View Javadoc
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 io.netty.channel;
17  
18  import io.netty.util.Attribute;
19  import io.netty.util.AttributeKey;
20  
21  import java.lang.annotation.Documented;
22  import java.lang.annotation.ElementType;
23  import java.lang.annotation.Inherited;
24  import java.lang.annotation.Retention;
25  import java.lang.annotation.RetentionPolicy;
26  import java.lang.annotation.Target;
27  
28  /**
29   * Handles an I/O event or intercepts an I/O operation, and forwards it to its next handler in
30   * its {@link ChannelPipeline}.
31   *
32   * <h3>Sub-types</h3>
33   * <p>
34   * {@link ChannelHandler} itself does not provide many methods, but you usually have to implement one of its subtypes:
35   * <ul>
36   * <li>{@link ChannelInboundHandler} to handle inbound I/O events, and</li>
37   * <li>{@link ChannelOutboundHandler} to handle outbound I/O operations.</li>
38   * </ul>
39   * </p>
40   * <p>
41   * Alternatively, the following adapter classes are provided for your convenience:
42   * <ul>
43   * <li>{@link ChannelInboundHandlerAdapter} to handle inbound I/O events,</li>
44   * <li>{@link ChannelOutboundHandlerAdapter} to handle outbound I/O operations, and</li>
45   * <li>{@link ChannelDuplexHandler} to handle both inbound and outbound events</li>
46   * </ul>
47   * </p>
48   * <p>
49   * For more information, please refer to the documentation of each subtype.
50   * </p>
51   *
52   * <h3>The context object</h3>
53   * <p>
54   * A {@link ChannelHandler} is provided with a {@link ChannelHandlerContext}
55   * object.  A {@link ChannelHandler} is supposed to interact with the
56   * {@link ChannelPipeline} it belongs to via a context object.  Using the
57   * context object, the {@link ChannelHandler} can pass events upstream or
58   * downstream, modify the pipeline dynamically, or store the information
59   * (using {@link AttributeKey}s) which is specific to the handler.
60   *
61   * <h3>State management</h3>
62   *
63   * A {@link ChannelHandler} often needs to store some stateful information.
64   * The simplest and recommended approach is to use member variables:
65   * <pre>
66   * public interface Message {
67   *     // your methods here
68   * }
69   *
70   * public class DataServerHandler extends {@link SimpleChannelInboundHandler}&lt;Message&gt; {
71   *
72   *     <b>private boolean loggedIn;</b>
73   *
74   *     {@code @Override}
75   *     public void channelRead0({@link ChannelHandlerContext} ctx, Message message) {
76   *         if (message instanceof LoginMessage) {
77   *             authenticate((LoginMessage) message);
78   *             <b>loggedIn = true;</b>
79   *         } else (message instanceof GetDataMessage) {
80   *             if (<b>loggedIn</b>) {
81   *                 ctx.writeAndFlush(fetchSecret((GetDataMessage) message));
82   *             } else {
83   *                 fail();
84   *             }
85   *         }
86   *     }
87   *     ...
88   * }
89   * </pre>
90   * Because the handler instance has a state variable which is dedicated to
91   * one connection, you have to create a new handler instance for each new
92   * channel to avoid a race condition where a unauthenticated client can get
93   * the confidential information:
94   * <pre>
95   * // Create a new handler instance per channel.
96   * // See {@link ChannelInitializer#initChannel(Channel)}.
97   * public class DataServerInitializer extends {@link ChannelInitializer}&lt;{@link Channel}&gt; {
98   *     {@code @Override}
99   *     public void initChannel({@link Channel} channel) {
100  *         channel.pipeline().addLast("handler", <b>new DataServerHandler()</b>);
101  *     }
102  * }
103  *
104  * </pre>
105  *
106  * <h4>Using {@link AttributeKey}s</h4>
107  *
108  * Although it's recommended to use member variables to store the state of a
109  * handler, for some reason you might not want to create many handler instances.
