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The Netty project is an effort to provide an asynchronous event-driven network application framework and tools for rapid development of maintainable high performance and high scalability protocol servers and clients. In other words, Netty is a NIO client server framework which enables quick and easy development of network applications such as protocol servers and clients. It greatly simplifies and streamlines network programming such as TCP and UDP socket server.

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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:
 *
 *   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 org.jboss.netty.channel;

import java.util.concurrent.Executor;

import org.jboss.netty.handler.execution.ExecutionHandler;


/**
 * Handles or intercepts an upstream {@link ChannelEvent}, and sends a
 * {@link ChannelEvent} to the next handler in a {@link ChannelPipeline}.
 * 
 * The most common use case of this interface is to intercept an I/O event
 * generated by I/O workers to transform the received messages or execute
 * the relevant business logic.
 *
 * 
{@link SimpleChannelUpstreamHandler}
 * 
 * In most cases, you will get to use a {@link SimpleChannelUpstreamHandler} to
 * implement an upstream handler because it provides an individual handler
 * method for each event type.  You might want to implement this interface
 * directly though if you want to handle various types of events in more
 * generic way.
 *
 * 
Firing an event to the next handler
 * 
 * You can forward the received event upstream or downstream.  In most cases,
 * {@link ChannelUpstreamHandler} will send the event upstream (i.e. inbound)
 * although it is legal to send the event downstream (i.e. outbound):
 *
 * 
 * // Sending the event upstream (inbound)
 * void handleUpstream({@link ChannelHandlerContext} ctx, {@link ChannelEvent} e) throws Exception {
 *     ...
 *     ctx.sendUpstream(e);
 *     ...
 * }
 *
 * // Sending the event downstream (outbound)
 * void handleDownstream({@link ChannelHandlerContext} ctx, {@link ChannelEvent} e) throws Exception {
 *     ...
 *     ctx.sendDownstream(new {@link DownstreamMessageEvent}(...));
 *     ...
 * }
 * 
 *
 * Using the helper class to send an event
 * 
 * You will also find various helper methods in {@link Channels} to be useful
 * to generate and send an artificial or manipulated event.
 *
 * 
State management
 *
 * Please refer to {@link ChannelHandler}.
 *
 * Thread safety
 * 
 * {@link #handleUpstream(ChannelHandlerContext, ChannelEvent) handleUpstream}
 * will be invoked sequentially by the same thread (i.e. an I/O thread) and
 * therefore a handler does not need to worry about being invoked with a new
 * upstream event before the previous upstream event is finished.
 * 

 * This does not necessarily mean that there's a dedicated thread per
 * {@link Channel}; the I/O thread of some transport can serve more than one
 * {@link Channel} (e.g. NIO transport), while the I/O thread of other
 * transports can serve only one (e.g. OIO transport).
 * 
 * However, if you add an {@link ExecutionHandler} to a {@link ChannelPipeline},
 * this behavior changes depending on what {@link Executor} was employed to
 * dispatch the events.  Please refer to {@link ExecutionHandler} for more
 * information.
 *
 * @apiviz.exclude ^org\.jboss\.netty\.handler\..*$
 */
public interface ChannelUpstreamHandler extends ChannelHandler {

    /**
     * Handles the specified upstream event.
     *
     * @param ctx  the context object for this handler
     * @param e    the upstream event to process or intercept
     */
    void handleUpstream(ChannelHandlerContext ctx, ChannelEvent e) throws Exception;
}