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   /*
    * Copyright 2015 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;
  
  import io.netty.util.concurrent.EventExecutor;
  import io.netty.util.concurrent.Future;
  
  import java.net.InetAddress;
  import java.net.InetSocketAddress;
  
  /**
   * A skeletal {@link NameResolver} implementation that resolves {@link InetAddress}.
   */
  public abstract class InetNameResolver extends SimpleNameResolver<InetAddress> {
      private volatile AddressResolver<InetSocketAddress> addressResolver;
  
      /**
       * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned
       *                 by {@link #resolve(String)}
       */
      protected InetNameResolver(EventExecutor executor) {
          super(executor);
      }
  
      /**
       * Return a {@link AddressResolver} that will use this name resolver underneath.
       * It's cached internally, so the same instance is always returned.
       */
      public AddressResolver<InetSocketAddress> asAddressResolver() {
          AddressResolver<InetSocketAddress> result = addressResolver;
          if (result == null) {
              synchronized (this) {
                  result = addressResolver;
                  if (result == null) {
                      addressResolver = result = new InetSocketAddressResolver(executor(), this);
                  }
              }
          }
          return result;
      }
  }