    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 *     https://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 added to a
    025 * {@link ConfigurationInterpolator} object. Whenever the {@code ConfigurationInterpolator} encounters a property value
    026 * referencing a variable, for example {@code ${prefix:variableName}}, it extracts the prefix and finds the matching
    027 * {@code Lookup} object. Then this object is asked to resolve the variable name and provide the corresponding value.
    028 * </p>
    029 * <p>
    030 * This interface defines a single method for performing variable lookup. It is passed the name of a variable and has to
    031 * return the corresponding value. It is of course up to a specific implementation how this is done. If the variable
    032 * name cannot be resolved, an implementation has to return <strong>null</strong>.
    033 * </p>
    034 * <p>
    035 * Note: Implementations must be aware that they can be accessed concurrently. This is for instance the case if a
    036 * configuration object is read by multiple threads or if a {@code Lookup} object is shared by multiple configurations.
    037 * </p>
    038 *
    039 * @since 2.0
    040 */
    041public interface Lookup {
    042    /**
    043     * Looks up the value of the specified variable. This method is called by {@link ConfigurationInterpolator} with the
    044     * variable name extracted from the expression to interpolate (i.e. the prefix name has already been removed). A
    045     * concrete implementation has to return the value of this variable or <strong>null</strong> if the variable name is unknown.
    046     *
    047     * @param variable the name of the variable to be resolved
    048     * @return the value of this variable or <strong>null</strong>
    049     */
    050    Object lookup(String variable);
    051}