src/main/java/org/apache/commons/configuration2/Configuration.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.commons.configuration2;

import java.util.Collection;
import java.util.Map;

import org.apache.commons.configuration2.interpol.ConfigurationInterpolator;
import org.apache.commons.configuration2.interpol.Lookup;
import org.apache.commons.configuration2.sync.SynchronizerSupport;

/**
 * <p>
 * The main Configuration interface.
 * </p>
 * <p>
 * This interface allows accessing and manipulating a configuration object. The major part of the methods defined in
 * this interface deals with accessing properties of various data types. There is a generic {@code getProperty()}
 * method, which returns the value of the queried property in its raw data type. Other getter methods try to convert
 * this raw data type into a specific data type. If this fails, a {@code ConversionException} will be thrown.
 * </p>
 * <p>
 * For most of the property getter methods an overloaded version exists that allows to specify a default value, which
 * will be returned if the queried property cannot be found in the configuration. The behavior of the methods that do
 * not take a default value in case of a missing property is not defined by this interface and depends on a concrete
 * implementation. E.g. the {@link AbstractConfiguration} class, which is the base class of most configuration
 * implementations provided by this package, per default returns <strong>null</strong> if a property is not found, but provides
 * the {@link AbstractConfiguration#setThrowExceptionOnMissing(boolean) setThrowExceptionOnMissing()} method, with which
 * it can be configured to throw a {@code NoSuchElementException} exception in that case. (Note that getter methods for
 * primitive types in {@code AbstractConfiguration} always throw an exception for missing properties because there is no
 * way of overloading the return value.)
 * </p>
 * <p>
 * With the {@code addProperty()} and {@code setProperty()} methods new properties can be added to a configuration or
 * the values of properties can be changed. With {@code clearProperty()} a property can be removed. Other methods allow
 * to iterate over the contained properties or to create a subset configuration.
 * </p>
 */
public interface Configuration extends ImmutableConfiguration, SynchronizerSupport {

    /**
     * Add a property to the configuration. If it already exists then the value stated here will be added to the
     * configuration entry. For example, if the property:
     *
     * <pre>
     * resource.loader = file
     * </pre>
     *
     * is already present in the configuration and you call
     *
     * <pre>
     * addProperty("resource.loader", "classpath")
     * </pre>
     *
     * Then you will end up with a List like the following:
     *
     * <pre>
     * ["file", "classpath"]
     * </pre>
     *
     * @param key The key to add the property to.
     * @param value The value to add.
     */
    void addProperty(String key, Object value);

    /**
     * Remove all properties from the configuration.
     */
    void clear();

    /**
     * Remove a property from the configuration.
     *
     * @param key the key to remove along with corresponding value.
     */
    void clearProperty(String key);

    /**
     * Gets the {@code ConfigurationInterpolator} object used by this {@code Configuration}. This object is responsible
     * for variable substitution.
     *
     * @return the {@code ConfigurationInterpolator} (can be <strong>null</strong>)
     */
    ConfigurationInterpolator getInterpolator();

    /**
     * Creates and installs a new {@code ConfigurationInterpolator} for this {@code Configuration} based on the passed in
     * arguments. This method creates a default {@code ConfigurationInterpolator} instance and initializes it with the
     * passed in {@code Lookup} objects. It also adds a special default {@code Lookup} object that tries to resolve
     * variables by matching them with properties contained in this {@code Configuration}. This is also the main difference
     * to the {@link #setInterpolator(ConfigurationInterpolator)} method which sets the passed in object as is without
     * adding this special lookup.
     *
     * @param prefixLookups the map with {@code Lookup} objects associated with specific prefixes (can be <strong>null</strong>)
     * @param defLookups a collection with default {@code Lookup} objects (can be <strong>null</strong>)
     * @see ConfigurationInterpolator
     */
    void installInterpolator(Map<String, ? extends Lookup> prefixLookups, Collection<? extends Lookup> defLookups);

    /**
     * Sets the {@code ConfigurationInterpolator} object to be used by this {@code Configuration}. This object is invoked
     * for each access of a string property in order to substitute variables which may be contained. The argument can be
     * <strong>null</strong> to disable interpolation at all.
     *
     * @param ci the new {@code ConfigurationInterpolator}
     */
    void setInterpolator(ConfigurationInterpolator ci);

    /**
     * Sets a property, this will replace any previously set values. Set values is implicitly a call to clearProperty(key),
     * addProperty(key, value).
     *
     * @param key The key of the property to change
     * @param value The new value
     */
    void setProperty(String key, Object value);

    /**
     * Return a decorator Configuration containing every key from the current Configuration that starts with the specified
     * prefix. The prefix is removed from the keys in the subset. For example, if the configuration contains the following
     * properties:
     *
     * <pre>
     *    prefix.number = 1
     *    prefix.string = Apache
     *    prefixed.foo = bar
     *    prefix = Jakarta
     * </pre>
     *
     * the Configuration returned by {@code subset("prefix")} will contain the properties:
     *
     * <pre>
     *    number = 1
     *    string = Apache
     *    = Jakarta
     * </pre>
     *
     * (The key for the value "Jakarta" is an empty string)
     * <p>
     * Since the subset is a decorator and not a modified copy of the initial Configuration, any change made to the subset
     * is available to the Configuration, and reciprocally.
     *
     * @param prefix The prefix used to select the properties.
     * @return a subset configuration
     * @see SubsetConfiguration
     */
    Configuration subset(String prefix);
}