netty - 从 LengthFieldBasedFrameDecoder 返回的缓冲区太小

Question

我目前正在评估 Netty 以处理 Java 客户端与 C++ 服务器集成的套接字通信。消息传递协议(protocol)具有以下结构 -

(Type)(SubType)(Length)(MessageBody) 大小为

<4bytes><4bytes><4bytes><...> - 长度包括标题。

在 Netty api 之后，我将 LengthFieldBasedFrameDecoder 子类化以接收有效的完整数据包，然后根据接收到的类型对每个数据包进行解码。从我正在使用的文档中 -

lengthFieldOffset = 8

长度字段长度 = 4

lengthAdjustment = -12(= HDR1 + LEN 的长度，负数)

initialBytesToStrip = 0

它可以正常工作大约 5 分钟(我每 5 秒左右收到一条消息)，然后解码事件包含一个比消息大小短得多的 ChannelBuffer。 (我在崩溃前多次收到此消息)。然后我显然在我的内部解码代码中得到了一个 BufferUnderflowException 。难道我做错了什么？使用 LengthFieldBasedFrameDecoder 时，是否应该保证消息的缓冲区大小正确？

LengthFieldBasedFrameDecoder 类 -

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;
    }

}

并在这里实例化。

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

您需要调用 super.decode(..) 并在 decode(..) 方法中对返回的 ChannelBuffer 进行操作。

所以它会是这样的；

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;
    }
}

请务必检查大写注释。