001    /*
002     * Licensed to the Apache Software Foundation (ASF) under one or more
003     * contributor license agreements. See the NOTICE file distributed with
004     * this work for additional information regarding copyright ownership.
005     * The ASF licenses this file to You under the Apache license, Version 2.0
006     * (the "License"); you may not use this file except in compliance with
007     * the License. You may obtain a copy of the License at
008     *
009     *      http://www.apache.org/licenses/LICENSE-2.0
010     *
011     * Unless required by applicable law or agreed to in writing, software
012     * distributed under the License is distributed on an "AS IS" BASIS,
013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014     * See the license for the specific language governing permissions and
015     * limitations under the license.
016     */
017    package org.apache.logging.log4j.core.lookup;
018    
019    import org.apache.logging.log4j.core.LogEvent;
020    
021    /**
022     * Lookup a String key to a String value.
023     * <p>
024     * This class represents the simplest form of a string to string map.
025     * It has a benefit over a map in that it can create the result on
026     * demand based on the key.
027     * </p>
028     * <p>
029     * This class comes complete with various factory methods.
030     * If these do not suffice, you can subclass and implement your own matcher.
031     * </p>
032     * <p>
033     * For example, it would be possible to implement a lookup that used the
034     * key as a primary key, and looked up the value on demand from the database
035     * </p>
036     */
037    public interface StrLookup {
038    
039        /**
040         * Main plugin category for StrLookup plugins.
041         *
042         * @since 2.1
043         */
044        String CATEGORY = "Lookup";
045    
046        /**
047         * Looks up a String key to a String value.
048         * <p>
049         * The internal implementation may use any mechanism to return the value.
050         * The simplest implementation is to use a Map. However, virtually any
051         * implementation is possible.
052         * </p>
053         * <p>
054         * For example, it would be possible to implement a lookup that used the
055         * key as a primary key, and looked up the value on demand from the database
056         * Or, a numeric based implementation could be created that treats the key
057         * as an integer, increments the value and return the result as a string -
058         * converting 1 to 2, 15 to 16 etc.
059         * </p>
060         * <p>
061         * This method always returns a String, regardless of
062         * the underlying data, by converting it as necessary. For example:
063         * </p>
064         * <pre>
065         * Map&lt;String, Object&gt; map = new HashMap&lt;String, Object&gt;();
066         * map.put("number", new Integer(2));
067         * assertEquals("2", StrLookup.mapLookup(map).lookup("number"));
068         * </pre>
069         * @param key  the key to be looked up, may be null
070         * @return the matching value, null if no match
071         */
072        String lookup(String key);
073    
074        /**
075         * Looks up a String key to a String value possibly using the current LogEvent.
076         * <p>
077         * The internal implementation may use any mechanism to return the value.
078         * The simplest implementation is to use a Map. However, virtually any
079         * implementation is possible.
080         * </p>
081         * <p>
082         * For example, it would be possible to implement a lookup that used the
083         * key as a primary key, and looked up the value on demand from the database
084         * Or, a numeric based implementation could be created that treats the key
085         * as an integer, increments the value and return the result as a string -
086         * converting 1 to 2, 15 to 16 etc.
087         * </p>
088         * <p>
089         * This method always returns a String, regardless of
090         * the underlying data, by converting it as necessary. For example:
091         * </p>
092         * <pre>
093         * Map&lt;String, Object&gt; map = new HashMap&lt;String, Object&gt;();
094         * map.put("number", new Integer(2));
095         * assertEquals("2", StrLookup.mapLookup(map).lookup("number"));
096         * </pre>
097         * @param event The current LogEvent.
098         * @param key  the key to be looked up, may be null
099         * @return the matching value, null if no match
100         */
101        String lookup(LogEvent event, String key);
102    }