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   /*
    * Copyright 2019 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.handler.address;
  
  import io.netty.channel.ChannelFuture;
  import io.netty.channel.ChannelFutureListener;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.channel.ChannelOutboundHandler;
  import io.netty.channel.ChannelOutboundHandlerAdapter;
  import io.netty.channel.ChannelPromise;
  
  import java.net.NetworkInterface;
  import java.net.SocketAddress;
  
  /**
   * {@link ChannelOutboundHandler} implementation which allows to dynamically replace the used
   * {@code remoteAddress} and / or {@code localAddress} when making a connection attempt.
   * <p>
   * This can be useful to for example bind to a specific {@link NetworkInterface} based on
   * the {@code remoteAddress}.
   */
  public abstract class DynamicAddressConnectHandler extends ChannelOutboundHandlerAdapter {
  
      @Override
      public final void connect(ChannelHandlerContext ctx, SocketAddress remoteAddress,
                                SocketAddress localAddress, ChannelPromise promise) {
          final SocketAddress remote;
          final SocketAddress local;
          try {
              remote = remoteAddress(remoteAddress, localAddress);
              local = localAddress(remoteAddress, localAddress);
          } catch (Exception e) {
              promise.setFailure(e);
              return;
          }
          ctx.connect(remote, local, promise).addListener(new ChannelFutureListener() {
              @Override
              public void operationComplete(ChannelFuture future) {
                  if (future.isSuccess()) {
                      // We only remove this handler from the pipeline once the connect was successful as otherwise
                      // the user may try to connect again.
                      future.channel().pipeline().remove(DynamicAddressConnectHandler.this);
                  }
              }
          });
      }
  
      /**
       * Returns the local {@link SocketAddress} to use for
       * {@link ChannelHandlerContext#connect(SocketAddress, SocketAddress)} based on the original {@code remoteAddress}
       * and {@code localAddress}.
       * By default, this method returns the given {@code localAddress}.
       */
      protected SocketAddress localAddress(
              @SuppressWarnings("unused") SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
          return localAddress;
      }
  
      /**
       * Returns the remote {@link SocketAddress} to use for
       * {@link ChannelHandlerContext#connect(SocketAddress, SocketAddress)} based on the original {@code remoteAddress}
       * and {@code localAddress}.
       * By default, this method returns the given {@code remoteAddress}.
       */
      protected SocketAddress remoteAddress(
              SocketAddress remoteAddress, @SuppressWarnings("unused") SocketAddress localAddress) throws Exception {
          return remoteAddress;
      }
  }