/*
* 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:
*
* http://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.util.Attribute;
import io.netty.util.AttributeKey;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* Handles or intercepts a {@link ChannelInboundInvoker} or {@link ChannelOutboundInvoker} operation, and forwards it
* to the next handler in a {@link ChannelPipeline}.
*
* <h3>Sub-types</h3>
* <p>
* {@link ChannelHandler} itself does not provide many methods. To handle a
* a {@link ChannelInboundInvoker} or {@link ChannelOutboundInvoker} operation
* you need to implement its sub-interfaces. There are many different sub-interfaces
* which handles inbound and outbound operations.
*
* But the most useful for developers may be:
* <ul>
* <li>{@link ChannelInboundHandlerAdapter} handles and intercepts inbound operations</li>
* <li>{@link ChannelOutboundHandlerAdapter} handles and intercepts outbound operations</li>
* </ul>
*
* You will also find more detailed explanation from the documentation of
* each sub-interface on how an event is interpreted when it goes upstream and
* downstream respectively.
*
* <h3>The context object</h3>
* <p>
* A {@link ChannelHandler} is provided with a {@link ChannelHandlerContext}
* object. A {@link ChannelHandler} is supposed to interact with the
* {@link ChannelPipeline} it belongs to via a context object. Using the
* context object, the {@link ChannelHandler} can pass events upstream or
* downstream, modify the pipeline dynamically, or store the information
* (using {@link AttributeKey}s) which is specific to the handler.
*
* <h3>State management</h3>
*
* A {@link ChannelHandler} often needs to store some stateful information.
* The simplest and recommended approach is to use member variables:
* <pre>
* public interface Message {
* // your methods here
* }
*
* public class DataServerHandler extends {@link SimpleChannelInboundHandler}<Message> {
*
* <b>private boolean loggedIn;</b>
*
* {@code @Override}
* public void channelRead0({@link ChannelHandlerContext} ctx, Message message) {
* {@link Channel} ch = e.getChannel();
* if (message instanceof LoginMessage) {
* authenticate((LoginMessage) message);
* <b>loggedIn = true;</b>
* } else (message instanceof GetDataMessage) {
* if (<b>loggedIn</b>) {
* ch.write(fetchSecret((GetDataMessage) message));
* } else {
* fail();
* }
* }
* }
* ...
* }
* </pre>
* Because the handler instance has a state variable which is dedicated to
* one connection, you have to create a new handler instance for each new
* channel to avoid a race condition where a unauthenticated client can get
* the confidential information:
* <pre>
* // Create a new handler instance per channel.
* // See {@link ChannelInitializer#initChannel(Channel)}.
* public class DataServerInitializer extends {@link ChannelInitializer}<{@link Channel}> {
* {@code @Override}
* public void initChannel({@link Channel} channel) {
* channel.pipeline().addLast("handler", <b>new DataServerHandler()</b>);
* }
* }
*
* </pre>
*
* <h4>Using {@link AttributeKey}</h4>
*
* Although it's recommended to use member variables to store the state of a
* handler, for some reason you might not want to create many handler instances.
* In such a case, you can use {@link AttributeKey}s which is provided by
* {@link ChannelHandlerContext}:
* <pre>
* public interface Message {
* // your methods here
* }
*
* {@code @Sharable}
* public class DataServerHandler extends {@link SimpleChannelInboundHandler}<Message> {
* private final {@link AttributeKey}<{@link Boolean}> auth =
* {@link AttributeKey#valueOf(String) AttributeKey.valueOf("auth")};
*
* {@code @Override}
* public void channelRead({@link ChannelHandlerContext} ctx, Message message) {
* {@link Attribute}<{@link Boolean}> attr = ctx.attr(auth);
* {@link Channel} ch = ctx.channel();
* if (message instanceof LoginMessage) {
* authenticate((LoginMessage) o);
* <b>attr.set(true)</b>;
* } else (message instanceof GetDataMessage) {
* if (<b>Boolean.TRUE.equals(attr.get())</b>) {
* ch.write(fetchSecret((GetDataMessage) o));
* } else {
* fail();
* }
* }
* }
* ...
* }
* </pre>
* Now that the state of the handler isattached to the {@link ChannelHandlerContext}, you can add the
* same handler instance to different pipelines:
* <pre>
* public class DataServerInitializer extends {@link ChannelInitializer}<{@link Channel}> {
*
* private static final DataServerHandler <b>SHARED</b> = new DataServerHandler();
*
* {@code @Override}
* public void initChannel({@link Channel} channel) {
* channel.pipeline().addLast("handler", <b>SHARED</b>);
* }
* }
* </pre>
*
*
* <h4>The {@code @Sharable} annotation</h4>
* <p>
* In the example above which used an {@link AttributeKey},
* you might have noticed the {@code @Sharable} annotation.
* <p>
* If a {@link ChannelHandler} is annotated with the {@code @Sharable}
* annotation, it means you can create an instance of the handler just once and
* add it to one or more {@link ChannelPipeline}s multiple times without
* a race condition.
* <p>
* If this annotation is not specified, you have to create a new handler
* instance every time you add it to a pipeline because it has unshared state
* such as member variables.
* <p>
* This annotation is provided for documentation purpose, just like
* <a href="http://www.javaconcurrencyinpractice.com/annotations/doc/">the JCIP annotations</a>.
*
* <h3>Additional resources worth reading</h3>
* <p>
* Please refer to the {@link ChannelHandler}, and
* {@link ChannelPipeline} to find out more about inbound and outbound operations,
* what fundamental differences they have, how they flow in a pipeline, and how to handle
* the operation in your application.
*/
public interface ChannelHandler {
/**
* Gets called after the {@link ChannelHandler} was added to the actual context and it's ready to handle events.
*/
void handlerAdded(ChannelHandlerContext ctx) throws Exception;
/**
* Gets called after the {@link ChannelHandler} was removed from the actual context and it doesn't handle events
* anymore.
*/
void handlerRemoved(ChannelHandlerContext ctx) throws Exception;
/**
* Gets called if a {@link Throwable} was thrown.
*/
void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;
/**
* Indicates that the same instance of the annotated {@link ChannelHandler}
* can be added to one or more {@link ChannelPipeline}s multiple times
* without a race condition.
* <p>
* If this annotation is not specified, you have to create a new handler
* instance every time you add it to a pipeline because it has unshared
* state such as member variables.
* <p>
* This annotation is provided for documentation purpose, just like
* <a href="http://www.javaconcurrencyinpractice.com/annotations/doc/">the JCIP annotations</a>.
*/
@Inherited
@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Sharable {
// no value
}
}