ConfigSource.java

/*
 ******************************************************************************
 * Copyright (c) 2009-2018 Contributors to the Eclipse Foundation
 *
 * See the NOTICE file(s) distributed with this work for additional
 * information regarding copyright ownership.
 *
 * Licensed 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
 *
 *     http://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.
 *
 * Contributors:
 *   2009       - Mark Struberg
 *      Ordinal solution in Apache OpenWebBeans
 *   2011-12-28 - Mark Struberg & Gerhard Petracek
 *      Contributed to Apache DeltaSpike fb0131106481f0b9a8fd
 *   2016-07-14 - Mark Struberg
 *      Extracted the Config part out of DeltaSpike and proposed as Microprofile-Config cf41cf130bcaf5447ff8
 *   2016-11-14 - Emily Jiang / IBM Corp
 *      Methods renamed, JavaDoc and cleanup
 *
 *******************************************************************************/
package jakarta.config.spi;

import java.util.Map;
import java.util.Set;
import java.util.function.Consumer;

/**
 * <p>Implement this interfaces to provide a ConfigSource.
 * A ConfigSource provides configuration values from a specific place, like JNDI configuration, a properties file, etc.
 * A ConfigSource is always read-only, any potential updates of the configured values must be handled directly inside each ConfigSource.
 *
 * <p>
 * The default config sources always available by default are:
 * <ol>
 * <li>System properties (ordinal=400)</li>
 * <li>Environment properties (ordinal=300)
 *    <p>Some operating systems allow only alphabetic characters or an underscore(_), in environment variables.
 *    Other characters such as ., /, etc may be disallowed.
 *    In order to set a value for a config property that has a name containing such disallowed characters from an environment variable,
 *    the following rules are used.
 *    This ConfigSource searches for potentially 3 environment variables with a given property name (e.g. {@code "com.ACME.size"}):</p>
 *        <ol>
 *            <li>Exact match (i.e. {@code "com.ACME.size"})</li>
 *            <li>Replace the character that is neither alphanumeric nor '_' with '_' (i.e. {@code "com_ACME_size"})</li>
 *            <li>Replace the character that is neither alphanumeric nor '_' with '_' and convert to upper case 
 *            (i.e. {@code "COM_ACME_SIZE"})</li>
 *        </ol>
 *    <p>The first environment variable that is found is returned by this ConfigSource.</p>
 * </li>
 * <li>/META-INF/javaconfig.properties (ordinal=100)</li>
 * </ol>
 *
 * <p>Custom ConfigSource will get picked up via the {@link java.util.ServiceLoader} mechanism and and can be registered by
 * providing a file
 * <pre>
 *     META-INF/services/ConfigSource
 * </pre>
 * which contains the fully qualified {@code ConfigSource} implementation class name as content.
 *
 * <p>Adding a dynamic amount of custom config sources can be done programmatically via
 * {@link ConfigSourceProvider}.
 *
 *  <p>If a ConfigSource implements the {@link AutoCloseable} interface
 *  then the {@link AutoCloseable#close()} method will be called when
 *  the underlying {@link jakarta.config.Config} is being released.
 *
 *  <p> If you register the same {@code ConfigSource} instance within multiple {@link jakarta.config.Config} then non-portable behaviour results.
 *
 * @author <a href="mailto:struberg@apache.org">Mark Struberg</a>
 * @author <a href="mailto:gpetracek@apache.org">Gerhard Petracek</a>
 * @author <a href="mailto:emijiang@uk.ibm.com">Emily Jiang</a>
 * @author <a href="mailto:john.d.ament@gmail.com">John D. Ament</a>
 * @author <a href="mailto:tomas.langer@oracle.com">Tomas Langer</a>
 *
 */
public interface ConfigSource {
    String CONFIG_ORDINAL = "config_ordinal";
    int DEFAULT_ORDINAL = 100;

