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1   /*
2    * Copyright 2012 The Netty Project
3    *
4    * The Netty Project licenses this file to you under the Apache License,
5    * version 2.0 (the "License"); you may not use this file except in compliance
6    * with the License. You may obtain a copy of the License at:
7    *
8    *   http://www.apache.org/licenses/LICENSE-2.0
9    *
10   * Unless required by applicable law or agreed to in writing, software
11   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
12   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
13   * License for the specific language governing permissions and limitations
14   * under the License.
15   */
16  package org.jboss.netty.channel;
17  
18  /**
19   * Handles or intercepts a downstream {@link ChannelEvent}, and sends a
20   * {@link ChannelEvent} to the next handler in a {@link ChannelPipeline}.
21   * <p>
22   * The most common use case of this interface is to intercept an I/O request
23   * such as {@link Channel#write(Object)} and {@link Channel#close()}.
24   *
25   * <h3>{@link SimpleChannelDownstreamHandler}</h3>
26   * <p>
27   * In most cases, you will get to use a {@link SimpleChannelDownstreamHandler}
28   * to implement a downstream handler because it provides an individual handler
29   * method for each event type.  You might want to implement this interface
30   * directly though if you want to handle various types of events in more
31   * generic way.
32   *
33   * <h3>Firing an event to the next handler</h3>
34   * <p>
35   * You can forward the received event downstream or upstream.  In most cases,
36   * {@link ChannelDownstreamHandler} will send the event downstream
37   * (i.e. outbound) although it is legal to send the event upstream (i.e. inbound):
38   *
39   * <pre>
40   * // Sending the event downstream (outbound)
41   * void handleDownstream({@link ChannelHandlerContext} ctx, {@link ChannelEvent} e) throws Exception {
42   *     ...
43   *     ctx.sendDownstream(e);
44   *     ...
45   * }
46   *
47   * // Sending the event upstream (inbound)
48   * void handleDownstream({@link ChannelHandlerContext} ctx, {@link ChannelEvent} e) throws Exception {
49   *     ...
50   *     ctx.sendUpstream(new {@link UpstreamChannelStateEvent}(...));
51   *     ...
52   * }
53   * </pre>
54   *
55   * <h4>Using the helper class to send an event</h4>
56   * <p>
57   * You will also find various helper methods in {@link Channels} to be useful
58   * to generate and send an artificial or manipulated event.
59   * <p>
60   * <strong>Caution:</strong>
61   * <p>
62   * Use the *Later(..) methods of the {@link Channels} class if you want to send an upstream event
63   * from a {@link ChannelDownstreamHandler} otherwise you may run into threading issues.
64   *
65   * <h3>State management</h3>
66   *
67   * Please refer to {@link ChannelHandler}.
68   *
69   * <h3>Thread safety</h3>
70   * <p>
71   * {@link #handleDownstream(ChannelHandlerContext, ChannelEvent) handleDownstream}
72   * may be invoked by more than one thread simultaneously.  If the handler
73   * accesses a shared resource or stores stateful information, you might need
74   * proper synchronization in the handler implementation.
75   *
76   * @apiviz.exclude ^org\.jboss\.netty\.handler\..*$
77   */
78  public interface ChannelDownstreamHandler extends ChannelHandler {
79  
80      /**
81       * Handles the specified downstream event.
82       *
83       * @param ctx  the context object for this handler
84       * @param e    the downstream event to process or intercept
85       */
86      void handleDownstream(ChannelHandlerContext ctx, ChannelEvent e) throws Exception;
87  }