如何从Netty向客户发送一个以上的回复

Question

I want to send more than one response to client based on back end process. But in Netty examples I saw echo server is sending back the response at the same time.

My requirement is, I need to validate the client and send him OK response, then send him the DB updates when available.

How can I send more responses to client ? Pls direct me to an example or any guide?

Answer 1

at every point in your pipeline you can get the pipeline Channel object from the MessageEvent object (or ChannelEvent ) which is passed from handler to handler. you can use this information to send multiple responses at different points in the pipeline.

if we take the echo server example as a base, we can add a handler which send the echo again (that can be done also in the same handler, but the example is to show that multiple handlers can respond).

    public class EchoServerHandler extends ChannelHandlerAdapter {

        public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
           Channel ch = e.getChannel();
           // first message
           ch.write(e.getMessage());
        }
        // ...
    }

    public class EchoServerHandler2 extends ChannelHandlerAdapter {

           public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
           Channel ch = e.getChannel();
           // send second message     
           ch.write(e.getMessage());
        }
        // ...
    }

Answer 2

You can do that as long as you have the reference to the relevant Channel (or ChannelHandlerContext ). For example, you can do this:

public class MyHandler extends ChannelHandlerAdapter {
  ...
  public void channelRead(ctx, msg) {
    MyRequest req = (MyRequest) msg;
    ctx.write(new MyFirstResponse(..));
    executor.execute(new Runnable() {
      public void run() {
        // Perform database operation
        ..
        ctx.write(new MySecondResponse(...));
      }
    }
  }
  ...
}

Answer 3

You can do this as long as Netty doesn't close the Channel. Its better you call close() yourself when you're done.

Here's a sample: https://stackoverflow.com/a/48128514/2557517