org.eclipse.microprofile.config.spi.ConfigProviderResolver Maven / Gradle / Ivy

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/*
 *******************************************************************************
 * Copyright (c) 2016-2020 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.
 *******************************************************************************/

package org.eclipse.microprofile.config.spi;

import java.util.Iterator;
import java.util.ServiceLoader;

import org.eclipse.microprofile.config.Config;

/**
 * This class is not intended to be used by end-users but for
 * portable container integration purpose only.
 *
 * Service provider for ConfigProviderResolver. The implementation registers
 * itself via the {@link java.util.ServiceLoader} mechanism.
 *
 * @author Mark Struberg
 * @author Romain Manni-Bucau
 * @author Emily Jiang
 */
public abstract class ConfigProviderResolver {
    protected ConfigProviderResolver() {
    }

    private static volatile ConfigProviderResolver instance = null;

    /**
     * @see org.eclipse.microprofile.config.ConfigProvider#getConfig()
     * @return config the config object for the Thread Context Classloader
     */
    public abstract Config getConfig();

    /**
     * @see org.eclipse.microprofile.config.ConfigProvider#getConfig(ClassLoader)
     * @param loader the classloader
     * @return config the config object for the specified classloader
     */
    public abstract Config getConfig(ClassLoader loader);

    /**
     * Create a fresh {@link ConfigBuilder} instance.
     *
     * This ConfigBuilder will initially contain no {@link ConfigSource}. The other {@link ConfigSource} will have
     * to be added manually or discovered by calling {@link ConfigBuilder#addDiscoveredSources()}.
     *
     * This ConfigBuilder will initially contain default {@link Converter Converters}. Any other converters will need to
     * be added manually.
     *
     * The ConfigProvider will not manage the Config instance internally
     * @return a fresh ConfigBuilder
     */
    public abstract ConfigBuilder getBuilder();

    /**
     * Register a given {@link Config} within the Application (or Module) identified by the given ClassLoader.
     * If the ClassLoader is {@code null} then the current Application will be used.
     *
     * @param config
     *          which should get registered
     * @param classLoader
     *          which identifies the Application or Module the given Config should get associated with.
     *
     * @throws IllegalStateException
     *          if there is already a Config registered within the Application.
     *          A user could explicitly use {@link #releaseConfig(Config)} for this case.
     */
    public abstract void registerConfig(Config config, ClassLoader classLoader);

    /**
     * A {@link Config} normally gets released if the Application it is associated with gets destroyed.
     * Invoke this method if you like to destroy the Config prematurely.
     *
     * If the given Config is associated within an Application then it will be unregistered.
     * @param config the config to be released
     */
    public abstract void releaseConfig(Config config);

    /**
     * Creates a ConfigProviderResolver object
     * Only used internally from within {@link org.eclipse.microprofile.config.ConfigProvider}
     * @return ConfigProviderResolver an instance of ConfigProviderResolver
     */
    public static ConfigProviderResolver instance() {
        if (instance == null) {
            synchronized (ConfigProviderResolver.class) {
                if (instance != null) {
                    return instance;
                }
                instance = loadSpi(ConfigProviderResolver.class.getClassLoader());
            }
        }

        return instance;
    }


    private static ConfigProviderResolver loadSpi(ClassLoader cl) {
        ServiceLoader sl = ServiceLoader.load(
                        ConfigProviderResolver.class, cl);
        final Iterator iterator = sl.iterator();
        if (iterator.hasNext()) {
            return iterator.next();
        }
        throw new IllegalStateException(
                        "No ConfigProviderResolver implementation found!");
    }

    /**
     * Set the instance. It is used by OSGi environment while service loader
     * pattern is not supported.
     *
     * @param resolver
     *            set the instance.
     */
    public static void setInstance(ConfigProviderResolver resolver) {
        instance = resolver;
    }
}