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   /*
    * Copyright 2023 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.bootstrap;
  
  import io.netty.channel.Channel;
  import io.netty.channel.ServerChannel;
  
  /**
   * A channel initializer extension make it possible to enforce rules and apply modifications across multiple,
   * disconnected uses of Netty within the same JVM process.
   * <p>
   * For instance, application-level firewall rules can be injected into all uses of Netty within an application,
   * without making changes to such uses that are otherwise outside the purview of the application code,
   * such as 3rd-party libraries.
   * <p>
   * Channel initializer extensions are <em>not</em> enabled by default, because of their power to influence Netty
   * pipelines across libraries, frameworks, and use-cases.
   * Extensions must be explicitly enabled by setting the {@value #EXTENSIONS_SYSTEM_PROPERTY} to {@code serviceload}.
   * <p>
   * All channel initializer extensions that are available on the classpath will be
   * {@linkplain java.util.ServiceLoader#load(Class) service-loaded} and used by all {@link AbstractBootstrap} subclasses.
   * <p>
   * Note that this feature will not work for Netty uses that are shaded <em>and relocated</em> into other libraries.
   * The classes in a relocated Netty library are technically distinct and incompatible types. This means the
   * service-loader in non-relocated Netty will not see types from a relocated Netty, and vice versa.
   */
  public abstract class ChannelInitializerExtension {
      /**
       * The name of the system property that control initializer extensions.
       * <p>
       * These extensions can potentially be a security liability, so they are disabled by default.
       * <p>
       * To enable the extensions, application operators can explicitly opt in by setting this system property to the
       * value {@code serviceload}. This will enable all the extensions that are available through the service loader
       * mechanism.
       * <p>
       * To load and log (at INFO level) all available extensions without actually running them, set this system property
       * to the value {@code log}.
       */
      public static final String EXTENSIONS_SYSTEM_PROPERTY = "io.netty.bootstrap.extensions";
  
      /**
       * Get the "priority" of this extension. If multiple extensions are avilable, then they will be called in their
       * priority order, from lowest to highest.
       * <p>
       * Implementers are encouraged to pick a number between {@code -100.0} and {@code 100.0}, where extensions that have
       * no particular opinion on their priority are encouraged to return {@code 0.0}.
       * <p>
       * Extensions with lower priority will get called first, while extensions with greater priority may be able to
       * observe the effects of extensions with lesser priority.
       * <p>
       * Note that if multiple extensions have the same priority, then their relative order will be unpredictable.
       * As such, implementations should always take into consideration that other extensions might be called before
       * or after them.
       * <p>
       * Override this method to specify your own priority.
       * The default implementation just returns {@code 0}.
       *
       * @return The priority.
       */
      public double priority() {
          return 0;
      }
  
      /**
       * Called by {@link Bootstrap} after the initialization of the given client channel.
       * <p>
       * The method is allowed to modify the handlers in the pipeline, the channel attributes, or the channel options.
       * The method must refrain from doing any I/O, or from closing the channel.
       * <p>
       * Override this method to add your own callback logic.
       * The default implementation does nothing.
       *
       * @param channel The channel that was initialized.
       */
      public void postInitializeClientChannel(Channel channel) {
      }
  
      /**
       * Called by {@link ServerBootstrap} after the initialization of the given server listener channel.
       * The listener channel is responsible for invoking the {@code accept(2)} system call,
       * and for producing child channels.
       * <p>
       * The method is allowed to modify the handlers in the pipeline, the channel attributes, or the channel options.
       * The method must refrain from doing any I/O, or from closing the channel.
       * <p>
      * Override this method to add your own callback logic.
      * The default implementation does nothing.
      *
      * @param channel The channel that was initialized.
      */
     public void postInitializeServerListenerChannel(ServerChannel channel) {
     }
 
     /**
      * Called by {@link ServerBootstrap} after the initialization of the given child channel.
      * A child channel is a newly established connection from a client to the server.
      * <p>
      * The method is allowed to modify the handlers in the pipeline, the channel attributes, or the channel options.
      * The method must refrain from doing any I/O, or from closing the channel.
      * <p>
      * Override this method to add your own callback logic.
      * The default implementation does nothing.
      *
      * @param channel The channel that was initialized.
      */
     public void postInitializeServerChildChannel(Channel channel) {
     }
 }