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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.buffer.ByteBuf;
  import io.netty.buffer.ByteBufAllocator;
  import io.netty.channel.socket.DatagramChannel;
  import io.netty.channel.socket.DatagramPacket;
  import io.netty.channel.socket.ServerSocketChannel;
  import io.netty.channel.socket.SocketChannel;
  import io.netty.util.AttributeMap;
  
  import java.net.InetSocketAddress;
  import java.net.SocketAddress;
  
  
  /**
   * A nexus to a network socket or a component which is capable of I/O
   * operations such as read, write, connect, and bind.
   * <p>
   * A channel provides a user:
   * <ul>
   * <li>the current state of the channel (e.g. is it open? is it connected?),</li>
   * <li>the {@linkplain ChannelConfig configuration parameters} of the channel (e.g. receive buffer size),</li>
   * <li>the I/O operations that the channel supports (e.g. read, write, connect, and bind), and</li>
   * <li>the {@link ChannelPipeline} which handles all I/O events and requests
   *     associated with the channel.</li>
   * </ul>
   *
   * <h3>All I/O operations are asynchronous.</h3>
   * <p>
   * All I/O operations in Netty are asynchronous.  It means any I/O calls will
   * return immediately with no guarantee that the requested I/O operation has
   * been completed at the end of the call.  Instead, you will be returned with
   * a {@link ChannelFuture} instance which will notify you when the requested I/O
   * operation has succeeded, failed, or canceled.
   *
   * <h3>Channels are hierarchical</h3>
   * <p>
   * A {@link Channel} can have a {@linkplain #parent() parent} depending on
   * how it was created.  For instance, a {@link SocketChannel}, that was accepted
   * by {@link ServerSocketChannel}, will return the {@link ServerSocketChannel}
   * as its parent on {@link #parent()}.
   * <p>
   * The semantics of the hierarchical structure depends on the transport
   * implementation where the {@link Channel} belongs to.  For example, you could
   * write a new {@link Channel} implementation that creates the sub-channels that
   * share one socket connection, as <a href="http://beepcore.org/">BEEP</a> and
   * <a href="https://en.wikipedia.org/wiki/Secure_Shell">SSH</a> do.
   *
   * <h3>Downcast to access transport-specific operations</h3>
   * <p>
   * Some transports exposes additional operations that is specific to the
   * transport.  Down-cast the {@link Channel} to sub-type to invoke such
   * operations.  For example, with the old I/O datagram transport, multicast
   * join / leave operations are provided by {@link DatagramChannel}.
   *
   * <h3>Release resources</h3>
   * <p>
   * It is important to call {@link #close()} or {@link #close(ChannelPromise)} to release all
   * resources once you are done with the {@link Channel}. This ensures all resources are
   * released in a proper way, i.e. filehandles.
   */
  public interface Channel extends AttributeMap, ChannelOutboundInvoker, Comparable<Channel> {
  
      /**
       * Returns the globally unique identifier of this {@link Channel}.
       */
      ChannelId id();
  
      /**
       * Return the {@link EventLoop} this {@link Channel} was registered to.
       */
      EventLoop eventLoop();
  
      /**
       * Returns the parent of this channel.
       *
       * @return the parent channel.
       *         {@code null} if this channel does not have a parent channel.
       */
      Channel parent();
  
      /**
       * Returns the configuration of this channel.
       */
     ChannelConfig config();
 
     /**
      * Returns {@code true} if the {@link Channel} is open and may get active later
      */
     boolean isOpen();
 
     /**
      * Returns {@code true} if the {@link Channel} is registered with an {@link EventLoop}.
      */
     boolean isRegistered();
 
     /**
      * Return {@code true} if the {@link Channel} is active and so connected.
      */
     boolean isActive();
 
     /**
      * Return the {@link ChannelMetadata} of the {@link Channel} which describe the nature of the {@link Channel}.
      */
     ChannelMetadata metadata();
 
     /**
      * Returns the local address where this channel is bound to.  The returned
      * {@link SocketAddress} is supposed to be down-cast into more concrete
      * type such as {@link InetSocketAddress} to retrieve the detailed
      * information.
      *
      * @return the local address of this channel.
      *         {@code null} if this channel is not bound.
      */
     SocketAddress localAddress();
 
     /**
      * Returns the remote address where this channel is connected to.  The
      * returned {@link SocketAddress} is supposed to be down-cast into more
      * concrete type such as {@link InetSocketAddress} to retrieve the detailed
      * information.
      *
      * @return the remote address of this channel.
      *         {@code null} if this channel is not connected.
      *         If this channel is not connected but it can receive messages
      *         from arbitrary remote addresses (e.g. {@link DatagramChannel},
      *         use {@link DatagramPacket#recipient()} to determine
      *         the origination of the received message as this method will
      *         return {@code null}.
      */
     SocketAddress remoteAddress();
 
     /**
      * Returns the {@link ChannelFuture} which will be notified when this
      * channel is closed.  This method always returns the same future instance.
      */
     ChannelFuture closeFuture();
 