    /**
     * Return the properties in this config source.
     *
     * @return the map containing the properties in this config source if these can be scanned, or empty Map
     * @see #isScannable()
     */
    Map<String, String> getProperties();

    /**
     * Gets all property names known to this config source, without evaluating the values.
     *
     * For backwards compatibility, there is a default implementation that just returns the keys of {@code getProperties()}
     * slower ConfigSource implementations should replace this with a more performant implementation
     *
     * @return the set of property keys that are known to this config source if these can be scanned or empty Set
     * @see #isScannable()
     */
    default Set<String> getPropertyNames() {
        return getProperties().keySet();
    }

    /**
     * Return the ordinal for this config source. If a property is specified in multiple config sources, the value
     * in the config source with the highest ordinal takes precedence.
     * For the config sources with the same ordinal value, the config source names will
     * be used for sorting according to string sorting criteria.
     * Note that this property only gets evaluated during ConfigSource discovery.
     *
     * The default ordinals for the default config sources:
     * <ol>
     *  <li>System properties (default ordinal=400)</li>
     *  <li>Environment properties (default ordinal=300)</li>
     *  <li>/META-INF/javaconfig.properties (default ordinal=100)</li>
     * </ol>
     *
     *
     * Any ConfigSource part of an application will typically use an ordinal between 0 and 200.
     * ConfigSource provided by the container or 'environment' typically use an ordinal higher than 200.
     * A framework which intends have values overwritten by the application will use ordinals between 0 and 100.
     * The property "config_ordinal" can be specified to override the default value.
     *
     * @return the ordinal value
     */
    default int getOrdinal() {
        String configOrdinal = getValue(CONFIG_ORDINAL);
        if (configOrdinal != null) {
            try {
                return Integer.parseInt(configOrdinal);
            }
            catch (NumberFormatException ignored) {

            }
        }
        return DEFAULT_ORDINAL;
    }

    /**
     * Return the value for the specified property in this config source.
     *
     * @param propertyName the property name
     * @return the property value, or {@code null} when property is not defined by this config source
     */
    String getValue(String propertyName);

    /**
     * The name of this config source might be used for logging or analysis of configured values.
     *
     * @return the 'name' of the configuration source, e.g. 'property-file mylocation/myproperty.properties'
     */
    String getName();


    /**
     * Determines if this config source should be scanned for its list of properties.
     *
     * Generally, slow ConfigSources should return false here.
     *
     * @return {@code true} if this ConfigSource should be scanned for its list of properties,
     *         {@code false} if it should not be scanned.
     */
    default boolean isScannable() {
        return true;
    }

    /**
     * The callback should get invoked if an attribute change got detected inside the ConfigSource.
     *
     * @param callback will be set by the {@link jakarta.config.Config} after this
     *                 {@code ConfigSource} got created and before any configured values
     *                 get served.
     * @return ChangeSupport informing the {@link jakarta.config.Config} implementation about support for changes by this source
     * @see ChangeSupport
     */
    default ChangeSupport setAttributeChangeCallback(Consumer<Set<String>> callback) {
        // do nothing by default. Just for compat with older ConfigSources.
        // return unsupported to tell config that it must re-query this source every time
        return ChangeSupport.UNSUPPORTED;
    }

    /**
     * What kind of change support this config source has.
     * <p>
     * {@link jakarta.config.Config} implementations may use this information for internal optimizations.
     */
    enum ChangeSupport {
        /**
         * Config change is supported, this config source will invoke the callback provided by
         * {@link ConfigSource#setAttributeChangeCallback(Consumer)}.
         * <p>
         * Example: File based config source that watches the file for changes
         */
        SUPPORTED,
        /**
         * Config change is not supported. Configuration values can change, though this change is not reported back.
         * <p>
         * Example: LDAP based config source
         */
        UNSUPPORTED,
        /**
         * Configuration values cannot change for the lifetime of this {@link ConfigSource}.
         * <p>
         * Example: Environment variables config source, classpath resource config source
         */
        IMMUTABLE
    }
}