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.configuration.interpol;
018
019import org.apache.commons.configuration.EnvironmentConfiguration;
020import org.apache.commons.lang.text.StrLookup;
021
022/**
023 * <p>
024 * A specialized lookup implementation that allows access to environment
025 * variables.
026 * </p>
027 * <p>
028 * This implementation relies on {@link EnvironmentConfiguration} to resolve
029 * environment variables. It can be used for referencing environment variables
030 * in configuration files in an easy way, for instance:
031 *
032 * <pre>
033 * java.home = ${env:JAVA_HOME}
034 * </pre>
035 *
036 * </p>
037 * <p>
038 * {@code EnvironmentLookup} is one of the standard lookups that is
039 * registered per default for each configuration.
040 * </p>
041 *
042 * @author <a
043 *         href="http://commons.apache.org/configuration/team-list.html">Commons
044 *         Configuration team</a>
045 * @since 1.7
046 * @version $Id: EnvironmentLookup.java 1210620 2011-12-05 20:57:31Z oheger $
047 */
048public class EnvironmentLookup extends StrLookup
049{
050    /** Stores the underlying {@code EnvironmentConfiguration}. */
051    private final EnvironmentConfiguration environmentConfig = new EnvironmentConfiguration();
052
053    /**
054     * Performs a lookup for the specified variable. This implementation
055     * directly delegates to a {@code EnvironmentConfiguration}.
056     *
057     * @param key the key to lookup
058     * @return the value of this key or <b>null</b> if it cannot be resolved
059     */
060    @Override
061    public String lookup(String key)
062    {
063        return environmentConfig.getString(key);
064    }
065}