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   /*
    * Copyright 2017 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.dns;
  
  import java.util.List;
  
  /**
   * A {@link DnsServerAddressStreamProvider} which iterates through a collection of
   * {@link DnsServerAddressStreamProvider} until the first non-{@code null} result is found.
   */
  public final class MultiDnsServerAddressStreamProvider implements DnsServerAddressStreamProvider {
      private final DnsServerAddressStreamProvider[] providers;
  
      /**
       * Create a new instance.
       * @param providers The providers to use for DNS resolution. They will be queried in order.
       */
      public MultiDnsServerAddressStreamProvider(List<DnsServerAddressStreamProvider> providers) {
          this.providers = providers.toArray(new DnsServerAddressStreamProvider[0]);
      }
  
      /**
       * Create a new instance.
       * @param providers The providers to use for DNS resolution. They will be queried in order.
       */
      public MultiDnsServerAddressStreamProvider(DnsServerAddressStreamProvider... providers) {
          this.providers = providers.clone();
      }
  
      @Override
      public DnsServerAddressStream nameServerAddressStream(String hostname) {
          for (DnsServerAddressStreamProvider provider : providers) {
              DnsServerAddressStream stream = provider.nameServerAddressStream(hostname);
              if (stream != null) {
                  return stream;
              }
          }
          return null;
      }
  }