110  * In such a case, you can use {@link AttributeKey}s which is provided by
111  * {@link ChannelHandlerContext}:
112  * <pre>
113  * public interface Message {
114  *     // your methods here
115  * }
116  *
117  * {@code @Sharable}
118  * public class DataServerHandler extends {@link SimpleChannelInboundHandler}&lt;Message&gt; {
119  *     private final {@link AttributeKey}&lt;{@link Boolean}&gt; auth =
120  *           {@link AttributeKey#valueOf(String) AttributeKey.valueOf("auth")};
121  *
122  *     {@code @Override}
123  *     public void channelRead({@link ChannelHandlerContext} ctx, Message message) {
124  *         {@link Attribute}&lt;{@link Boolean}&gt; attr = ctx.attr(auth);
125  *         if (message instanceof LoginMessage) {
126  *             authenticate((LoginMessage) o);
127  *             <b>attr.set(true)</b>;
128  *         } else (message instanceof GetDataMessage) {
129  *             if (<b>Boolean.TRUE.equals(attr.get())</b>) {
130  *                 ctx.writeAndFlush(fetchSecret((GetDataMessage) o));
131  *             } else {
132  *                 fail();
133  *             }
134  *         }
135  *     }
136  *     ...
137  * }
138  * </pre>
139  * Now that the state of the handler is attached to the {@link ChannelHandlerContext}, you can add the
140  * same handler instance to different pipelines:
141  * <pre>
142  * public class DataServerInitializer extends {@link ChannelInitializer}&lt;{@link Channel}&gt; {
143  *
144  *     private static final DataServerHandler <b>SHARED</b> = new DataServerHandler();
145  *
146  *     {@code @Override}
147  *     public void initChannel({@link Channel} channel) {
148  *         channel.pipeline().addLast("handler", <b>SHARED</b>);
149  *     }
150  * }
151  * </pre>
152  *
153  *
154  * <h4>The {@code @Sharable} annotation</h4>
155  * <p>
156  * In the example above which used an {@link AttributeKey},
157  * you might have noticed the {@code @Sharable} annotation.
158  * <p>
159  * If a {@link ChannelHandler} is annotated with the {@code @Sharable}
160  * annotation, it means you can create an instance of the handler just once and
161  * add it to one or more {@link ChannelPipeline}s multiple times without
162  * a race condition.
163  * <p>
164  * If this annotation is not specified, you have to create a new handler
165  * instance every time you add it to a pipeline because it has unshared state
166  * such as member variables.
167  * <p>
168  * This annotation is provided for documentation purpose, just like
169  * <a href="http://www.javaconcurrencyinpractice.com/annotations/doc/">the JCIP annotations</a>.
170  *
171  * <h3>Additional resources worth reading</h3>
172  * <p>
173  * Please refer to the {@link ChannelHandler}, and
174  * {@link ChannelPipeline} to find out more about inbound and outbound operations,
175  * what fundamental differences they have, how they flow in a  pipeline,  and how to handle
176  * the operation in your application.
177  */
178 public interface ChannelHandler {
179 
180     /**
181      * Gets called after the {@link ChannelHandler} was added to the actual context and it's ready to handle events.
182      */
183     void handlerAdded(ChannelHandlerContext ctx) throws Exception;
184 
185     /**
186      * Gets called after the {@link ChannelHandler} was removed from the actual context and it doesn't handle events
187      * anymore.
188      */
189     void handlerRemoved(ChannelHandlerContext ctx) throws Exception;
190 
191     /**
192      * Gets called if a {@link Throwable} was thrown.
193      *
194      * @deprecated is part of {@link ChannelInboundHandler}
195      */
196     @Deprecated
197     void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;
198 
199     /**
200      * Indicates that the same instance of the annotated {@link ChannelHandler}
201      * can be added to one or more {@link ChannelPipeline}s multiple times
202      * without a race condition.
203      * <p>
204      * If this annotation is not specified, you have to create a new handler
205      * instance every time you add it to a pipeline because it has unshared
206      * state such as member variables.
207      * <p>
208      * This annotation is provided for documentation purpose, just like
209      * <a href="http://www.javaconcurrencyinpractice.com/annotations/doc/">the JCIP annotations</a>.
210      */
211     @Inherited
212     @Documented
213     @Target(ElementType.TYPE)
214     @Retention(RetentionPolicy.RUNTIME)
215     @interface Sharable {
216         // no value
217     }
218 }