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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.protobuf;

import static org.jboss.netty.buffer.ChannelBuffers.*;

import org.jboss.netty.buffer.ChannelBuffer;
import org.jboss.netty.buffer.ChannelBufferOutputStream;
import org.jboss.netty.channel.Channel;
import org.jboss.netty.channel.ChannelHandler.Sharable;
import org.jboss.netty.channel.ChannelHandlerContext;
import org.jboss.netty.handler.codec.oneone.OneToOneEncoder;

import com.google.protobuf.CodedOutputStream;

/**
 * An encoder that prepends the the Google Protocol Buffers
 * Base
 * 128 Varints integer length field.  For example:
 *  * BEFORE DECODE (300 bytes)       AFTER DECODE (302 bytes)
 * +---------------+               +--------+---------------+
 * | Protobuf Data |-------------->| Length | Protobuf Data |
 * |  (300 bytes)  |               | 0xAC02 |  (300 bytes)  |
 * +---------------+               +--------+---------------+
 *  *
 *
 * @see CodedOutputStream
 */
@Sharable
public class ProtobufVarint32LengthFieldPrepender extends OneToOneEncoder {

    @Override
    protected Object encode(ChannelHandlerContext ctx, Channel channel,
            Object msg) throws Exception {
        if (!(msg instanceof ChannelBuffer)) {
            return msg;
        }

        ChannelBuffer body = (ChannelBuffer) msg;
        int length = body.readableBytes();
        ChannelBuffer header =
            channel.getConfig().getBufferFactory().getBuffer(
                    body.order(),
                    CodedOutputStream.computeRawVarint32Size(length));
        CodedOutputStream codedOutputStream = CodedOutputStream
                .newInstance(new ChannelBufferOutputStream(header));
        codedOutputStream.writeRawVarint32(length);
        codedOutputStream.flush();
        return wrappedBuffer(header, body);
    }

}