    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     */
    017    package org.apache.commons.configuration.interpol;
    018
    019    import org.apache.commons.configuration.EnvironmentConfiguration;
    020    import 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     */
    048    public 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    }