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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.channel.Channel;
import org.jboss.netty.channel.ChannelEvent;
import org.jboss.netty.channel.ChannelHandlerContext;
import org.jboss.netty.channel.ChannelPipeline;
import org.jboss.netty.channel.ChannelUpstreamHandler;
import org.jboss.netty.channel.MessageEvent;
import org.jboss.netty.handler.codec.frame.DelimiterBasedFrameDecoder;
import org.jboss.netty.handler.codec.frame.Delimiters;
import org.jboss.netty.handler.codec.frame.FrameDecoder;

/**
 * Transforms a received message into another message.  Please note that this
 * decoder must be used with a proper {@link FrameDecoder} such as
 * {@link DelimiterBasedFrameDecoder} or you must implement proper framing
 * mechanism by yourself if you are using a stream-based transport such as
 * TCP/IP.  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 OneToOneDecoder implements ChannelUpstreamHandler {

    /**
     * Creates a new instance with the current system character set.
     */
    protected OneToOneDecoder() {
    }

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

        MessageEvent e = (MessageEvent) evt;
        Object originalMessage = e.getMessage();
        Object decodedMessage = decode(ctx, e.getChannel(), originalMessage);
        if (originalMessage == decodedMessage) {
            ctx.sendUpstream(evt);
        } else if (decodedMessage != null) {
            fireMessageReceived(ctx, decodedMessage, e.getRemoteAddress());
        }
    }

    /**
     * Transforms the specified received message into another message and return
     * the transformed message.  Return {@code null} if the received message
     * is supposed to be discarded.
     */
    protected abstract Object decode(
            ChannelHandlerContext ctx, Channel channel, Object msg) throws Exception;
}