Java tutorial
/* * Copyright 2016 - 2018 Anton Tananaev (anton@traccar.org) * * Licensed 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.traccar.protocol; import io.netty.buffer.ByteBuf; import io.netty.buffer.Unpooled; import io.netty.channel.Channel; import io.netty.channel.ChannelHandlerContext; import org.traccar.BaseFrameDecoder; import org.traccar.NetworkMessage; public class At2000FrameDecoder extends BaseFrameDecoder { private static final int BLOCK_LENGTH = 16; private static final int ACK_LENGTH = 496; private boolean firstPacket = true; private ByteBuf currentBuffer; private int acknowledgedBytes; private void sendResponse(Channel channel) { if (channel != null) { ByteBuf response = Unpooled.buffer(2 * BLOCK_LENGTH); response.writeByte(At2000ProtocolDecoder.MSG_ACKNOWLEDGEMENT); response.writeMedium(1); response.writeByte(0x00); // success response.writerIndex(2 * BLOCK_LENGTH); channel.writeAndFlush(new NetworkMessage(response, channel.remoteAddress())); } } @Override protected Object decode(ChannelHandlerContext ctx, Channel channel, ByteBuf buf) throws Exception { if (buf.readableBytes() < 5) { return null; } int length; if (firstPacket) { firstPacket = false; length = buf.getUnsignedMediumLE(buf.readerIndex() + 2); } else { length = buf.getUnsignedMediumLE(buf.readerIndex() + 1); } length += BLOCK_LENGTH; if (length % BLOCK_LENGTH != 0) { length = (length / BLOCK_LENGTH + 1) * BLOCK_LENGTH; } if ((buf.readableBytes() >= length || buf.readableBytes() % ACK_LENGTH == 0) && (buf != currentBuffer || buf.readableBytes() > acknowledgedBytes)) { sendResponse(channel); currentBuffer = buf; acknowledgedBytes = buf.readableBytes(); } if (buf.readableBytes() >= length) { return buf.readRetainedSlice(length); } return null; } }