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 */
017package org.apache.commons.configuration2.interpol;
018
019/**
020 * <p>
021 * Definition of an interface for looking up variables during interpolation.
022 * </p>
023 * <p>
024 * Objects implementing this interface can be assigned a variable prefix and
025 * added to a {@link ConfigurationInterpolator} object. Whenever the
026 * {@code ConfigurationInterpolator} encounters a property value referencing a
027 * variable, e.g. {@code ${prefix:variableName}}, it extracts the prefix
028 * and finds the matching {@code Lookup} object. Then this object is asked to
029 * resolve the variable name and provide the corresponding value.
030 * </p>
031 * <p>
032 * This interface defines a single method for performing variable lookup. It is
033 * passed the name of a variable and has to return the corresponding value. It
034 * is of course up to a specific implementation how this is done. If the
035 * variable name cannot be resolved, an implementation has to return
036 * <b>null</b>.
037 * </p>
038 * <p>
039 * Note: Implementations must be aware that they can be accessed concurrently.
040 * This is for instance the case if a configuration object is read by multiple
041 * threads or if a {@code Lookup} object is shared by multiple configurations.
042 * </p>
043 *
044 * @since 2.0
045 */
046public interface Lookup
047{
048    /**
049     * Looks up the value of the specified variable. This method is called by
050     * {@link ConfigurationInterpolator} with the variable name extracted from
051     * the expression to interpolate (i.e. the prefix name has already been
052     * removed). A concrete implementation has to return the value of this
053     * variable or <b>null</b> if the variable name is unknown.
054     *
055     * @param variable the name of the variable to be resolved
056     * @return the value of this variable or <b>null</b>
057     */
058    Object lookup(String variable);
059}