How to read/write with netty when other side is using readUTF/writeUTF?

Question

I'm trying to communicate with a server that uses DataInputStream.readUTF and DataOutputStream.writeUTF.

I did the usual bootstrapping code to setup my client, and set the following pipelinefactory

    bootstrap.setPipelineFactory(new ChannelPipelineFactory() {

        @Override
        public ChannelPipeline getPipeline() throws Exception {
            return Channels.pipeline(
                    new LengthFieldBasedFrameDecoder(65536, 0, 2),
                    new StringDecoder(CharsetUtil.UTF_8),
                    new StringEncoder(CharsetUtil.UTF_8),
                    new MyClientHandler());
        }
    });

in MyClientHandler which extends SimpleChannelUpstreamHandler, I have the following:

    boolean sent = false; //is this thread safe?

    @Override
    public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
        logger.log(Level.INFO, e.getMessage().toString());
        e.getChannel().write("Hello over there!");

        if(!sent){
             //do something and set sent
        }
    }

I managed to receive messages from server successfully, but server is not receiving my "hello over there" message. Not sure what I might have overlooked.

Also, notice the boolean sent, can I add such fields and work with them without threading concerns?

Answer 1

• I managed to receive messages from server successfully, but server is not receiving my "hello over there" message. Not sure what I might have overlooked.

Because the message from the server was able to be received by using LengthFieldBasedFrameDecoder, the message has a length field.

 +--------+----------+
 | Length | Message  |
 +--------+----------+

Therefore, There is a possibility that the server will expect the received message has the length field. How if the length field is written as follows?

 +--------+---------------------+
 | 0x0011 | "Hello over there!" |
 +--------+---------------------+

[sample]

@Override
public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
    logger.log(Level.INFO, e.getMessage().toString());
    byte[] message = "Hello over there!".getBytes("UTF-8");

    ChannelBuffer buf = ChannelBuffers.buffer(message.length + 2);
    buf.clear();
    short len = (short)message.length;
    buf.writeShort(len);
    buf.writeBytes(message);
    e.getChannel().write(buf);

    if(!sent){
        //do something and set sent
    }
}

• Also, notice the boolean sent, can I add such fields and work with them without threading concerns?

Yes, you can add the fields to store some state. And you need not consider the synchronization of the thread.