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 * A specialized lookup implementation that allows access to environment variables.
022 * </p>
023 * <p>
024 * This implementation relies on {@link System#getenv()} to resolve environment variables. It can be used for
025 * referencing environment variables in configuration files in an easy way, for instance:
026 * </p>
027 *
028 * <pre>
029 * java.home = ${env:JAVA_HOME}
030 * </pre>
031 *
032 * <p>
033 * {@code EnvironmentLookup} is one of the standard lookups that is registered per default for each configuration.
034 * </p>
035 *
036 * @since 1.7
037 * @deprecated Use {@link DefaultLookups#ENVIRONMENT}.
038 */
039@Deprecated
040public class EnvironmentLookup implements Lookup {
041    /**
042     * Performs a lookup for the specified variable. This implementation directly delegates to a {@code System.getenv()}.
043     *
044     * @param key the key to lookup
045     * @return the value of this key or <b>null</b> if it cannot be resolved
046     */
047    @Override
048    public String lookup(final String key) {
049        return System.getenv(key);
050    }
051}