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