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1   /*
2    * Copyright 2017 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.dns;
17  
18  import io.netty.util.internal.UnstableApi;
19  
20  import java.util.List;
21  
22  /**
23   * A {@link DnsServerAddressStreamProvider} which iterates through a collection of
24   * {@link DnsServerAddressStreamProvider} until the first non-{@code null} result is found.
25   */
26  @UnstableApi
27  public final class MultiDnsServerAddressStreamProvider implements DnsServerAddressStreamProvider {
28      private final DnsServerAddressStreamProvider[] providers;
29  
30      /**
31       * Create a new instance.
32       * @param providers The providers to use for DNS resolution. They will be queried in order.
33       */
34      public MultiDnsServerAddressStreamProvider(List<DnsServerAddressStreamProvider> providers) {
35          this.providers = providers.toArray(new DnsServerAddressStreamProvider[0]);
36      }
37  
38      /**
39       * Create a new instance.
40       * @param providers The providers to use for DNS resolution. They will be queried in order.
41       */
42      public MultiDnsServerAddressStreamProvider(DnsServerAddressStreamProvider... providers) {
43          this.providers = providers.clone();
44      }
45  
46      @Override
47      public DnsServerAddressStream nameServerAddressStream(String hostname) {
48          for (DnsServerAddressStreamProvider provider : providers) {
49              DnsServerAddressStream stream = provider.nameServerAddressStream(hostname);
50              if (stream != null) {
51                  return stream;
52              }
53          }
54          return null;
55      }
56  }