237 lines
8.8 KiB
Java
237 lines
8.8 KiB
Java
/*
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* Copyright 2009 Red Hat, Inc.
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*
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* Red Hat licenses this file to you under the Apache License, version 2.0
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* (the "License"); you may not use this file except in compliance with the
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* License. You may obtain a copy of the License at:
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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* License for the specific language governing permissions and limitations
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* under the License.
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*/
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package io.netty.channel;
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import java.lang.annotation.Documented;
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import java.lang.annotation.ElementType;
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import java.lang.annotation.Inherited;
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import java.lang.annotation.Retention;
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import java.lang.annotation.RetentionPolicy;
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import java.lang.annotation.Target;
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import io.netty.bootstrap.Bootstrap;
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import io.netty.channel.group.ChannelGroup;
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/**
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* Handles or intercepts a {@link ChannelEvent}, and sends a
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* {@link ChannelEvent} to the next handler in a {@link ChannelPipeline}.
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*
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* <h3>Sub-types</h3>
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* <p>
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* {@link ChannelHandler} itself does not provide any method. To handle a
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* {@link ChannelEvent} you need to implement its sub-interfaces. There are
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* two sub-interfaces which handles a received event, one for upstream events
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* and the other for downstream events:
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* <ul>
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* <li>{@link ChannelUpstreamHandler} handles and intercepts an upstream {@link ChannelEvent}.</li>
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* <li>{@link ChannelDownstreamHandler} handles and intercepts a downstream {@link ChannelEvent}.</li>
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* </ul>
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*
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* You will also find more detailed explanation from the documentation of
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* each sub-interface on how an event is interpreted when it goes upstream and
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* downstream respectively.
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*
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* <h3>The context object</h3>
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* <p>
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* A {@link ChannelHandler} is provided with a {@link ChannelHandlerContext}
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* object. A {@link ChannelHandler} is supposed to interact with the
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* {@link ChannelPipeline} it belongs to via a context object. Using the
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* context object, the {@link ChannelHandler} can pass events upstream or
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* downstream, modify the pipeline dynamically, or store the information
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* (attachment) which is specific to the handler.
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*
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* <h3>State management</h3>
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*
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* A {@link ChannelHandler} often needs to store some stateful information.
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* The simplest and recommended approach is to use member variables:
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* <pre>
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* public class DataServerHandler extends {@link SimpleChannelHandler} {
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*
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* <b>private boolean loggedIn;</b>
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*
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* {@code @Override}
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* public void messageReceived({@link ChannelHandlerContext} ctx, {@link MessageEvent} e) {
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* {@link Channel} ch = e.getChannel();
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* Object o = e.getMessage();
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* if (o instanceof LoginMessage) {
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* authenticate((LoginMessage) o);
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* <b>loggedIn = true;</b>
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* } else (o instanceof GetDataMessage) {
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* if (<b>loggedIn</b>) {
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* ch.write(fetchSecret((GetDataMessage) o));
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* } else {
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* fail();
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* }
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* }
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* }
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* ...
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* }
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* </pre>
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* Because the handler instance has a state variable which is dedicated to
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* one connection, you have to create a new handler instance for each new
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* channel to avoid a race condition where a unauthenticated client can get
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* the confidential information:
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* <pre>
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* // Create a new handler instance per channel.
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* // See {@link Bootstrap#setPipelineFactory(ChannelPipelineFactory)}.
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* public class DataServerPipelineFactory implements {@link ChannelPipelineFactory} {
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* public {@link ChannelPipeline} getPipeline() {
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* return {@link Channels}.pipeline(<b>new DataServerHandler()</b>);
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* }
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* }
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* </pre>
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*
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* <h4>Using an attachment</h4>
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*
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* Although it's recommended to use member variables to store the state of a
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* handler, for some reason you might not want to create many handler instances.
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* In such a case, you can use an <em>attachment</em> which is provided by
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* {@link ChannelHandlerContext}:
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* <pre>
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* {@code @Sharable}
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* public class DataServerHandler extends {@link SimpleChannelHandler} {
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*
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* {@code @Override}
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* public void messageReceived({@link ChannelHandlerContext} ctx, {@link MessageEvent} e) {
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* {@link Channel} ch = e.getChannel();
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* Object o = e.getMessage();
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* if (o instanceof LoginMessage) {
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* authenticate((LoginMessage) o);
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* <b>ctx.setAttachment(true)</b>;
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* } else (o instanceof GetDataMessage) {
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* if (<b>Boolean.TRUE.equals(ctx.getAttachment())</b>) {
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* ch.write(fetchSecret((GetDataMessage) o));
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* } else {
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* fail();
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* }
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* }
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* }
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* ...
