Netty server send a byte[] encoded by Protobuf, but C# client Socket.Receive keeps being 0

Question

I am trying to accomplish an Unity game demo with network function, using C# for programming of client, and Java for server. To be specific, server communication is implemented by Netty. I also brought in Protobuf, which helps me define protocols of messages. As I am new to server programming, dealing with packet merging and loss in TCP has not been considered in my code yet.

When I created sockets from client, and sent message to server, everything went well. Problem happened when server replied: In the client, an async method is ready to receive message. When I simply sent a string-format message from server, the method were able to get it. But when I replaced the message with a 4-length byte[], which encoded from a Protobuf Message object, client just showed that it received NOTHING.

when I print what I've sent in the server console, it is like this: 00001000 00000001 00010000 00000001

My server code overrides channelRead and channelReadComplete functions of Netty. In channelRead, ChannelHandlerContext.write was invoked to write the message to the transmission cache. And in channelReadComplete, ChannelHandlerContext.flush was invoked, so that the message could be sent finally.

channelRead()

@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) {
   Request.MsgPack msgPack = (Request.MsgPack) msg;
   Request.MsgPack.MsgType type = msgPack.getType();
   switch (type)
   {
       case GetServerState:
            final Request.GetServerState gssbody = msgPack.getGetServerState();
            System.out.println("收到类型为" + type + "的消息，内容为：" +
                      "\nrequestId = " + gssbody.getRequestId()
                );
            byte[] bytes = ServerStateManager.getState(gssbody.getRequestId());
            ctx.write(bytes);
            break;

getState(): including Protobuf-encoding procedure

public static byte[] getState(int requestId)
{
     ReturnServerState.Message.Builder replyBuilder = ReturnServerState.Message.newBuilder();
     replyBuilder.setRequestId(requestId);
     replyBuilder.setIsIdle(new ServerStateManager().isIdle());
     return replyBuilder.build().toByteArray();
}

channelReadComplete()

@Override
public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
   try
   {
      ctx.flush();
   }
   finally
   {
      ctx.close();
   }
}

Client code:

public class ShortLink
{
    Socket clientSocket = null;
    static byte[] result = new byte[1024];

    Task ReceiveAsync<T>(string ip, int port)
    {
        return Task.Run(() =>
        {
           T component = default(T);
           while (clientSocket.Receive(result) == 0)
           {
               break;

ReceiveAsync is invoked in the way of:

await ReceiveAsync<ReturnServerState>(ip, port);

when I found clientSocket.Receive(result) always output 0, I tried to log result[0], result[1], result[2], result[3] like this:

Debug.Log(Convert.ToString(result[0]) + ", " +
          Convert.ToString(result[1]) + ", " +
          Convert.ToString(result[2]) + ", " +
          Convert.ToString(result[3]));

And the log turned to be 0,0,0,0.

I will be grateful for any idea of "why the client socket received nothing", and the solution. Since I come from Asia, there may be a time lag between your reply and mine, and also English is not my mother tongue. However, I will try my best to reply in time. Thanks a lot!

Answer 1

Okay..I have finally solve it myself

1.The usage "return replyBuilder.build().toByteArray()" is wrong because ProtoEncoder has already do toByteArray() for me:

public class ProtobufEncoder extends MessageToMessageEncoder<MessageLiteOrBuilder> {
    public ProtobufEncoder() {
    }

    protected void encode(ChannelHandlerContext ctx, MessageLiteOrBuilder msg, List<Object> out) throws Exception {
        if (msg instanceof MessageLite) {
            out.add(Unpooled.wrappedBuffer(((MessageLite)msg).toByteArray()));
        } else {
            if (msg instanceof Builder) {
                out.add(Unpooled.wrappedBuffer(((Builder)msg).build().toByteArray()));
            }

        }
    }
}

So once I registered "new ProtobufEncoder()" in the Netty Channel Pipeline, I can just use "return replyBuilder.build()" - that is correct.

2.In "static byte[] result = new byte[1024];", The length of received message is defined casually, and it doesn't matter - until it really receives a message.

When receiving message, I shall always copy the message bytes to a new byte[] with a correct length firstly - or there will be just a 1024-length bytes[], with the data I need at the beginning, and several zeroes following, which will certainly fail to be decoded.