     /**
      * Returns {@code true} if and only if the I/O thread will perform the
      * requested write operation immediately.  Any write requests made when
      * this method returns {@code false} are queued until the I/O thread is
      * ready to process the queued write requests.
      *
      * {@link WriteBufferWaterMark} can be used to configure on which condition
      * the write buffer would cause this channel to change writability.
      */
     boolean isWritable();
 
     /**
      * Get how many bytes can be written until {@link #isWritable()} returns {@code false}.
      * This quantity will always be non-negative. If {@link #isWritable()} is {@code false} then 0.
      *
      * {@link WriteBufferWaterMark} can be used to define writability settings.
      */
     long bytesBeforeUnwritable();
 
     /**
      * Get how many bytes must be drained from underlying buffers until {@link #isWritable()} returns {@code true}.
      * This quantity will always be non-negative. If {@link #isWritable()} is {@code true} then 0.
      *
      * {@link WriteBufferWaterMark} can be used to define writability settings.
      */
     long bytesBeforeWritable();
 
     /**
      * Returns an <em>internal-use-only</em> object that provides unsafe operations.
      */
     Unsafe unsafe();
 
     /**
      * Return the assigned {@link ChannelPipeline}.
      */
     ChannelPipeline pipeline();
 
     /**
      * Return the assigned {@link ByteBufAllocator} which will be used to allocate {@link ByteBuf}s.
      */
     ByteBufAllocator alloc();
 
     @Override
     Channel read();
 
     @Override
     Channel flush();
 
     /**
      * <em>Unsafe</em> operations that should <em>never</em> be called from user-code. These methods
      * are only provided to implement the actual transport, and must be invoked from an I/O thread except for the
      * following methods:
      * <ul>
      *   <li>{@link #localAddress()}</li>
      *   <li>{@link #remoteAddress()}</li>
      *   <li>{@link #closeForcibly()}</li>
      *   <li>{@link #register(EventLoop, ChannelPromise)}</li>
      *   <li>{@link #deregister(ChannelPromise)}</li>
      *   <li>{@link #voidPromise()}</li>
      * </ul>
      */
     interface Unsafe {
 
         /**
          * Return the assigned {@link RecvByteBufAllocator.Handle} which will be used to allocate {@link ByteBuf}'s when
          * receiving data.
          */
         RecvByteBufAllocator.Handle recvBufAllocHandle();
 
         /**
          * Return the {@link SocketAddress} to which is bound local or
          * {@code null} if none.
          */
         SocketAddress localAddress();
 
         /**
          * Return the {@link SocketAddress} to which is bound remote or
          * {@code null} if none is bound yet.
          */
         SocketAddress remoteAddress();
 
         /**
          * Register the {@link Channel} of the {@link ChannelPromise} and notify
          * the {@link ChannelFuture} once the registration was complete.
          */
         void register(EventLoop eventLoop, ChannelPromise promise);
 
         /**
          * Bind the {@link SocketAddress} to the {@link Channel} of the {@link ChannelPromise} and notify
          * it once its done.
          */
         void bind(SocketAddress localAddress, ChannelPromise promise);
 
         /**
          * Connect the {@link Channel} of the given {@link ChannelFuture} with the given remote {@link SocketAddress}.
          * If a specific local {@link SocketAddress} should be used it need to be given as argument. Otherwise just
          * pass {@code null} to it.
          *
          * The {@link ChannelPromise} will get notified once the connect operation was complete.
          */
         void connect(SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise);
 
         /**
          * Disconnect the {@link Channel} of the {@link ChannelFuture} and notify the {@link ChannelPromise} once the
          * operation was complete.
          */
         void disconnect(ChannelPromise promise);
 
         /**
          * Close the {@link Channel} of the {@link ChannelPromise} and notify the {@link ChannelPromise} once the
          * operation was complete.
          */
         void close(ChannelPromise promise);
 
         /**
          * Closes the {@link Channel} immediately without firing any events.  Probably only useful
          * when registration attempt failed.
          */
         void closeForcibly();
 
         /**
          * Deregister the {@link Channel} of the {@link ChannelPromise} from {@link EventLoop} and notify the
          * {@link ChannelPromise} once the operation was complete.
          */
         void deregister(ChannelPromise promise);
 
         /**
          * Schedules a read operation that fills the inbound buffer of the first {@link ChannelInboundHandler} in the
          * {@link ChannelPipeline}.  If there's already a pending read operation, this method does nothing.
          */
         void beginRead();
 
         /**
          * Schedules a write operation.
          */
         void write(Object msg, ChannelPromise promise);
 
         /**
          * Flush out all write operations scheduled via {@link #write(Object, ChannelPromise)}.
          */
         void flush();
 
         /**
          * Return a special ChannelPromise which can be reused and passed to the operations in {@link Unsafe}.
          * It will never be notified of a success or error and so is only a placeholder for operations
          * that take a {@link ChannelPromise} as argument but for which you not want to get notified.
          */
         ChannelPromise voidPromise();
 
         /**
          * Returns the {@link ChannelOutboundBuffer} of the {@link Channel} where the pending write requests are stored.
          */
         ChannelOutboundBuffer outboundBuffer();
     }
 }