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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    *   https://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  
21  import java.net.InetAddress;
22  import java.net.InetSocketAddress;
23  
24  /**
25   * A skeletal {@link NameResolver} implementation that resolves {@link InetAddress}.
26   */
27  public abstract class InetNameResolver extends SimpleNameResolver<InetAddress> {
28      private volatile AddressResolver<InetSocketAddress> addressResolver;
29  
30      /**
31       * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned
32       *                 by {@link #resolve(String)}
33       */
34      protected InetNameResolver(EventExecutor executor) {
35          super(executor);
36      }
37  
38      /**
39       * Return a {@link AddressResolver} that will use this name resolver underneath.
40       * It's cached internally, so the same instance is always returned.
41       */
42      public AddressResolver<InetSocketAddress> asAddressResolver() {
43          AddressResolver<InetSocketAddress> result = addressResolver;
44          if (result == null) {
45              synchronized (this) {
46                  result = addressResolver;
47                  if (result == null) {
48                      addressResolver = result = new InetSocketAddressResolver(executor(), this);
49                  }
50              }
51          }
52          return result;
53      }
54  }