    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}</code>, 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}