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.juneau.commons.settings;
018
019import java.util.*;
020
021/**
022 * Interface for pluggable property sources used by {@link Settings}.
023 *
024 * <p>
025 * A property source provides a way to retrieve property values.
026 * Sources are checked in reverse order (last added is checked first) when looking up properties.
027 *
028 * <p>
029 * For writable sources that support modifying property values, see {@link PropertyStore}.
030 */
031public interface PropertySource {
032
033   /**
034    * Returns a property in this property source.
035    *
036    * @param name The property name.
037    * @return The property lookup result:
038    *    <ul class='spaced-list'>
039    *       <li>{@link PropertyLookupResult#missing()} if this source does not define the key.
040    *       <li>{@link PropertyLookupResult#present(Optional)} with {@link Optional#empty()} if the key exists with a null value.
041    *       <li>{@link PropertyLookupResult#present(Optional)} with a value if the key exists with a non-null value.
042    *    </ul>
043    */
044   PropertyLookupResult get(String name);
045}