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* }
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* </pre>
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* Now that the state of the handler is stored as an attachment, you can add the
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* same handler instance to different pipelines:
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* <pre>
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* public class DataServerPipelineFactory implements {@link ChannelPipelineFactory} {
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*
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* private static final DataServerHandler <b>SHARED</b> = new DataServerHandler();
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*
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* public {@link ChannelPipeline} getPipeline() {
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* return {@link Channels}.pipeline(<b>SHARED</b>);
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* }
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* }
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* </pre>
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*
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* <h4>Using a {@link ChannelLocal}</h4>
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*
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* If you have a state variable which needs to be accessed either from other
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* handlers or outside handlers, you can use {@link ChannelLocal}:
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* <pre>
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* public final class DataServerState {
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*
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* <b>public static final {@link ChannelLocal}<Boolean> loggedIn = new {@link ChannelLocal}<Boolean>() {
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* protected Boolean initialValue(Channel channel) {
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* return false;
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* }
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* }</b>
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* ...
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* }
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*
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* {@code @Sharable}
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* public class DataServerHandler extends {@link SimpleChannelHandler} {
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*
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* {@code @Override}
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* public void messageReceived({@link ChannelHandlerContext} ctx, {@link MessageEvent} e) {
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* Channel ch = e.getChannel();
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* Object o = e.getMessage();
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* if (o instanceof LoginMessage) {
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* authenticate((LoginMessage) o);
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* <b>DataServerState.loggedIn.set(ch, true);</b>
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* } else (o instanceof GetDataMessage) {
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* if (<b>DataServerState.loggedIn.get(ch)</b>) {
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* ctx.getChannel().write(fetchSecret((GetDataMessage) o));
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* } else {
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* fail();
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* }
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* }
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* }
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* ...
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* }
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*
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* // Print the remote addresses of the authenticated clients:
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* {@link ChannelGroup} allClientChannels = ...;
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* for ({@link Channel} ch: allClientChannels) {
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* if (<b>DataServerState.loggedIn.get(ch)</b>) {
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* System.out.println(ch.getRemoteAddress());
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* }
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* }
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* </pre>
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*
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* <h4>The {@code @Sharable} annotation</h4>
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* <p>
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* In the examples above which used an attachment or a {@link ChannelLocal},
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* you might have noticed the {@code @Sharable} annotation.
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* <p>
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* If a {@link ChannelHandler} is annotated with the {@code @Sharable}
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* annotation, it means you can create an instance of the handler just once and
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* add it to one or more {@link ChannelPipeline}s multiple times without
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* a race condition.
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* <p>
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* If this annotation is not specified, you have to create a new handler
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* instance every time you add it to a pipeline because it has unshared state
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* such as member variables.
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* <p>
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* This annotation is provided for documentation purpose, just like
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* <a href="http://www.javaconcurrencyinpractice.com/annotations/doc/">the JCIP annotations</a>.
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*
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* <h3>Additional resources worth reading</h3>
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* <p>
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* Please refer to the {@link ChannelEvent} and {@link ChannelPipeline} to find
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* out what a upstream event and a downstream event are, what fundamental
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* differences they have, and how they flow in a pipeline.
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*
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* @author <a href="http://netty.io/">The Netty Project</a>
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* @author <a href="http://gleamynode.net/">Trustin Lee</a>
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*
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* @apiviz.landmark
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* @apiviz.exclude ^org\.jboss\.netty\.handler\..*$
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*/
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public interface ChannelHandler {
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/**
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* Indicates that the same instance of the annotated {@link ChannelHandler}
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* can be added to one or more {@link ChannelPipeline}s multiple times
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* without a race condition.
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* <p>
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* If this annotation is not specified, you have to create a new handler
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* instance every time you add it to a pipeline because it has unshared
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* state such as member variables.
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* <p>
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* This annotation is provided for documentation purpose, just like
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* <a href="http://www.javaconcurrencyinpractice.com/annotations/doc/">the JCIP annotations</a>.
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*
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* @author <a href="http://netty.io/">The Netty Project</a>
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* @author <a href="http://gleamynode.net/">Trustin Lee</a>
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*/
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@Inherited
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@Documented
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@Target(ElementType.TYPE)
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@Retention(RetentionPolicy.RUNTIME)
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@interface Sharable {
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// no value
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}
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}
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