Question

I am currently evaluating Netty to handle socket comms for a Java client to integrate with a C++ server. The messaging protocol has the following structure –

• (Type)(SubType)(Length)(MessageBody) with size
• <4bytes><4bytes><4bytes><…> – The Length includes the header.

Following the Netty api I subclass LengthFieldBasedFrameDecoder to receive a valid full packet and then decode each packet depending on the type received. From docs I’m using –

• lengthFieldOffset = 8
• lengthFieldLength = 4
• lengthAdjustment = -12 (= the length of HDR1 + LEN, negative)
• initialBytesToStrip = 0

It works fine for about 5 minutes (I’m getting one message every 5 seconds or so) and then the decode event contains a ChannelBuffer that is much shorter than the size of the message. (I have received this message multiple times before the crash). I then obviously get an BufferUnderflowException in my internal decode code. Am I doing something wrong? Should I be guaranteed the correct sized buffer for the message when using LengthFieldBasedFrameDecoder?

LengthFieldBasedFrameDecoder class –

public class CisPacketDecoder extends LengthFieldBasedFrameDecoder
{
    public CisPacketDecoder(int maxFrameLength, int lengthFieldOffset,
            int lengthFieldLength, int lengthAdjustment, int initialBytesToStrip) {
        super(maxFrameLength, lengthFieldOffset, lengthFieldLength, lengthAdjustment,
                initialBytesToStrip);       
    }

    @Override
    protected Object decode(ChannelHandlerContext ctx, Channel channel, ChannelBuffer buf)
            throws Exception 
    {       
        CisMessage message = null;      
        int type = buf.getInt(0); //Type is always first int
        CisMessageType messageType = CisMessageType.fromIntToType(type);
        if(messageType != null)
        {
            message = messageType.getObject();
            if(message != null)
            {
                message.decode(buf.toByteBuffer());
            }
            else
            {
                System.out.println("Unable to create message for type " + type);
            }
        }

        //mark the Channel buf as read by moving reader index
        buf.readerIndex(buf.capacity());
        return message;
    }

}

And instantiated here.

public class PmcPipelineFactory implements ChannelPipelineFactory 
{

    @Override
    public ChannelPipeline getPipeline() throws Exception
    {
        ChannelPipeline pipeline = Channels.pipeline();

        pipeline.addLast("encoder", new CisPacketEncoder());
        pipeline.addLast("decoder", new CisPacketDecoder(1024, 8, 4, -12, 0));      
        pipeline.addLast("handler", new MsgClientHandler());
        return pipeline;
    }


} 

Answer 1

You need to call super.decode(..) and operate on the returned ChannelBuffer in your decode(..) method.

So it would be like this;

public class CisPacketDecoder extends LengthFieldBasedFrameDecoder {
    public CisPacketDecoder(int maxFrameLength, int lengthFieldOffset,
            int lengthFieldLength, int lengthAdjustment, int initialBytesToStrip) {
        super(maxFrameLength, lengthFieldOffset, lengthFieldLength, lengthAdjustment,
            initialBytesToStrip);       
    }

    @Override
    protected Object decode(ChannelHandlerContext ctx, Channel channel, ChannelBuffer buf)
        throws Exception {

        // THIS IS IMPORTANT!!!!!
        ChannelBuffer decoded = (ChannelBuffer) super.decode(ctx, channel, buf);
        if (decoded == null) {
            return null;
        }

        // NOW ONLY OPERATE ON decoded 
        CisMessage message = null;      
        int type = decoded.getInt(0); //Type is always first int
        CisMessageType messageType = CisMessageType.fromIntToType(type);
        if(messageType != null) {
            message = messageType.getObject();
            if(message != null) {
                message.decode(decoded.toByteBuffer());
            } else {
                System.out.println("Unable to create message for type " + type);
            }
        }
        return message;
    }
}

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