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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.frame;

import org.jboss.netty.buffer.ChannelBuffer;
import org.jboss.netty.buffer.ChannelBufferFactory;
import org.jboss.netty.channel.Channel;
import org.jboss.netty.channel.ChannelHandlerContext;

/**
 * A decoder that splits the received {@link ChannelBuffer}s by the fixed number
 * of bytes. For example, if you received the following four fragmented packets:
 *  * +---+----+------+----+
 * | A | BC | DEFG | HI |
 * +---+----+------+----+
 * 
 * A {@link FixedLengthFrameDecoder}{@code (3)} will decode them into the
 * following three packets with the fixed length:
 *  * +-----+-----+-----+
 * | ABC | DEF | GHI |
 * +-----+-----+-----+
 * 
 */
public class FixedLengthFrameDecoder extends FrameDecoder {

    private final int frameLength;
    private final boolean allocateFullBuffer;

    /**
     * Calls {@link #FixedLengthFrameDecoder(int, boolean)} with {@code false}
     */
    public FixedLengthFrameDecoder(int frameLength) {
        this(frameLength, false);
    }

    /**
     * Creates a new instance.
     *
     * @param frameLength
     *        the length of the frame
     * @param allocateFullBuffer
     *        {@code true} if the cumulative {@link ChannelBuffer} should use the
     *        {@link #frameLength} as its initial size
     */
    public FixedLengthFrameDecoder(int frameLength, boolean allocateFullBuffer) {
        if (frameLength <= 0) {
            throw new IllegalArgumentException(
                    "frameLength must be a positive integer: " + frameLength);
        }
        this.frameLength = frameLength;
        this.allocateFullBuffer = allocateFullBuffer;
    }

    @Override
    protected Object decode(
            ChannelHandlerContext ctx, Channel channel, ChannelBuffer buffer) throws Exception {
        if (buffer.readableBytes() < frameLength) {
            return null;
        } else {
            ChannelBuffer frame = extractFrame(buffer, buffer.readerIndex(), frameLength);
            buffer.skipBytes(frameLength);
            return frame;
        }
    }

    @Override
    protected ChannelBuffer newCumulationBuffer(ChannelHandlerContext ctx, int minimumCapacity) {
        ChannelBufferFactory factory = ctx.getChannel().getConfig().getBufferFactory();
        if (allocateFullBuffer) {
            return factory.getBuffer(frameLength);
        }
        return super.newCumulationBuffer(ctx, minimumCapacity);
    }
}