks up a String key to a String value possibly using the current LogEvent.
64       * <p>
65       * The internal implementation may use any mechanism to return the value.
66       * The simplest implementation is to use a Map. However, virtually any
67       * implementation is possible.
68       * <p>
69       * For example, it would be possible to implement a lookup that used the
70       * key as a primary key, and looked up the value on demand from the database
71       * Or, a numeric based implementation could be created that treats the key
72       * as an integer, increments the value and return the result as a string -
73       * converting 1 to 2, 15 to 16 etc.
74       * <p>
75       * The {@link #lookup(String)} method always returns a String, regardless of
76       * the underlying data, by converting it as necessary. For example:
77       * <pre>
78       * Map<String, Object> map = new HashMap<String, Object>();
79       * map.put("number", new Integer(2));
80       * assertEquals("2", StrLookup.mapLookup(map).lookup("number"));
81       * </pre>
82       * @param event The current LogEvent.
83       * @param key  the key to be looked up, may be null
84       * @return the matching value, null if no match
85       */
86      String lookup(LogEvent event, String key);
87  }