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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;

/**
 * Handles or intercepts a downstream {@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 request
 * such as {@link Channel#write(Object)} and {@link Channel#close()}.
 *
 * 
{@link SimpleChannelDownstreamHandler}
 * 
 * In most cases, you will get to use a {@link SimpleChannelDownstreamHandler}
 * to implement a downstream 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 downstream or upstream.  In most cases,
 * {@link ChannelDownstreamHandler} will send the event downstream
 * (i.e. outbound) although it is legal to send the event upstream (i.e. inbound):
 *
 * 
 * // Sending the event downstream (outbound)
 * void handleDownstream({@link ChannelHandlerContext} ctx, {@link ChannelEvent} e) throws Exception {
 *     ...
 *     ctx.sendDownstream(e);
 *     ...
 * }
 *
 * // Sending the event upstream (inbound)
 * void handleDownstream({@link ChannelHandlerContext} ctx, {@link ChannelEvent} e) throws Exception {
 *     ...
 *     ctx.sendUpstream(new {@link UpstreamChannelStateEvent}(...));
 *     ...
 * }
 * 
 *
 * 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.
 * 

 * Caution:
 * 

 * Use the *Later(..) methods of the {@link Channels} class if you want to send an upstream event
 * from a {@link ChannelDownstreamHandler} otherwise you may run into threading issues.
 *
 * 
State management
 *
 * Please refer to {@link ChannelHandler}.
 *
 * Thread safety
 * 
 * {@link #handleDownstream(ChannelHandlerContext, ChannelEvent) handleDownstream}
 * may be invoked by more than one thread simultaneously.  If the handler
 * accesses a shared resource or stores stateful information, you might need
 * proper synchronization in the handler implementation.
 *
 * @apiviz.exclude ^org\.jboss\.netty\.handler\..*$
 */
public interface ChannelDownstreamHandler extends ChannelHandler {

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