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 */
017
018package org.apache.commons.configuration2;
019
020import java.util.HashMap;
021
022/**
023 * <p>
024 * A Configuration implementation that reads the platform specific environment variables using the map returned by
025 * {@link System#getenv()}.
026 * </p>
027 *
028 * <p>
029 * This configuration implementation is read-only. It allows read access to the defined OS environment variables, but
030 * their values cannot be changed. Any attempts to add or remove a property will throw an
031 * {@link UnsupportedOperationException}
032 * </p>
033 *
034 * <p>
035 * Usage of this class is easy: After an instance has been created the get methods provided by the {@code Configuration}
036 * interface can be used for querying environment variables, for example:
037 * </p>
038 *
039 * <pre>
040 * Configuration envConfig = new EnvironmentConfiguration();
041 * System.out.println("JAVA_HOME=" + envConfig.getString("JAVA_HOME");
042 * </pre>
043 *
044 * @since 1.5
045 */
046public class EnvironmentConfiguration extends MapConfiguration {
047
048    /**
049     * Create a Configuration based on the environment variables.
050     *
051     * @see System#getenv()
052     */
053    public EnvironmentConfiguration() {
054        super(new HashMap<>(System.getenv()));
055    }
056
057    /**
058     * Adds a property to this configuration. Because this configuration is read-only, this operation is not allowed and
059     * will cause an exception.
060     *
061     * @param key the key of the property to be added
062     * @param value the property value
063     */
064    @Override
065    protected void addPropertyDirect(final String key, final Object value) {
066        throw new UnsupportedOperationException("EnvironmentConfiguration is read-only!");
067    }
068
069    /**
070     * Removes all properties from this configuration. Because this configuration is read-only, this operation is not
071     * allowed and will cause an exception.
072     */
073    @Override
074    protected void clearInternal() {
075        throw new UnsupportedOperationException("EnvironmentConfiguration is read-only!");
076    }
077
078    /**
079     * Removes a property from this configuration. Because this configuration is read-only, this operation is not allowed
080     * and will cause an exception.
081     *
082     * @param key the key of the property to be removed
083     */
084    @Override
085    protected void clearPropertyDirect(final String key) {
086        throw new UnsupportedOperationException("EnvironmentConfiguration is read-only!");
087    }
088}