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   /*
    * Copyright 2012 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.channel;
  
  import io.netty.bootstrap.Bootstrap;
  import io.netty.bootstrap.ServerBootstrap;
  import io.netty.channel.ChannelHandler.Sharable;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.util.Collections;
  import java.util.Set;
  import java.util.concurrent.ConcurrentHashMap;
  
  /**
   * A special {@link ChannelInboundHandler} which offers an easy way to initialize a {@link Channel} once it was
   * registered to its {@link EventLoop}.
   *
   * Implementations are most often used in the context of {@link Bootstrap#handler(ChannelHandler)} ,
   * {@link ServerBootstrap#handler(ChannelHandler)} and {@link ServerBootstrap#childHandler(ChannelHandler)} to
   * setup the {@link ChannelPipeline} of a {@link Channel}.
   *
   * <pre>
   *
   * public class MyChannelInitializer extends {@link ChannelInitializer} {
   *     public void initChannel({@link Channel} channel) {
   *         channel.pipeline().addLast("myHandler", new MyHandler());
   *     }
   * }
   *
   * {@link ServerBootstrap} bootstrap = ...;
   * ...
   * bootstrap.childHandler(new MyChannelInitializer());
   * ...
   * </pre>
   * Be aware that this class is marked as {@link Sharable} and so the implementation must be safe to be re-used.
   *
   * @param <C>   A sub-type of {@link Channel}
   */
  @Sharable
  public abstract class ChannelInitializer<C extends Channel> extends ChannelInboundHandlerAdapter {
  
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(ChannelInitializer.class);
      // We use a Set as a ChannelInitializer is usually shared between all Channels in a Bootstrap /
      // ServerBootstrap. This way we can reduce the memory usage compared to use Attributes.
      private final Set<ChannelHandlerContext> initMap = Collections.newSetFromMap(
              new ConcurrentHashMap<ChannelHandlerContext, Boolean>());
  
      /**
       * This method will be called once the {@link Channel} was registered. After the method returns this instance
       * will be removed from the {@link ChannelPipeline} of the {@link Channel}.
       *
       * @param ch            the {@link Channel} which was registered.
       * @throws Exception    is thrown if an error occurs. In that case it will be handled by
       *                      {@link #exceptionCaught(ChannelHandlerContext, Throwable)} which will by default close
       *                      the {@link Channel}.
       */
      protected abstract void initChannel(C ch) throws Exception;
  
      @Override
      @SuppressWarnings("unchecked")
      public final void channelRegistered(ChannelHandlerContext ctx) throws Exception {
          // Normally this method will never be called as handlerAdded(...) should call initChannel(...) and remove
          // the handler.
          if (initChannel(ctx)) {
              // we called initChannel(...) so we need to call now pipeline.fireChannelRegistered() to ensure we not
              // miss an event.
              ctx.pipeline().fireChannelRegistered();
  
              // We are done with init the Channel, removing all the state for the Channel now.
              removeState(ctx);
          } else {
              // Called initChannel(...) before which is the expected behavior, so just forward the event.
              ctx.fireChannelRegistered();
          }
      }
  
      /**
       * Handle the {@link Throwable} by logging and closing the {@link Channel}. Sub-classes may override this.
       */
      @Override
      public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
          if (logger.isWarnEnabled()) {
              logger.warn("Failed to initialize a channel. Closing: " + ctx.channel(), cause);
          }
          ctx.close();
     }
 
     /**
      * {@inheritDoc} If override this method ensure you call super!
      */
     @Override
     public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
         if (ctx.channel().isRegistered()) {
             // This should always be true with our current DefaultChannelPipeline implementation.
             // The good thing about calling initChannel(...) in handlerAdded(...) is that there will be no ordering
             // surprises if a ChannelInitializer will add another ChannelInitializer. This is as all handlers
             // will be added in the expected order.
             if (initChannel(ctx)) {
 
                 // We are done with init the Channel, removing the initializer now.
                 removeState(ctx);
             }
         }
     }
 
     @Override
     public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
         initMap.remove(ctx);
     }
 
     @SuppressWarnings("unchecked")
     private boolean initChannel(ChannelHandlerContext ctx) throws Exception {
         if (initMap.add(ctx)) { // Guard against re-entrance.
             try {
                 initChannel((C) ctx.channel());
             } catch (Throwable cause) {
                 // Explicitly call exceptionCaught(...) as we removed the handler before calling initChannel(...).
                 // We do so to prevent multiple calls to initChannel(...).
                 exceptionCaught(ctx, cause);
             } finally {
                 if (!ctx.isRemoved()) {
                     ctx.pipeline().remove(this);
                 }
             }
             return true;
         }
         return false;
     }
 
     private void removeState(final ChannelHandlerContext ctx) {
         // The removal may happen in an async fashion if the EventExecutor we use does something funky.
         if (ctx.isRemoved()) {
             initMap.remove(ctx);
         } else {
             // The context is not removed yet which is most likely the case because a custom EventExecutor is used.
             // Let's schedule it on the EventExecutor to give it some more time to be completed in case it is offloaded.
             ctx.executor().execute(new Runnable() {
                 @Override
                 public void run() {
                     initMap.remove(ctx);
                 }
             });
         }
     }
 }