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   /*
    * Copyright 2017 The Netty Project
    *
    * The Netty Project 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:
    *
    *   https://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 io.netty.resolver.dns;
  
  import io.netty.util.internal.PlatformDependent;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.lang.reflect.Constructor;
  import java.lang.reflect.InvocationTargetException;
  import java.security.AccessController;
  import java.security.PrivilegedAction;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicLong;
  
  /**
   * Utility methods related to {@link DnsServerAddressStreamProvider}.
   */
  public final class DnsServerAddressStreamProviders {
  
      private static final InternalLogger LOGGER =
              InternalLoggerFactory.getInstance(DnsServerAddressStreamProviders.class);
      private static final Constructor<? extends DnsServerAddressStreamProvider> STREAM_PROVIDER_CONSTRUCTOR;
      private static final String MACOS_PROVIDER_CLASS_NAME =
              "io.netty.resolver.dns.macos.MacOSDnsServerAddressStreamProvider";
  
      static {
          Constructor<? extends DnsServerAddressStreamProvider> constructor = null;
          if (PlatformDependent.isOsx()) {
              try {
                  // As MacOSDnsServerAddressStreamProvider is contained in another jar which depends on this jar
                  // we use reflection to use it if its on the classpath.
                  Object maybeProvider = AccessController.doPrivileged(new PrivilegedAction<Object>() {
                      @Override
                      public Object run() {
                          try {
                              return Class.forName(
                                      MACOS_PROVIDER_CLASS_NAME,
                                      true,
                                      DnsServerAddressStreamProviders.class.getClassLoader());
                          } catch (Throwable cause) {
                              return cause;
                          }
                      }
                  });
                  if (maybeProvider instanceof Class) {
                      @SuppressWarnings("unchecked")
                      Class<? extends DnsServerAddressStreamProvider> providerClass =
                              (Class<? extends DnsServerAddressStreamProvider>) maybeProvider;
                      constructor = providerClass.getConstructor();
                      constructor.newInstance();  // ctor ensures availability
                      LOGGER.debug("{}: available", MACOS_PROVIDER_CLASS_NAME);
                  } else {
                      throw (Throwable) maybeProvider;
                  }
              } catch (ClassNotFoundException cause) {
                  LOGGER.warn("Can not find {} in the classpath, fallback to system defaults. This may result in "
                          + "incorrect DNS resolutions on MacOS. Check whether you have a dependency on "
                          + "'io.netty:netty-resolver-dns-native-macos'", MACOS_PROVIDER_CLASS_NAME);
              } catch (Throwable cause) {
                  if (LOGGER.isDebugEnabled()) {
                      LOGGER.error("Unable to load {}, fallback to system defaults. This may result in "
                              + "incorrect DNS resolutions on MacOS. Check whether you have a dependency on "
                              + "'io.netty:netty-resolver-dns-native-macos'", MACOS_PROVIDER_CLASS_NAME, cause);
                  } else {
                      LOGGER.error("Unable to load {}, fallback to system defaults. This may result in "
                              + "incorrect DNS resolutions on MacOS. Check whether you have a dependency on "
                              + "'io.netty:netty-resolver-dns-native-macos'. Use DEBUG level to see the full stack: {}",
                              MACOS_PROVIDER_CLASS_NAME,
                              cause.getCause() != null ? cause.getCause().toString() : cause.toString());
                  }
                  constructor = null;
              }
          }
          STREAM_PROVIDER_CONSTRUCTOR = constructor;
      }
  
      private DnsServerAddressStreamProviders() {
      }
  
      /**
       * A {@link DnsServerAddressStreamProvider} which inherits the DNS servers from your local host's configuration.
       * <p>
       * Note that only macOS and Linux are currently supported.
       * @return A {@link DnsServerAddressStreamProvider} which inherits the DNS servers from your local host's
       * configuration.
      */
     public static DnsServerAddressStreamProvider platformDefault() {
         if (STREAM_PROVIDER_CONSTRUCTOR != null) {
             try {
                 return STREAM_PROVIDER_CONSTRUCTOR.newInstance();
             } catch (IllegalAccessException e) {
                 // ignore
             } catch (InstantiationException e) {
                 // ignore
             } catch (InvocationTargetException e) {
                 // ignore
             }
         }
         return unixDefault();
     }
 
     public static DnsServerAddressStreamProvider unixDefault() {
         return DefaultProviderHolder.DEFAULT_DNS_SERVER_ADDRESS_STREAM_PROVIDER;
     }
 
     // We use a Holder class to only initialize DEFAULT_DNS_SERVER_ADDRESS_STREAM_PROVIDER if we really
     // need it.
     private static final class DefaultProviderHolder {
         // We use 5 minutes which is the same as what OpenJDK is using in sun.net.dns.ResolverConfigurationImpl.
         private static final long REFRESH_INTERVAL = TimeUnit.MINUTES.toNanos(5);
 
         // TODO(scott): how is this done on Windows? This may require a JNI call to GetNetworkParams
         // https://msdn.microsoft.com/en-us/library/aa365968(VS.85).aspx.
         static final DnsServerAddressStreamProvider DEFAULT_DNS_SERVER_ADDRESS_STREAM_PROVIDER =
                 new DnsServerAddressStreamProvider() {
                     private volatile DnsServerAddressStreamProvider currentProvider = provider();
                     private final AtomicLong lastRefresh = new AtomicLong(System.nanoTime());
 
                     @Override
                     public DnsServerAddressStream nameServerAddressStream(String hostname) {
                         long last = lastRefresh.get();
                         DnsServerAddressStreamProvider current = currentProvider;
                         if (System.nanoTime() - last > REFRESH_INTERVAL) {
                             // This is slightly racy which means it will be possible still use the old configuration
                             // for a small amount of time, but that's ok.
                             if (lastRefresh.compareAndSet(last, System.nanoTime())) {
                                 current = currentProvider = provider();
                             }
                         }
                         return current.nameServerAddressStream(hostname);
                     }
 
                     private DnsServerAddressStreamProvider provider() {
                         // If on windows just use the DefaultDnsServerAddressStreamProvider.INSTANCE as otherwise
                         // we will log some error which may be confusing.
                         return PlatformDependent.isWindows() ? DefaultDnsServerAddressStreamProvider.INSTANCE :
                                 UnixResolverDnsServerAddressStreamProvider.parseSilently();
                     }
                 };
     }
 }