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Pax Logging backend implementation based on Apache Log4J2. It provides Log4J2 specific implementation of PaxLoggingService interface and Log4J2 specific configuration methods. Users may customize Log4J2 behaviour (appenders, layouts) by creating fragment attached to this bundle.

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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.Properties;
import java.util.concurrent.CopyOnWriteArraySet;

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

/**
 * Consider this class private.
 */
public final class ProviderUtil {

    /**
     * Resource name for a Log4j 2 provider properties file.
     */
    protected static final String PROVIDER_RESOURCE = "META-INF/log4j-provider.properties";
    private static final String API_VERSION = "Log4jAPIVersion";

    private static final String[] COMPATIBLE_API_VERSIONS = {
        "2.0.0"
    };

    private static final Logger LOGGER = StatusLogger.getLogger();

    private static final Collection PROVIDERS = new CopyOnWriteArraySet();

    private ProviderUtil() {
    }

    static {
        final ClassLoader cl = findClassLoader();
        Enumeration enumResources = null;
        try {
            enumResources = cl.getResources(PROVIDER_RESOURCE);
        } catch (final IOException e) {
            LOGGER.fatal("Unable to locate {}", PROVIDER_RESOURCE, e);
        }
        loadProviders(enumResources, cl);
    }

    protected static void loadProviders(final Enumeration enumResources, ClassLoader cl) {
        if (enumResources != null) {
            while (enumResources.hasMoreElements()) {
                final URL url = enumResources.nextElement();
                loadProvider(url, cl);
            }
        }
    }

    /**
     * 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);
        }
    }

    public static Iterable getProviders() {
        return PROVIDERS;
    }

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

    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;
    }
}