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1   /*
2    * Copyright 2015 The Netty Project
3    *
4    * The Netty Project licenses this file to you under the Apache License,
5    * version 2.0 (the "License"); you may not use this file except in compliance
6    * with the License. You may obtain a copy of the License at:
7    *
8    *   http://www.apache.org/licenses/LICENSE-2.0
9    *
10   * Unless required by applicable law or agreed to in writing, software
11   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
12   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
13   * License for the specific language governing permissions and limitations
14   * under the License.
15   */
16  package io.netty.resolver;
17  
18  import io.netty.util.concurrent.EventExecutor;
19  import io.netty.util.concurrent.Future;
20  import io.netty.util.internal.UnstableApi;
21  
22  import java.net.InetAddress;
23  import java.net.InetSocketAddress;
24  
25  /**
26   * A skeletal {@link NameResolver} implementation that resolves {@link InetAddress}.
27   */
28  @UnstableApi
29  public abstract class InetNameResolver extends SimpleNameResolver<InetAddress> {
30      private volatile AddressResolver<InetSocketAddress> addressResolver;
31  
32      /**
33       * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned
34       *                 by {@link #resolve(String)}
35       */
36      protected InetNameResolver(EventExecutor executor) {
37          super(executor);
38      }
39  
40      /**
41       * Return a {@link AddressResolver} that will use this name resolver underneath.
42       * It's cached internally, so the same instance is always returned.
43       */
44      public AddressResolver<InetSocketAddress> asAddressResolver() {
45          AddressResolver<InetSocketAddress> result = addressResolver;
46          if (result == null) {
47              synchronized (this) {
48                  result = addressResolver;
49                  if (result == null) {
50                      addressResolver = result = new InetSocketAddressResolver(executor(), this);
51                  }
52              }
53          }
54          return result;
55      }
56  }