/*
    * 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;
  
  /**
   * <em>Consider this class private.</em>
   * 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";
      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();
  
      protected static final Collection<Provider> PROVIDERS = new HashSet<Provider>();
  
      /**
       * 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();
      // 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<URL> urls, final ClassLoader cl) {
          if (urls != null) {
              while (urls.hasMoreElements()) {
                  loadProvider(urls.nextElement(), cl);
              }
          }
      }
  
     public static Iterable<Provider> 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;
     }
 }