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/** 021 * <p> 022 * A specialized lookup implementation that allows access to environment 023 * variables. 024 * </p> 025 * <p> 026 * This implementation relies on {@link System#getenv()} to resolve 027 * environment variables. It can be used for referencing environment variables 028 * in configuration files in an easy way, for instance: 029 * </p> 030 * 031 * <pre> 032 * java.home = ${env:JAVA_HOME} 033 * </pre> 034 * 035 * <p> 036 * {@code EnvironmentLookup} is one of the standard lookups that is 037 * registered per default for each configuration. 038 * </p> 039 * 040 * @author <a 041 * href="http://commons.apache.org/configuration/team-list.html">Commons 042 * Configuration team</a> 043 * @since 1.7 044 * @version $Id: EnvironmentLookup.java 1679784 2015-05-16 17:46:21Z oheger $ 045 */ 046public class EnvironmentLookup implements Lookup 047{ 048 /** 049 * Performs a lookup for the specified variable. This implementation 050 * directly delegates to a {@code System.getenv()}. 051 * 052 * @param key the key to lookup 053 * @return the value of this key or <b>null</b> if it cannot be resolved 054 */ 055 @Override 056 public String lookup(String key) 057 { 058 return System.getenv(key); 059 } 060}