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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.handler.codec.oneone;

import static org.jboss.netty.channel.Channels.*;

import org.jboss.netty.buffer.ChannelBuffers;
import org.jboss.netty.channel.Channel;
import org.jboss.netty.channel.ChannelDownstreamHandler;
import org.jboss.netty.channel.ChannelEvent;
import org.jboss.netty.channel.ChannelHandlerContext;
import org.jboss.netty.channel.ChannelPipeline;
import org.jboss.netty.channel.MessageEvent;
import org.jboss.netty.handler.codec.frame.DelimiterBasedFrameDecoder;
import org.jboss.netty.handler.codec.frame.Delimiters;

/**
 * Transforms a write request into another write request.  A typical setup for
 * TCP/IP would be:
 *  * {@link ChannelPipeline} pipeline = ...;
 *
 * // Decoders
 * pipeline.addLast("frameDecoder", new {@link DelimiterBasedFrameDecoder}(80, {@link Delimiters#nulDelimiter()}));
 * pipeline.addLast("customDecoder", new {@link OneToOneDecoder}() { ... });
 *
 * // Encoder
 * pipeline.addLast("customEncoder", new {@link OneToOneEncoder}() { ... });
 * 
 *
 * @apiviz.landmark
 */
public abstract class OneToOneEncoder implements ChannelDownstreamHandler {

    protected OneToOneEncoder() {
    }

    public void handleDownstream(
            ChannelHandlerContext ctx, ChannelEvent evt) throws Exception {
        if (!(evt instanceof MessageEvent)) {
            ctx.sendDownstream(evt);
            return;
        }

        MessageEvent e = (MessageEvent) evt;
        if (!doEncode(ctx, e)) {
            ctx.sendDownstream(e);
        }
    }

    protected boolean doEncode(ChannelHandlerContext ctx, MessageEvent e) throws Exception {
        Object originalMessage = e.getMessage();
        Object encodedMessage = encode(ctx, e.getChannel(), originalMessage);
        if (originalMessage == encodedMessage) {
            return false;
        } else if (encodedMessage != null) {
            write(ctx, e.getFuture(), encodedMessage, e.getRemoteAddress());
        }
        return true;
    }

    /**
     * Transforms the specified message into another message and return the
     * transformed message.  Note that you can not return {@code null}, unlike
     * you can in {@link OneToOneDecoder#decode(ChannelHandlerContext, Channel, Object)};
     * you must return something, at least {@link ChannelBuffers#EMPTY_BUFFER}.
     */
    protected abstract Object encode(
            ChannelHandlerContext ctx, Channel channel, Object msg) throws Exception;
}