rgoers/log4j2-core/src/main/java/org/apache/logging/log4j/core/lookup/StrLookup.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache license, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the license for the specific language governing permissions and
 * limitations under the license.
 */
package org.apache.logging.log4j.core.lookup;

import org.apache.logging.log4j.core.LogEvent;

/**
 * Lookup a String key to a String value.
 * <p>
 * This class represents the simplest form of a string to string map.
 * It has a benefit over a map in that it can create the result on
 * demand based on the key.
 * <p>
 * This class comes complete with various factory methods.
 * If these do not suffice, you can subclass and implement your own matcher.
 * <p>
 * For example, it would be possible to implement a lookup that used the
 * key as a primary key, and looked up the value on demand from the database
 *
 * @author Apache Software Foundation
 * @version $Id$
 *
 * @param <V> The type of the value that is being queried.
 */
public interface StrLookup<V> {
    /**
     * Looks up a String key to a String value.
     * <p>
     * The internal implementation may use any mechanism to return the value.
     * The simplest implementation is to use a Map. However, virtually any
     * implementation is possible.
     * <p>
     * For example, it would be possible to implement a lookup that used the
     * key as a primary key, and looked up the value on demand from the database
     * Or, a numeric based implementation could be created that treats the key
     * as an integer, increments the value and return the result as a string -
     * converting 1 to 2, 15 to 16 etc.
     * <p>
     * The {@link #lookup(String)} method always returns a String, regardless of
     * the underlying data, by converting it as necessary. For example:
     * <pre>
     * Map<String, Object> map = new HashMap<String, Object>();
     * map.put("number", new Integer(2));
     * assertEquals("2", StrLookup.mapLookup(map).lookup("number"));
     * </pre>
     * @param key  the key to be looked up, may be null
     * @return the matching value, null if no match
     */
    String lookup(String key);

    /**
     * Looks up a String key to a String value possibly using the current LogEvent.
     * <p>
     * The internal implementation may use any mechanism to return the value.
     * The simplest implementation is to use a Map. However, virtually any
     * implementation is possible.
     * <p>
     * For example, it would be possible to implement a lookup that used the
     * key as a primary key, and looked up the value on demand from the database
     * Or, a numeric based implementation could be created that treats the key
     * as an integer, increments the value and return the result as a string -
     * converting 1 to 2, 15 to 16 etc.
     * <p>
     * The {@link #lookup(String)} method always returns a String, regardless of
     * the underlying data, by converting it as necessary. For example:
     * <pre>
     * Map<String, Object> map = new HashMap<String, Object>();
     * map.put("number", new Integer(2));
     * assertEquals("2", StrLookup.mapLookup(map).lookup("number"));
     * </pre>
     * @param event The current LogEvent.
     * @param key  the key to be looked up, may be null
     * @return the matching value, null if no match
     */
    String lookup(LogEvent event, String key);
}