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   /*
    * Copyright 2015 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;
  
  import io.netty.util.concurrent.EventExecutor;
  import io.netty.util.concurrent.Future;
  import io.netty.util.concurrent.FutureListener;
  import io.netty.util.concurrent.Promise;
  import io.netty.util.internal.ObjectUtil;
  
  import java.util.Arrays;
  import java.util.List;
  
  import static io.netty.util.internal.ObjectUtil.checkNotNull;
  
  /**
   * A composite {@link SimpleNameResolver} that resolves a host name against a sequence of {@link NameResolver}s.
   *
   * In case of a failure, only the last one will be reported.
   */
  public final class CompositeNameResolver<T> extends SimpleNameResolver<T> {
  
      private final NameResolver<T>[] resolvers;
  
      /**
       * @param executor the {@link EventExecutor} which is used to notify the listeners of the {@link Future} returned
       *                 by {@link #resolve(String)}
       * @param resolvers the {@link NameResolver}s to be tried sequentially
       */
      public CompositeNameResolver(EventExecutor executor, NameResolver<T>... resolvers) {
          super(executor);
          checkNotNull(resolvers, "resolvers");
          for (int i = 0; i < resolvers.length; i++) {
              ObjectUtil.checkNotNull(resolvers[i], "resolvers[" + i + ']');
          }
          if (resolvers.length < 2) {
              throw new IllegalArgumentException("resolvers: " + Arrays.asList(resolvers) +
                      " (expected: at least 2 resolvers)");
          }
          this.resolvers = resolvers.clone();
      }
  
      @Override
      protected void doResolve(String inetHost, Promise<T> promise) throws Exception {
          doResolveRec(inetHost, promise, 0, null);
      }
  
      private void doResolveRec(final String inetHost,
                                final Promise<T> promise,
                                final int resolverIndex,
                                Throwable lastFailure) throws Exception {
          if (resolverIndex >= resolvers.length) {
              promise.setFailure(lastFailure);
          } else {
              NameResolver<T> resolver = resolvers[resolverIndex];
              resolver.resolve(inetHost).addListener(new FutureListener<T>() {
                  @Override
                  public void operationComplete(Future<T> future) throws Exception {
                      if (future.isSuccess()) {
                          promise.setSuccess(future.getNow());
                      } else {
                          doResolveRec(inetHost, promise, resolverIndex + 1, future.cause());
                      }
                  }
              });
          }
      }
  
      @Override
      protected void doResolveAll(String inetHost, Promise<List<T>> promise) throws Exception {
          doResolveAllRec(inetHost, promise, 0, null);
      }
  
      private void doResolveAllRec(final String inetHost,
                                final Promise<List<T>> promise,
                                final int resolverIndex,
                                Throwable lastFailure) throws Exception {
          if (resolverIndex >= resolvers.length) {
              promise.setFailure(lastFailure);
          } else {
              NameResolver<T> resolver = resolvers[resolverIndex];
              resolver.resolveAll(inetHost).addListener(new FutureListener<List<T>>() {
                  @Override
                  public void operationComplete(Future<List<T>> future) throws Exception {
                      if (future.isSuccess()) {
                          promise.setSuccess(future.getNow());
                     } else {
                         doResolveAllRec(inetHost, promise, resolverIndex + 1, future.cause());
                     }
                 }
             });
         }
     }
 }