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The Pax Logging API Library is to allow for the Pax Logging Service to be reloaded without stopping the many dependent bundles. It also contains the OSGi Log Service API and the Knopflerfish Log API.
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/*
 * 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
 *
 *      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.apache.logging.log4j.util;

import java.io.IOException;
import java.net.URL;
import java.util.Collection;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.Properties;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.spi.Provider;
import org.apache.logging.log4j.status.StatusLogger;

/**
 * Consider this class private. Utility class for Log4j {@link Provider}s. When integrating with an application
 * container framework, any Log4j Providers not accessible through standard classpath scanning should
 * {@link #loadProvider(java.net.URL, ClassLoader)} a classpath accordingly.
 */
public final class ProviderUtil {

    /**
     * Resource name for a Log4j 2 provider properties file.
     */
    protected static final String PROVIDER_RESOURCE = "META-INF/log4j-provider.properties";

    /**
     * Loaded providers.
     */
    protected static final Collection PROVIDERS = new HashSet<>();

    /**
     * Guards the ProviderUtil singleton instance from lazy initialization. This is primarily used for OSGi support.
     *
     * @since 2.1
     */
    protected static final Lock STARTUP_LOCK = new ReentrantLock();

    private static final String API_VERSION = "Log4jAPIVersion";
    private static final String[] COMPATIBLE_API_VERSIONS = {"2.0.0", "2.1.0"};
    private static final Logger LOGGER = StatusLogger.getLogger();

    // STARTUP_LOCK guards INSTANCE for lazy initialization; this allows the OSGi Activator to pause the startup and
    // wait for a Provider to be installed. See LOG4J2-373
    private static volatile ProviderUtil instance;

    private ProviderUtil() {
        for (final LoaderUtil.UrlResource resource : LoaderUtil.findUrlResources(PROVIDER_RESOURCE)) {
            loadProvider(resource.getUrl(), resource.getClassLoader());
        }
    }

    /**
     * Loads an individual Provider implementation. This method is really only useful for the OSGi bundle activator and
     * this class itself.
     *
     * @param url the URL to the provider properties file
     * @param cl the ClassLoader to load the provider classes with
     */
    protected static void loadProvider(final URL url, final ClassLoader cl) {
        try {
            final Properties props = PropertiesUtil.loadClose(url.openStream(), url);
            if (validVersion(props.getProperty(API_VERSION))) {
                PROVIDERS.add(new Provider(props, url, cl));
            }
        } catch (final IOException e) {
            LOGGER.error("Unable to open {}", url, e);
        }
    }

    /**
     * @deprecated Use {@link #loadProvider(java.net.URL, ClassLoader)} instead. Will be removed in 3.0.
     */
    @Deprecated
    protected static void loadProviders(final Enumeration urls, final ClassLoader cl) {
        if (urls != null) {
            while (urls.hasMoreElements()) {
                loadProvider(urls.nextElement(), cl);
            }
        }
    }

    public static Iterable getProviders() {
        lazyInit();
        return PROVIDERS;
    }

    public static boolean hasProviders() {
        lazyInit();
        return !PROVIDERS.isEmpty();
    }

    /**
     * Lazily initializes the ProviderUtil singleton.
     *
     * @since 2.1
     */
    protected static void lazyInit() {
        // noinspection DoubleCheckedLocking
        if (instance == null) {
            try {
                STARTUP_LOCK.lockInterruptibly();
                try {
                    if (instance == null) {
                        instance = new ProviderUtil();
                    }
                } finally {
                    STARTUP_LOCK.unlock();
                }
            } catch (final InterruptedException e) {
                LOGGER.fatal("Interrupted before Log4j Providers could be loaded.", e);
                Thread.currentThread().interrupt();
            }
        }
    }

    public static ClassLoader findClassLoader() {
        return LoaderUtil.getThreadContextClassLoader();
    }

    private static boolean validVersion(final String version) {
        for (final String v : COMPATIBLE_API_VERSIONS) {
            if (version.startsWith(v)) {
                return true;
            }
        }
        return false;
    }
}